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#161
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MOSFET output stage
Eeyore wrote:
Kevin Aylward wrote: Eeyore wrote: Kevin Aylward wrote: Anyone that claims that a general purpose PA amp, sounds bad or not good, if it has thd and imd 0.005% and slew rates of 100V/us, is pretty much delusional. Roll on the £200 oxygen free mains cable I say.... Why stop at £200 ? Oh ! http://www.asa.org.uk/asa/adjudicati..._ADJ_44177.htm This link seems to be bad. I did find this though Works here. worked now. http://divinecables.co.uk/mains-powe...i-power-cable# All I can say, is that I am still stunned..and shocked...shocked and stunned...all I said was that I was taller than Jesus, not that I was bigger than Jesus... I'd like to start a movement to have all these liars and fraudsters shut down. Indeed. If I had actually said that I was bigger than Jesus, I would have said so... As you may have noticed from visiting churches, the average height a while back, was a lot smaller... Kevin Aylward |
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#162
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Kevin Aylward wrote: Eeyore wrote: Kevin Aylward wrote: Eeyore wrote: Kevin Aylward wrote: Anyone that claims that a general purpose PA amp, sounds bad or not good, if it has thd and imd 0.005% and slew rates of 100V/us, is pretty much delusional. Roll on the £200 oxygen free mains cable I say.... Why stop at £200 ? Oh ! http://www.asa.org.uk/asa/adjudicati..._ADJ_44177.htm This link seems to be bad. I did find this though Works here. worked now. http://divinecables.co.uk/mains-powe...i-power-cable# All I can say, is that I am still stunned..and shocked...shocked and stunned...all I said was that I was taller than Jesus, not that I was bigger than Jesus... I'd like to start a movement to have all these liars and fraudsters shut down. Indeed. If I had actually said that I was bigger than Jesus, I would have said so... As you may have noticed from visiting churches, the average height a while back, was a lot smaller... By pure chance I just received some info from Russ Andrews (several booklets) on behalf of somone who didn't want to give their identity away. Cream of the bunch I think was a 'special' phono lead optimised for subwoofer connections in multi-speaker systems for deeper and richer bass or whatever nutcase story they were making up. I really would like to see that company go down along with all the fraudulent hi-fi rags too. Graham |
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#163
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
John Larkin wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. Fine. Can you elaborate some more on it ? Laterals have some truly lovely features for audio. The only downside being a slightly highish Ron. Not really a problem when (as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They also match beautifully with no need for source balance resistors (so some of the Ron loss 'goes away'). An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. Opamps are cheap, but fets and heat sinks are expensive. Power all those gate-drive opamps from a DC-DC converter floating on the output node; DC/DC bricks are cheap nowadays, too. Do a simple output current limit for fast overloads and back that up with a digital fet power dissipation simulation that provides the real protections. I've toyed in the past with doing device protection using an analogue multiplier actually funnily enough. That will optimize the hell out of the power supply, fets, and heat sink, giving a lot more safe power for the buck, especially in complex-signal non-sinusoidal apps like audio and NMR gradient drivers. Use a bunch of smaller fets rather than a few big ones; that speeds things up and spreads the heat out across the heat sinks better. For the audio version, use two "hot" heat sinks, with no insulators under the fets. Something I am very much in favour of. I like that junction to see cool aluminium as fast as possible. Include full BIST. It's worth it for the savings in production test alone. The output stage will be so quiet and linear that no overall feedback is needed or helpful. The audiophools will hate this. All the better ! Graham |
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#164
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On a sunny day (Fri, 19 Sep 2008 18:10:20 +0100) it happened Eeyore
wrote in : An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. If you loo kat the TDA9274 datahseet (the ST DMOS chip), then in the blockdiagram on page 2 you will see this is exactly what is done with the lower output MOSFET, combined with opamp makes unity gain. The top is already a source followwr. The over current protection same thing, opamps. It is likely this what makes the Boucherot network not needed. You could do the same thing with the top MOSFET in a discrete design, if you must. Opamps are cheap, but fets and heat sinks are expensive. Power all those gate-drive opamps from a DC-DC converter floating on the output node; DC/DC bricks are cheap nowadays, too. Do a simple output current limit for fast overloads and back that up with a digital fet power dissipation simulation that provides the real protections. I've toyed in the past with doing device protection using an analogue multiplier actually funnily enough. I only see over current protection in a lot of amps, plus thermal, the real thing was a bridge that caused foldback current limit, couple of resistors and diodes, makes it any load proof. |
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#165
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Eeyore wrote:
John Larkin wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. Fine. Can you elaborate some more on it ? Laterals have some truly lovely features for audio. The only downside being a slightly highish Ron. Not really a problem when (as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They also match beautifully with no need for source balance resistors (so some of the Ron loss 'goes away'). An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. I have a simple embodiment of that concept here, done a while ago, in virtual land;-) http://www.kevinaylward.co.uk/ee/cir...ortionAmp2.jpg Its a push/pull gain loop around the output devices, forcing them to be unity gain followers. You can get lower distortion, at the expense of speed, because you have to compensate earlier. Common mode feedback at the second stage, allows for enormous dc/lf gain. cascodes to allow the use of fast small transistors to do all the main work. Emitter follower buffer to reduce the current swing in the input pair, as per doug self. Spice says it should be in the 0.0001% , 20Khz range, maybe... Kevin Aylward |
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#166
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Jan Panteltje wrote:
On a sunny day (Fri, 19 Sep 2008 18:10:20 +0100) it happened Eeyore wrote in : An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. If you loo kat the TDA9274 datahseet (the ST DMOS chip), then in the blockdiagram on page 2 you will see this is exactly what is done with the lower output MOSFET, combined with opamp makes unity gain. The top is already a source followwr. The over current protection same thing, opamps. It is likely this what makes the Boucherot network not needed. I doubt it. I haven't seen the data sheet, but it is not usual to be able to stabilise an amp with what I believe you are describing here. Consider one amp feeding the other, both running open loop, with overall feedback, to the 1st. Now consider the case where the second amp is configured with local feedback, to make it a unity buffer, following the 1st amp. Naively , one might argue that the 2nd stage now has a wider bandwidth, due to feedback, such that the "new " system might be stable, i.e. one main rolloff due to the 1st amp. However, in realty, topologically, nothing changes. The determine the stability of the system, one needs to break the loop at a point that breaks *all* feedback paths at once. This point will be the point directly at the output of the 2nd amp. when this is done, it is clear that the stabiliy is still due to the total loop gain of both amops cascaded. Excepting for the special cases, e.g. , where feedback is used to neutralise r.f amps, feedback in general, cannot be used to widen bandwidth, if the purpose of that wider bandwidth is to achieve stability, in this type of arrangement To wit, There is no such thing as a free lunch... Kevin Aylward www.superspice.co.uk |
