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#1
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Need a good mosfet PN for a current sink
I'm wanting to use a FET as a current sink for a differential longtail.
This will be a balanced input front end using a 6922 with about a -60VDC on the tail. All the FETS I have around here seem to have the reverse protection diode built in. Can anyone recommend a good part number for this? I've been experimenting with the MJE350 bipolar part, but I want to try the FET and see if it will be more stable with temp and better PSRR. Any ideas appreciated. Thanks! RonL |
#2
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I'm wanting to use a FET as a current sink for a differential longtail. This will be a balanced input front end using a 6922 with about a -60VDC on the tail. ** So the FET connects from -60V to the two cathodes - right ? All the FETS I have around here seem to have the reverse protection diode built in. ** Every power MOSFET made is like that - so what ? Can anyone recommend a good part number for this? ** Almost any MOSFET with a Vds of 100 volts. I've been experimenting with the MJE350 bipolar part, but I want to try the FET and see if it will be more stable with temp and better PSRR. ** A MOSFET will be LESS stable with respect to temperature than a bi-bolar device. For bi-polars, the Vbe drops by 2mV per degree C rise for a given Ic - needing one silicon diode in the bias cct to correct. For a MOSFET , the Vgs drops by 6 mV per degree C rise for a given I ds - needing 3 silicon diodes in the bias cct to correct. ......... Phil |
#3
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Thanks Phil
That's good info. I've never compared them. But I noticed ARC used FETs Would it be a noise issue? Or a PSRR or compliance issue? I guess I need to build both and actually measure and compare. RonL I'm wanting to use a FET as a current sink for a differential longtail. This will be a balanced input front end using a 6922 with about a -60VDC on the tail. ** So the FET connects from -60V to the two cathodes - right ? All the FETS I have around here seem to have the reverse protection diode built in. ** Every power MOSFET made is like that - so what ? Can anyone recommend a good part number for this? ** Almost any MOSFET with a Vds of 100 volts. I've been experimenting with the MJE350 bipolar part, but I want to try the FET and see if it will be more stable with temp and better PSRR. ** A MOSFET will be LESS stable with respect to temperature than a bi-bolar device. For bi-polars, the Vbe drops by 2mV per degree C rise for a given Ic - needing one silicon diode in the bias cct to correct. For a MOSFET , the Vgs drops by 6 mV per degree C rise for a given I ds - needing 3 silicon diodes in the bias cct to correct. ........ Phil |
#4
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wrote: I'm wanting to use a FET as a current sink for a differential longtail. This will be a balanced input front end using a 6922 with about a -60VDC on the tail. All the FETS I have around here seem to have the reverse protection diode built in. Can anyone recommend a good part number for this? I've been experimenting with the MJE350 bipolar part, but I want to try the FET and see if it will be more stable with temp and better PSRR. Any ideas appreciated. You don't need a PNP fet. NPN fet is what you want, but they are fragile and expensive and no more effective than am MJE340, an npn bjt, as seen in the schematic at http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm The bjt CCS is seen in the middle of the schematic about 3/4 the way down the image. There would be zero benefit in using any other device because the stability of the current flow value etc is all very immune to temp changes since the current is set by the emitter resistor negative current feedback developed by the emitter resistor. And the simple MJE340 CCS is a better CCS than a pentode, unless the cathode resistor is indeed a large value, since the actual typical value of the current source resistance is above 20megohms which is enough for the source to be called a constant current source since the current changes such a tiny and totally insignificant amount. Patrick Turner. Thanks! RonL |
#6
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I have used STD1HNC60 but I notice they have been discontinued by
RS. I recall low capacitance with decent transconductance were key considerations. The smaller the better, probably. Check out those with the lowest current capacity and voltage limit you can get away with. Keep in mind that your CCS must be stable, and work well beyond the bandwidth of the amp. Patrick's presumably static measurements are not worth much. cheers, Ian wrote in message ... I'm wanting to use a FET as a current sink for a differential longtail. This will be a balanced input front end using a 6922 with about a -60VDC on the tail. All the FETS I have around here seem to have the reverse protection diode built in. Can anyone recommend a good part number for this? I've been experimenting with the MJE350 bipolar part, but I want to try the FET and see if it will be more stable with temp and better PSRR. Any ideas appreciated. Thanks! RonL |
#7
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Thanks Patrick!
I know I can always get my head on straight aftert reading your post. I have a MJE340 set up on the bench and it is solid as a rock. I had noticed that ARC used FET's and thought I might be missing something there. I still want to try the FET and make some test befor I commit to a PCB. Thanks Again RonL "Ian Iveson" wrote in message . uk... I have used STD1HNC60 but I notice they have been discontinued by RS. I recall low capacitance with decent transconductance were key considerations. The smaller the better, probably. Check out those with the lowest current capacity and voltage limit you can get away with. Keep in mind that your CCS must be stable, and work well beyond the bandwidth of the amp. Patrick's presumably static measurements are not worth much. cheers, Ian wrote in message ... I'm wanting to use a FET as a current sink for a differential longtail. This will be a balanced input front end using a 6922 with about a -60VDC on the tail. All the FETS I have around here seem to have the reverse protection diode built in. Can anyone recommend a good part number for this? I've been experimenting with the MJE350 bipolar part, but I want to try the FET and see if it will be more stable with temp and better PSRR. Any ideas appreciated. Thanks! RonL |
#8
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wrote: Thanks Patrick! I know I can always get my head on straight aftert reading your post. I have a MJE340 set up on the bench and it is solid as a rock. I had noticed that ARC used FET's and thought I might be missing something there. If you examine the circuit of the ARC SP11 preamp from around 1987, you will see how not to build a simple preamp IMHO because the sheer number of devices and bits and pieces and loops of high amounts of NFB are mind boggling. But the amp measures well; it measures astonishingly well for single ended triodes all hooked up one after the other. But just as good sound, or perhaps better sound many would say can be had from circuitry with 1/3 of the parts count. I still want to try the FET and make some test befor I commit to a PCB. Thanks Again RonL Allen Wright likes fets used for CCS, i suggest you take a wander around his site at http://www.vacuumstate.com He may not agree with an MJE340 for a preamp, maybe because there are lower noise fets which suit his preamps. Erno Borbelly also says a few words on fets.... Patrick Turner. |
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