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#168
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On Fri, 19 Sep 2008 18:10:20 +0100, Eeyore
wrote: John Larkin wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. Fine. Can you elaborate some more on it ? Laterals have some truly lovely features for audio. The only downside being a slightly highish Ron. Not really a problem when (as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They also match beautifully with no need for source balance resistors (so some of the Ron loss 'goes away'). An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. It needs a little loop tweaking, roughly.... V+ |\ | -----------| + | | d | out-------+------Rg--------g | | s +------| - | | | |/ Cf | | | | +--------------------+------Rf---------+ | | Rs | | +------------- output rail and an opamp that can slam the gate hard enough, not a big problem nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs can be small, and the quiescent bias voltage can be small, because the opamp offset voltage can be tiny. The fets share the load exactly, and the standing bias current can be designed in, exactly, with no adjustments. The driver stage sees only opamps, so doesn't have to work very hard. Opamps are cheap, but fets and heat sinks are expensive. Power all those gate-drive opamps from a DC-DC converter floating on the output node; DC/DC bricks are cheap nowadays, too. Do a simple output current limit for fast overloads and back that up with a digital fet power dissipation simulation that provides the real protections. I've toyed in the past with doing device protection using an analogue multiplier actually funnily enough. I digitize everything - heatsink temp, supply currents, load voltage - and run a realtime simulation of fet power dissipation and resulting junction temperature, with shutdown at, say, 140 C. An adaptive fan speed would be a nice touch... no fan until it's really needed. Maybe next time. John |
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#169
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
John Larkin wrote:
On Fri, 19 Sep 2008 18:10:20 +0100, Eeyore wrote: John Larkin wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. Fine. Can you elaborate some more on it ? Laterals have some truly lovely features for audio. The only downside being a slightly highish Ron. Not really a problem when (as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They also match beautifully with no need for source balance resistors (so some of the Ron loss 'goes away'). An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. It needs a little loop tweaking, roughly.... V+ |\ | -----------| + | | d | out-------+------Rg--------g | | s +------| - | | | |/ Cf | | | | +--------------------+------Rf---------+ | | Rs | | +------------- output rail and an opamp that can slam the gate hard enough, not a big problem nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs can be small, and the quiescent bias voltage can be small, because the opamp offset voltage can be tiny. The fets share the load exactly, and the standing bias current can be designed in, exactly, with no adjustments. The driver stage sees only opamps, so doesn't have to work very hard. However....there are some issues with using whole op-amps, rather than discrete transistors as I have in the noted circuit in my other post. You may need 200V+ ratings, and a very fast one at that!!! Kevin Aylward www.kevinaylward.co.uk |
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#170
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On a sunny day (Fri, 19 Sep 2008 19:39:51 GMT) it happened "Kevin Aylward"
wrote in : If you loo kat the TDA9274 datahseet (the ST DMOS chip), then in the blockdiagram on page 2 you will see this is exactly what is done with the lower output MOSFET, combined with opamp makes unity gain. The top is already a source followwr. The over current protection same thing, opamps. It is likely this what makes the Boucherot network not needed. I doubt it. I haven't seen the data sheet, but it is not usual to be able to stabilise an amp with what I believe you are describing here. It is always a good idea to lookuop what we are talking about. Consider one amp feeding the other, both running open loop, with overall feedback, to the 1st. Now consider the case where the second amp is configured with local feedback, to make it a unity buffer, following the 1st amp. Naively , one might argue that the 2nd stage now has a wider bandwidth, due to feedback, such that the "new " system might be stable, i.e. one main rolloff due to the 1st amp. However, in realty, topologically, nothing changes. Some people are really good at that stuff, sort of reasonin gI mean. But if you look at the combination MOSFET - opamp, as John Larkin is describing in an other post in this thread, then you can treat that as one 'perfect MOSFET'. Of course it is not really perfect, but you can make that as stable or unstable as you want. Then basically what you do is chaining stable blocks together. If you then apply feedback, you have to use the phase characteristic of all those, and, as long as you prevent too high frequencies from circulating, it should be stable, and largely independent of the load. I note the TDA9274 has one capacitor to roll of in the driver... This is normal, at least in the amps I designed. The determine the stability of the system, one needs to break the loop at a point that breaks *all* feedback paths at once. This point will be the point directly at the output of the 2nd amp. when this is done, it is clear that the stabiliy is still due to the total loop gain of both amops cascaded. You mean 'open loop gain?' Yes, but he second amp would have gain 1. Excepting for the special cases, e.g. , where feedback is used to neutralise r.f amps, feedback in general, cannot be used to widen bandwidth, if the purpose of that wider bandwidth is to achieve stability, in this type of arrangement I was not suggesting to widen bandwidth, although strong local feedback would of course widen the bandwidth of a stage, Honestly, I have to think about this a bit, maybe run it in spice. Fact remains that the TDA9274 is the only amp I know that needs no Boucherot circuit :-) To wit, There is no such thing as a free lunch... It seems to exist for US bankers ATM. |
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#171
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Kevin Aylward wrote:
Eeyore wrote: John Larkin wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. Fine. Can you elaborate some more on it ? Laterals have some truly lovely features for audio. The only downside being a slightly highish Ron. Not really a problem when (as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They also match beautifully with no need for source balance resistors (so some of the Ron loss 'goes away'). An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. I have a simple embodiment of that concept here, done a while ago, in virtual land;-) http://www.kevinaylward.co.uk/ee/cir...ortionAmp2.jpg Its a push/pull gain loop around the output devices, forcing them to be unity gain followers. You can get lower distortion, at the expense of speed, because you have to compensate earlier. Common mode feedback at the second stage, allows for enormous dc/lf gain. cascodes to allow the use of fast small transistors to do all the main work. Emitter follower buffer to reduce the current swing in the input pair, as per doug self. Spice says it should be in the 0.0001% , 20Khz range, maybe... Kevin Aylward Kevin - an interesting circuit, and I appreciate what you have done with the output stage, but I'm still wondering why you didn't include it within the global feedback loop - that could only have made it better, lower output impedance, more load insensitive etc etc etc. Can you explain the thinking? d |
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#172
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On Sep 18, Eeyore wrote:
Who do MOSFET sound better than bipolar, as an audio amp output driver? As a driver ? Now if you said as an output stage it might make sense. There's a difference? Damn right there is. I'm thinking of the bits that attach to the copper thingy which loops around the magnets which make the air move. - Rich |
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#173
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Jan Panteltje wrote: On a sunny day (Fri, 19 Sep 2008 18:10:20 +0100) it happened Eeyore wrote in : An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. If you loo kat the TDA9274 datahseet (the ST DMOS chip), then in the blockdiagram on page 2 you will see this is exactly what is done with the lower output MOSFET, combined with opamp makes unity gain. The top is already a source followwr. The over current protection same thing, opamps. It is likely this what makes the Boucherot network not needed. You could do the same thing with the top MOSFET in a discrete design, if you must. Thanks for the tip. Opamps are cheap, but fets and heat sinks are expensive. Power all those gate-drive opamps from a DC-DC converter floating on the output node; DC/DC bricks are cheap nowadays, too. Do a simple output current limit for fast overloads and back that up with a digital fet power dissipation simulation that provides the real protections. I've toyed in the past with doing device protection using an analogue multiplier actually funnily enough. I only see over current protection in a lot of amps, plus thermal, the real thing was a bridge that caused foldback current limit, couple of resistors and diodes, makes it any load proof. Except you don't want an audio amp to foldback, just shut down when it sees an 'impossible' load. Graham |
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#174
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Kevin Aylward wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. Fine. Can you elaborate some more on it ? Laterals have some truly lovely features for audio. The only downside being a slightly highish Ron. Not really a problem when (as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They also match beautifully with no need for source balance resistors (so some of the Ron loss 'goes away'). An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. I have a simple embodiment of that concept here, done a while ago, in virtual land;-) http://www.kevinaylward.co.uk/ee/cir...ortionAmp2.jpg Its a push/pull gain loop around the output devices, forcing them to be unity gain followers. You can get lower distortion, at the expense of speed, because you have to compensate earlier. Common mode feedback at the second stage, allows for enormous dc/lf gain. cascodes to allow the use of fast small transistors to do all the main work. Emitter follower buffer to reduce the current swing in the input pair, as per doug self. Spice says it should be in the 0.0001% , 20Khz range, maybe... Not exactly short of current mirrors ! ;~) What gave you the idea ? Graham |
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#175
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
John Larkin wrote: Spehro Pefhany wrote: Anything approaching 1600A would blow the leadwires off a TO-220. The fusing current of a long wire of that cross-section area is only in the 30-50A range. 100A-rated wire is around 6mm (1/4") in diameter. Their selection tables show this as a 280 amp TO-220: https://ec.irf.com/v6/en/US/adirect/...9+429 4852430 but the datasheet qualifies this as merely 75: http://www.irf.com/product-info/data...ta/irf2804.pdf They do rate it for 330 watts! Pulsed I hope ! Graham |
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#176
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
John Larkin wrote: Eeyore wrote: John Larkin wrote: An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. It needs a little loop tweaking, roughly.... V+ |\ | -----------| + | | d | out-------+------Rg--------g | | s +------| - | | | |/ Cf | | | | +--------------------+------Rf---------+ | | Rs | | +------------- output rail and an opamp that can slam the gate hard enough, not a big problem nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs can be small, and the quiescent bias voltage can be small, because the opamp offset voltage can be tiny. The fets share the load exactly, and the standing bias current can be designed in, exactly, with no adjustments. The driver stage sees only opamps, so doesn't have to work very hard. It's certainly interesting. I'm wondering what the transition from one side to the other would be like i.e the crossover point. I'm wondering if one might get a brief dead band. What would be ideal would be if the power device never fully turned off and left say 10mA Iq. Opamps are cheap, but fets and heat sinks are expensive. Power all those gate-drive opamps from a DC-DC converter floating on the output node; DC/DC bricks are cheap nowadays, too. Do a simple output current limit for fast overloads and back that up with a digital fet power dissipation simulation that provides the real protections. I've toyed in the past with doing device protection using an analogue multiplier actually funnily enough. I digitize everything - heatsink temp, supply currents, load voltage - and run a realtime simulation of fet power dissipation and resulting junction temperature, with shutdown at, say, 140 C. Nice if you have the budget. I found a cheap NJR / JRC multiplier I had in mind. An adaptive fan speed would be a nice touch... no fan until it's really needed. Maybe next time. Oh that's dead easy, transistor on the heatsink, measure Vbe for the constant blow rate plus add a rectified sniff of the HT line ripple. The fans powers up before the heatsink gets hot. Ordinary DC fans work fine off a variable voltage. (c) me. LOL. Graham |
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#177
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Kevin Aylward wrote: John Larkin wrote: Eeyore wrote: John Larkin wrote: An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. It needs a little loop tweaking, roughly.... V+ |\ | -----------| + | | d | out-------+------Rg--------g | | s +------| - | | | |/ Cf | | | | +--------------------+------Rf---------+ | | Rs | | +------------- output rail and an opamp that can slam the gate hard enough, not a big problem nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs can be small, and the quiescent bias voltage can be small, because the opamp offset voltage can be tiny. The fets share the load exactly, and the standing bias current can be designed in, exactly, with no adjustments. The driver stage sees only opamps, so doesn't have to work very hard. However....there are some issues with using whole op-amps, rather than discrete transistors as I have in the noted circuit in my other post. You may need 200V+ ratings, and a very fast one at that!!! If you could clamp the input, would that do it ? Graham |
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#178
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
RichD wrote: Eeyore wrote: Who do MOSFET sound better than bipolar, as an audio amp output driver? As a driver ? Now if you said as an output stage it might make sense. There's a difference? Damn right there is. I'm thinking of the bits that attach to the copper thingy which loops around the magnets which make the air move. An unusual design but the copper thingy is very likely a heatsink, in which case you're referring to the actual output devices. They in turn usually have devices called 'drivers' which precede them, although it's less necessary with mosfets, only for ultimate performance.. Graham |
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#179
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On Fri, 19 Sep 2008 23:29:40 +0100, Eeyore
wrote: John Larkin wrote: Spehro Pefhany wrote: Anything approaching 1600A would blow the leadwires off a TO-220. The fusing current of a long wire of that cross-section area is only in the 30-50A range. 100A-rated wire is around 6mm (1/4") in diameter. Their selection tables show this as a 280 amp TO-220: https://ec.irf.com/v6/en/US/adirect/...9+429 4852430 but the datasheet qualifies this as merely 75: http://www.irf.com/product-info/data...ta/irf2804.pdf They do rate it for 330 watts! Pulsed I hope ! Graham Fig 8: 4KW for 100 usec, which isn't as frightening as 330 watts CW. John |
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#180
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
John Larkin wrote: Eeyore wrote: John Larkin wrote: Spehro Pefhany wrote: Anything approaching 1600A would blow the leadwires off a TO-220. The fusing current of a long wire of that cross-section area is only in the 30-50A range. 100A-rated wire is around 6mm (1/4") in diameter. Their selection tables show this as a 280 amp TO-220: https://ec.irf.com/v6/en/US/adirect/...9+429 4852430 but the datasheet qualifies this as merely 75: http://www.irf.com/product-info/data...ta/irf2804.pdf They do rate it for 330 watts! Pulsed I hope ! Fig 8: 4KW for 100 usec, which isn't as frightening as 330 watts CW. You just reminded me. There was one bipolar design I developed of a range of powers which was used extensively across a range of products including rack mount amplifiers and powered mixers. We must have built tens of thousands of the amp modules. Late in the proving process I ran one up with no fan. The heatsink reached 150C and there was a strong smell of hot aluminium plus creaking noises from thermal expansion before I took pity on it and powered it down. Worked fine the next day. TO-3 devices you see. Can't beat them. Graham |
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#181
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On Fri, 19 Sep 2008 21:13:15 GMT, "Kevin Aylward"
wrote: John Larkin wrote: On Fri, 19 Sep 2008 18:10:20 +0100, Eeyore wrote: John Larkin wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. Fine. Can you elaborate some more on it ? Laterals have some truly lovely features for audio. The only downside being a slightly highish Ron. Not really a problem when (as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They also match beautifully with no need for source balance resistors (so some of the Ron loss 'goes away'). An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. It needs a little loop tweaking, roughly.... V+ |\ | -----------| + | | d | out-------+------Rg--------g | | s +------| - | | | |/ Cf | | | | +--------------------+------Rf---------+ | | Rs | | +------------- output rail and an opamp that can slam the gate hard enough, not a big problem nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs can be small, and the quiescent bias voltage can be small, because the opamp offset voltage can be tiny. The fets share the load exactly, and the standing bias current can be designed in, exactly, with no adjustments. The driver stage sees only opamps, so doesn't have to work very hard. However....there are some issues with using whole op-amps, rather than discrete transistors as I have in the noted circuit in my other post. You may need 200V+ ratings, and a very fast one at that!!! The opamp power supply can be a cheap isolated dc/dc converter, +-12 volts maybe, floating on the output rail, so the opamps never see a lot of swing. Of course, a real circuit needs some protections for overload and startup conditions, but that's just a few diodes. The opamp inputs need never go more than a few tenths of a volt above or below the output rail. The floating opamp supply allows one to truly saturate the fets and swing the output all the way to both supply rails. (I assume some number of N-channel and P-channel opamp-composite fets in a real amp.) That pays for a dinky DC-DC sip thingie all by itself. John |
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#182
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On Fri, 19 Sep 2008 23:39:10 +0100, Eeyore
wrote: John Larkin wrote: Eeyore wrote: John Larkin wrote: An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. It needs a little loop tweaking, roughly.... V+ |\ | -----------| + | | d | out-------+------Rg--------g | | s +------| - | | | |/ Cf | | | | +--------------------+------Rf---------+ | | Rs | | +------------- output rail and an opamp that can slam the gate hard enough, not a big problem nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs can be small, and the quiescent bias voltage can be small, because the opamp offset voltage can be tiny. The fets share the load exactly, and the standing bias current can be designed in, exactly, with no adjustments. The driver stage sees only opamps, so doesn't have to work very hard. It's certainly interesting. I'm wondering what the transition from one side to the other would be like i.e the crossover point. I'm wondering if one might get a brief dead band. What would be ideal would be if the power device never fully turned off and left say 10mA Iq. Assuming some number of composite N-channel and P-channel fet thingies, as a complementary follower, some bias voltage has to be applied between the pseudo-gates (N-bank and P-bank opamp inputs) to set the idle current. When the mess starts to drive a load in one direction, some means ought to make sure the other bank doesn't go off, but maintains its bias current. That takes a little more "analog logic", still floating with all the rest of the stuff. That's not a big deal. Opamps are cheap, but fets and heat sinks are expensive. Power all those gate-drive opamps from a DC-DC converter floating on the output node; DC/DC bricks are cheap nowadays, too. Do a simple output current limit for fast overloads and back that up with a digital fet power dissipation simulation that provides the real protections. I've toyed in the past with doing device protection using an analogue multiplier actually funnily enough. I digitize everything - heatsink temp, supply currents, load voltage - and run a realtime simulation of fet power dissipation and resulting junction temperature, with shutdown at, say, 140 C. Nice if you have the budget. I found a cheap NJR / JRC multiplier I had in mind. You can do that, simulate the Tj's, once for each bank... Tnfet = Theatsink + K * (lowpass filter of) { (V+ - Vout) * Iout } Tpfet = Theatsink + K * (lowpass filter of) { (V- - Vout) * Iout } (except you have to get all the signs right) where K relates to Theta-junction-heatsink and the lowpass filter simulates the thermal mass of the silicon. My NMR amps needed a uP and ADC and display anyhow, so the fancy protections were pretty much just more code. The code runs at a few KHz. A real PITA, but free in production. My big amps display everything... temperatures, power supply voltages, TRMS load current, output power, error messages, tons of stuff. John |
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#183
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On Sat, 20 Sep 2008 00:07:58 +0100, Eeyore
wrote: John Larkin wrote: Eeyore wrote: John Larkin wrote: Spehro Pefhany wrote: Anything approaching 1600A would blow the leadwires off a TO-220. The fusing current of a long wire of that cross-section area is only in the 30-50A range. 100A-rated wire is around 6mm (1/4") in diameter. Their selection tables show this as a 280 amp TO-220: https://ec.irf.com/v6/en/US/adirect/...9+429 4852430 but the datasheet qualifies this as merely 75: http://www.irf.com/product-info/data...ta/irf2804.pdf They do rate it for 330 watts! Pulsed I hope ! Fig 8: 4KW for 100 usec, which isn't as frightening as 330 watts CW. You just reminded me. There was one bipolar design I developed of a range of powers which was used extensively across a range of products including rack mount amplifiers and powered mixers. We must have built tens of thousands of the amp modules. Late in the proving process I ran one up with no fan. The heatsink reached 150C and there was a strong smell of hot aluminium plus creaking noises from thermal expansion before I took pity on it and powered it down. Worked fine the next day. TO-3 devices you see. Can't beat them. Graham We tested a bunch of TO-247 power mosfets to destruction, in various sadistic ways. One was pure temperature. Vgs-th drops as temp goes up. The fets seemed to turn on hard, with 0 gate voltage, at 300C, but recovered when cooled. After 330C, they died, on hard, and didn't recover. It's hard to buy TO-3 fets any more. John |
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#184
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
John Larkin wrote: "Kevin Aylward" wrote: However....there are some issues with using whole op-amps, rather than discrete transistors as I have in the noted circuit in my other post. You may need 200V+ ratings, and a very fast one at that!!! The opamp power supply can be a cheap isolated dc/dc converter, +-12 volts maybe, floating on the output rail, so the opamps never see a lot of swing. Of course, a real circuit needs some protections for overload and startup conditions, but that's just a few diodes. The opamp inputs need never go more than a few tenths of a volt above or below the output rail. The floating opamp supply allows one to truly saturate the fets and swing the output all the way to both supply rails. (I assume some number of N-channel and P-channel opamp-composite fets in a real amp.) That pays for a dinky DC-DC sip thingie all by itself. Yes you need some high side drive for that. I've used that in both my mosfet and bipolar designs (to oversome multiple Vbe's (some of the drivers ran off of the high side rail) AND a Baker clamp to stop the last voltage gain stage transistor saturating). You probably do only need input protection. Might need to be clever about start up / turn-off conditions. Maybe a gate control ? I used that one on my amps too. 100% effective against power-out damage even if the previous signal chain generates a 'thump'. Graham |
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#185
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
John Larkin wrote: Eeyore wrote: I'm wondering what the transition from one side to the other would be like i.e the crossover point. I'm wondering if one might get a brief dead band. What would be ideal would be if the power device never fully turned off and left say 10mA Iq. Assuming some number of composite N-channel and P-channel fet thingies, as a complementary follower, some bias voltage has to be applied between the pseudo-gates (N-bank and P-bank opamp inputs) to set the idle current. When the mess starts to drive a load in one direction, some means ought to make sure the other bank doesn't go off, but maintains its bias current. That takes a little more "analog logic", still floating with all the rest of the stuff. That's not a big deal. I wish it were that simple. It's defeated me for years. Maybe a new set of eyes can discover the trick ? Opamps are cheap, but fets and heat sinks are expensive. Power all those gate-drive opamps from a DC-DC converter floating on the output node; DC/DC bricks are cheap nowadays, too. Do a simple output current limit for fast overloads and back that up with a digital fet power dissipation simulation that provides the real protections. I've toyed in the past with doing device protection using an analogue multiplier actually funnily enough. I digitize everything - heatsink temp, supply currents, load voltage - and run a realtime simulation of fet power dissipation and resulting junction temperature, with shutdown at, say, 140 C. Nice if you have the budget. I found a cheap NJR / JRC multiplier I had in mind. You can do that, simulate the Tj's, once for each bank... Tnfet = Theatsink + K * (lowpass filter of) { (V+ - Vout) * Iout } Tpfet = Theatsink + K * (lowpass filter of) { (V- - Vout) * Iout } (except you have to get all the signs right) where K relates to Theta-junction-heatsink and the lowpass filter simulates the thermal mass of the silicon. Absolutely. You can get all the thermal time constants in there and so on. My NMR amps needed a uP and ADC and display anyhow, so the fancy protections were pretty much just more code. The code runs at a few KHz. A real PITA, but free in production. My big amps display everything... temperatures, power supply voltages, TRMS load current, output power, error messages, tons of stuff. Nice. The pro-audio market wouldn't buy it though. Price (and weight) is currently everything, although there are a very few high end niche products with ethernet (or proprietary) control and monitoring around. Graham |
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#186
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
John Larkin wrote: Eeyore wrote: You just reminded me. There was one bipolar design I developed of a range of powers which was used extensively across a range of products including rack mount amplifiers and powered mixers. We musthave built tens of thousands of the amp modules. Late in the proving process I ran one up with no fan. The heatsink reached 150C and there was a strong smell of hot aluminium plus creaking noises from thermal expansion before I took pity on it and powered it down. Worked fine the next day. TO-3 devices you see. Can't beat them. We tested a bunch of TO-247 power mosfets to destruction, in various sadistic ways. One was pure temperature. Vgs-th drops as temp goes up. The fets seemed to turn on hard, with 0 gate voltage, at 300C, but recovered when cooled. After 330C, they died, on hard, and didn't recover. Were you estimating Tj ? It's hard to buy TO-3 fets any more. More's the shame. Tj max is typically rated 50C higher. And there are TWO bolts to hold then down with too. Need I mention the advantages of that ? Graham |
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#187
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On Sat, 20 Sep 2008 01:27:33 +0100, Eeyore
wrote: John Larkin wrote: Eeyore wrote: You just reminded me. There was one bipolar design I developed of a range of powers which was used extensively across a range of products including rack mount amplifiers and powered mixers. We musthave built tens of thousands of the amp modules. Late in the proving process I ran one up with no fan. The heatsink reached 150C and there was a strong smell of hot aluminium plus creaking noises from thermal expansion before I took pity on it and powered it down. Worked fine the next day. TO-3 devices you see. Can't beat them. We tested a bunch of TO-247 power mosfets to destruction, in various sadistic ways. One was pure temperature. Vgs-th drops as temp goes up. The fets seemed to turn on hard, with 0 gate voltage, at 300C, but recovered when cooled. After 330C, they died, on hard, and didn't recover. Were you estimating Tj ? No, we were heating the fets externally. It's hard to buy TO-3 fets any more. More's the shame. Tj max is typically rated 50C higher. And there are TWO bolts to hold then down with too. Need I mention the advantages of that ? But you can put a lot more silicon into a TO-247, especially the version without the mounting hole. John |
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#188
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
John Larkin wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: You just reminded me. There was one bipolar design I developed of a range of powers which was used extensively across a range of products including rack mount amplifiers and powered mixers. We musthave built tens of thousands of the amp modules. Late in the proving process I ran one up with no fan. The heatsink reached 150C and there was a strong smell of hot aluminium plus creaking noises from thermal expansion before I took pity on it And my colleagues who were looking slightly anxious. and powered it down. Worked fine the next day. TO-3 devices you see. Can't beat them. We tested a bunch of TO-247 power mosfets to destruction, in various sadistic ways. One was pure temperature. Vgs-th drops as temp goes up. The fets seemed to turn on hard, with 0 gate voltage, at 300C, but recovered when cooled. After 330C, they died, on hard, and didn't recover. Were you estimating Tj ? No, we were heating the fets externally. Sorry, I didn't read it properly. How about by self-heating ? It's hard to buy TO-3 fets any more. More's the shame. Tj max is typically rated 50C higher. And there are TWO bolts to hold then down with too. Need I mention the advantages of that ? But you can put a lot more silicon into a TO-247, especially the version without the mounting hole. Well, at least clips or mounting bars won't bend the tab ! Semelab/Magnatec make lateral fets with 2 matched dies in TO-3. Beat that ! 250W true continuous Pd. http://www.magnatec-uk.co.uk/latmos.shtml And quads too by the look of it ! http://www.magnatec-uk.co.uk/mosdata.shtml BUZ901X4S 200V 32A 500W SOT227 BUZ906X4S -200V -32A 500W SOT227 Graham |
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#189
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On Sep 19, 7:37*am, Eeyore
wrote: MooseFET wrote: Bewa *Mosfets like the STW55NM60 have a decreasing threshold voltage for increasing temperature. *This means that biasing them to a low idle current isn't so easy. Indeed. You'll need lossy ballast resistors. Laterals are different that way. Yes, but once the STW55NM60 or equiv. is biased up, it makes a nice 200V at about 100KHz. Its not exactly audio but is sure isn't really RF either. Graham |
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#190
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
MooseFET wrote: On Sep 19, 7:37 am, Eeyore wrote: MooseFET wrote: Bewa Mosfets like the STW55NM60 have a decreasing threshold voltage for increasing temperature. This means that biasing them to a low idle current isn't so easy. Indeed. You'll need lossy ballast resistors. Laterals are different that way. Yes, but once the STW55NM60 or equiv. is biased up, it makes a nice 200V at about 100KHz. Its not exactly audio but is sure isn't really RF either. Tell that to WWVB, who transmits at 60 kHz. -- http://improve-usenet.org/index.html aioe.org, Goggle Groups, and Web TV users must request to be white listed, or I will not see your messages. If you have broadband, your ISP may have a NNTP news server included in your account: http://www.usenettools.net/ISP.htm There are two kinds of people on this earth: The crazy, and the insane. The first sign of insanity is denying that you're crazy. |
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#191
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Don Pearce wrote:
Kevin Aylward wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. Fine. Can you elaborate some more on it ? Laterals have some truly lovely features for audio. The only downside being a slightly highish Ron. Not really a problem when (as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They also match beautifully with no need for source balance resistors (so some of the Ron loss 'goes away'). An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. I have a simple embodiment of that concept here, done a while ago, in virtual land;-) http://www.kevinaylward.co.uk/ee/cir...ortionAmp2.jpg Its a push/pull gain loop around the output devices, forcing them to be unity gain followers. You can get lower distortion, at the expense of speed, because you have to compensate earlier. Common mode feedback at the second stage, allows for enormous dc/lf gain. cascodes to allow the use of fast small transistors to do all the main work. Emitter follower buffer to reduce the current swing in the input pair, as per doug self. Spice says it should be in the 0.0001% , 20Khz range, maybe... Kevin Aylward Kevin - an interesting circuit, and I appreciate what you have done with the output stage, but I'm still wondering why you didn't include it within the global feedback loop - that could only have made it better, lower output impedance, more load insensitive etc etc etc. It is.!!! I think the schematic is not as clear as it should be.. I have a zero volt source near the output devices in the feedback circuit to calculate LG. The overall loop feedback passes through this source!!! Regards Kevin Aylward www.blonddee.co.uk www.kevinaylward.co.uk |
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#192
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Jan Panteltje wrote:
On a sunny day (Fri, 19 Sep 2008 19:39:51 GMT) it happened "Kevin Aylward" wrote in : If you loo kat the TDA9274 datahseet (the ST DMOS chip), then in the blockdiagram on page 2 you will see this is exactly what is done with the lower output MOSFET, combined with opamp makes unity gain. The top is already a source followwr. The over current protection same thing, opamps. It is likely this what makes the Boucherot network not needed. I doubt it. I haven't seen the data sheet, but it is not usual to be able to stabilise an amp with what I believe you are describing here. It is always a good idea to lookuop what we are talking about. Indeed. Consider one amp feeding the other, both running open loop, with overall feedback, to the 1st. Now consider the case where the second amp is configured with local feedback, to make it a unity buffer, following the 1st amp. Naively , one might argue that the 2nd stage now has a wider bandwidth, due to feedback, such that the "new " system might be stable, i.e. one main rolloff due to the 1st amp. However, in realty, topologically, nothing changes. Some people are really good at that stuff, sort of reasonin gI mean. But if you look at the combination MOSFET - opamp, as John Larkin is describing in an other post in this thread, then you can treat that as one 'perfect MOSFET'. Of course it is not really perfect, but you can make that as stable or unstable as you want. Then basically what you do is chaining stable blocks together. If you then apply feedback, you have to use the phase characteristic of all those, and, as long as you prevent too high frequencies from circulating, it should be stable, and largely independent of the load. If we consider calculating the actual LG=B(s).A(s) of the whole system, then it is identical whther or not there is local feedback around the 2nd stage or not. Yes, you can analyse as you describe here, but the result must be identical, as breaking all loops (if it can actually be done that is). So, it can't be because there is a local loop that there is no zobel network. I note the TDA9274 has one capacitor to roll of in the driver... This is normal, at least in the amps I designed. The determine the stability of the system, one needs to break the loop at a point that breaks *all* feedback paths at once. This point will be the point directly at the output of the 2nd amp. when this is done, it is clear that the stabiliy is still due to the total loop gain of both amops cascaded. You mean 'open loop gain?' No. I mean the loop gain. The loop gain is the open loop gain X the beta factor (e.g. resistive divider). It is the gain around the loop that matters. Yes, but he second amp would have gain 1. But thats my point. Its irrelevent that the closed lop gain of the second stage is unity when the loop is closed. When you do the stability analyisis you need to break all the loops. As I said, if you break the loop directly at the output of the second amp, which breaks both feedback loops at once, it is obvious that there overal loop gain is not effected by having the 2nd stage. It is the same loop gain Excepting for the special cases, e.g. , where feedback is used to neutralise r.f amps, feedback in general, cannot be used to widen bandwidth, if the purpose of that wider bandwidth is to achieve stability, in this type of arrangement I was not suggesting to widen bandwidth, although strong local feedback would of course widen the bandwidth of a stage, But not the BW of the final, overall amp, so the BW of the internal stages is irrelevant if it is the result of feedback. What the local loop can buy you is reduced distortion at lower frequencies. I ran these two circuits up quite a while ago. One has the UGB at the output, one doesn't. http://www.kevinaylward.co.uk/ee/cir...ortionAmp1.jpg http://www.kevinaylward.co.uk/ee/cir...ortionAmp2.jpg The UGB version had to be compensated earlier in frequency, i.e the non UGB version was significantly faster. Unfortunately, I cant remember much of the data, and I seem to have lost the SS files;-) I think one had 0.0001% at 20khz. Honestly, I have to think about this a bit, maybe run it in spice. Fact remains that the TDA9274 is the only amp I know that needs no Boucherot circuit :-) I would wager it's "non-optimum" designed. You give up a bit if the output load is not defined, usually. Without the zobel, the load reflected to the gain stages is all over the place. My guess is that they had a design goal of minimising the number of external components, which is standard practice in designing ics, but consequently, gave up some potential performance improvement. To wit, There is no such thing as a free lunch... It seems to exist for US bankers ATM. Kevin Aylward www.kevinaylward.co.uk www.blonddee.co.uk |
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#193
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Eeyore wrote:
Kevin Aylward wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. Fine. Can you elaborate some more on it ? Laterals have some truly lovely features for audio. The only downside being a slightly highish Ron. Not really a problem when (as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They also match beautifully with no need for source balance resistors (so some of the Ron loss 'goes away'). An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. I have a simple embodiment of that concept here, done a while ago, in virtual land;-) http://www.kevinaylward.co.uk/ee/cir...ortionAmp2.jpg Its a push/pull gain loop around the output devices, forcing them to be unity gain followers. You can get lower distortion, at the expense of speed, because you have to compensate earlier. Common mode feedback at the second stage, allows for enormous dc/lf gain. cascodes to allow the use of fast small transistors to do all the main work. Emitter follower buffer to reduce the current swing in the input pair, as per doug self. Spice says it should be in the 0.0001% , 20Khz range, maybe... Not exactly short of current mirrors ! ;~) What gave you the idea ? Graham The loop around the output came about from studying a patent that could not work as claimed, around 1983. This patent claimed distortion reduction by eliminating miller effect. It had a standard class A gain stage, but used a cascode. However, the cascode transistor base was connected to the output in a local feedback loop. This was the claim for distortion reduction. However, to1st order, the voltage on the base of a cascode transistor, don't effect anything. The input is current feed, irespective of the base potential, therfore the could not be any signal feedback. This led me to consider a way of doing it properly. I also noted that as the base was connected to the output, the emmiter of the cascode, and the collecter of the driving transister. hence the collecter still swings the full output, hence, millor effect is still there. So, that patent was complete nonsense, and my final circuit was completly different, but it followed from the thought flow of that patent. The key point in this approach was to get a push pull drive to both outputs.,i.e. to avoid low turn off resisters, which kills gain and drive. Kevin Aylward www.blonddee.co.uk www.kevinaylward.co.uk |
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#194
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Kevin Aylward wrote:
Don Pearce wrote: Kevin Aylward wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. Fine. Can you elaborate some more on it ? Laterals have some truly lovely features for audio. The only downside being a slightly highish Ron. Not really a problem when (as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They also match beautifully with no need for source balance resistors (so some of the Ron loss 'goes away'). An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. I have a simple embodiment of that concept here, done a while ago, in virtual land;-) http://www.kevinaylward.co.uk/ee/cir...ortionAmp2.jpg Its a push/pull gain loop around the output devices, forcing them to be unity gain followers. You can get lower distortion, at the expense of speed, because you have to compensate earlier. Common mode feedback at the second stage, allows for enormous dc/lf gain. cascodes to allow the use of fast small transistors to do all the main work. Emitter follower buffer to reduce the current swing in the input pair, as per doug self. Spice says it should be in the 0.0001% , 20Khz range, maybe... Kevin Aylward Kevin - an interesting circuit, and I appreciate what you have done with the output stage, but I'm still wondering why you didn't include it within the global feedback loop - that could only have made it better, lower output impedance, more load insensitive etc etc etc. It is.!!! I think the schematic is not as clear as it should be.. I have a zero volt source near the output devices in the feedback circuit to calculate LG. The overall loop feedback passes through this source!!! Regards Kevin Aylward www.blonddee.co.uk www.kevinaylward.co.uk Ah, is that what it was? I thought you were putting actual voltage sources in there that would be replaced by some small circuit in an actual design. I hope you are connecting the sensing point of the feedback resistor to the final summed speaker connection, not some random point in amongst the bunch of fets (teaching granny to suck eggs?) ;-) Actually the "right" place to connect the feedback sensing resistor is right out at the speaker itself, via a third sensing wire. d |
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#195
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MOSFET output stage
Kevin Aylward wrote: Jan Panteltje wrote: Honestly, I have to think about this a bit, maybe run it in spice. Fact remains that the TDA9274 is the only amp I know that needs no Boucherot circuit :-) I would wager it's "non-optimum" designed. You give up a bit if the output load is not defined, usually. Without the zobel, the load reflected to the gain stages is all over the place. My guess is that they had a design goal of minimising the number of external components, which is standard practice in designing ics, but consequently, gave up some potential performance improvement. Absolutely. I always use an isolating inductor twixt amp and load and the series RC to GND to define an accurate HF load. Never heard it called a "Boucherot circuit" though. Zobel network is the popular one here, although technically isn't that across the load itself ? There's a lot to be said for impedance compensation of speaker drivers too. Graham |
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#196
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MOSFET output stage
Don Pearce wrote: Actually the "right" place to connect the feedback sensing resistor is right out at the speaker itself, via a third sensing wire. You don't realise just how true that is. I've tuned pcb layouts for THD just by moving that node. PCB layout guys look perplexed but thankfully usually do it. In fact there's loads of layout tricks the PCB guys are hopeless at, loops in particular. In my 1200B design, all the power fets were mounted on a separate pcb to which power, drive and feedback wires were attached, manually soldered. If you didn't get the feedback wire bang in the centre up went the 2nd harmonic THD. Graham |
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#197
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
Don Pearce wrote: Kevin Aylward wrote: Don Pearce wrote: Kevin Aylward wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: John Larkin wrote: Eeyore wrote: Learn something about LATERAL mosfets that were designed for audio. I've already given part number and links to data sheets. That doesn't really matter. The transfer function only needs to be continuous so that you can close a loop around it, and the fet needs to be able to stand the peak power dissipation. That can easily be done with vertical "switching" type fets. A modern FLOOD architecture [1] works great with most any kind of fet. Lots of things have changed in the last few decades. John [1] Of course you've never heard the term before. I just invented it. Fine. Can you elaborate some more on it ? Laterals have some truly lovely features for audio. The only downside being a slightly highish Ron. Not really a problem when (as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They also match beautifully with no need for source balance resistors (so some of the Ron loss 'goes away'). An opamp per fet, closing a local loop, feedback from the fet source, makes each fet look like a perfect unity-gain, fast, zero-offset device. Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp output overshoot. I have a simple embodiment of that concept here, done a while ago, in virtual land;-) http://www.kevinaylward.co.uk/ee/cir...ortionAmp2.jpg Its a push/pull gain loop around the output devices, forcing them to be unity gain followers. You can get lower distortion, at the expense of speed, because you have to compensate earlier. Common mode feedback at the second stage, allows for enormous dc/lf gain. cascodes to allow the use of fast small transistors to do all the main work. Emitter follower buffer to reduce the current swing in the input pair, as per doug self. Spice says it should be in the 0.0001% , 20Khz range, maybe... Kevin Aylward Kevin - an interesting circuit, and I appreciate what you have done with the output stage, but I'm still wondering why you didn't include it within the global feedback loop - that could only have made it better, lower output impedance, more load insensitive etc etc etc. It is.!!! I think the schematic is not as clear as it should be.. I have a zero volt source near the output devices in the feedback circuit to calculate LG. The overall loop feedback passes through this source!!! Ah, is that what it was? I thought you were putting actual voltage sources in there that would be replaced by some small circuit in an actual design. I hope you are connecting the sensing point of the feedback resistor to the final summed speaker connection, not some random point in amongst the bunch of fets (teaching granny to suck eggs?) ;-) Actually the "right" place to connect the feedback sensing resistor is right out at the speaker itself, via a third sensing wire. Better still, have differential sensing, to compensate for the volt drop in the groundy side wire too ! Graham |
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#198
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On a sunny day (Sat, 20 Sep 2008 09:11:49 GMT) it happened "Kevin Aylward"
wrote in : I was not suggesting to widen bandwidth, although strong local feedback would of course widen the bandwidth of a stage, But not the BW of the final, overall amp, so the BW of the internal stages is irrelevant if it is the result of feedback. What the local loop can buy you is reduced distortion at lower frequencies. I ran these two circuits up quite a while ago. One has the UGB at the output, one doesn't. http://www.kevinaylward.co.uk/ee/cir...ortionAmp1.jpg Now that gets complicated, Q4 and the 2 diodes in the emitter... http://www.kevinaylward.co.uk/ee/cir...ortionAmp2.jpg The UGB version had to be compensated earlier in frequency, i.e the non UGB version was significantly faster. Unfortunately, I cant remember much of the data, and I seem to have lost the SS files;-) I think one had 0.0001% at 20khz. Yes, very good. Why use dotted paper ;-) |
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#199
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MOSFET output stage
On a sunny day (Sat, 20 Sep 2008 10:22:09 +0100) it happened Don Pearce
wrote in : Actually the "right" place to connect the feedback sensing resistor is right out at the speaker itself, via a third sensing wire. Ha, why did I never think of that... This will eliminate my massive gold feed rods to the woofer. You will need 2 sensing wires, and a diff amp. |
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#200
Posted to sci.electronics.design,rec.audio.tech
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MOSFET output stage
On a sunny day (Fri, 19 Sep 2008 23:24:02 +0100) it happened Eeyore
wrote in : I only see over current protection in a lot of amps, plus thermal, the real thing was a bridge that caused fold back current limit, couple of resistors and diodes, makes it any load proof. Except you don't want an audio amp to foldback, just shut down when it sees an 'impossible' load. Graham That depends, if you just current limit, and have thermal protection, then that takes time, perhaps enough time to melt some silicon. If an amp is designed for 4 Ohm minimum load, and somebody connects 2 speakers in parallel (for example), then current limit will step in. The voltage drop will be small over the 2 Ohm (or lower, in case of a near short), so say the transistors will have near full voltage at max current. It is clear that when current limiting, then you can lower the reference for the current limit circuit for low output voltages. This reduces dissipation. In fact (but this may hurt audio-people's egos perhaps), an audio amp is nothing but 2 symmetrical regulated power supplies ;-) In such a power supply one often also uses fold back for protection against bad loads, example 7805 regulator... |
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