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#161
Posted to rec.audio.tubes
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Distortion in amplifiers / distortion in 2N3904 vs tubes.
Patrick Turner wrote:
If we had a pure CCS load to the 2N3904 collector, its open loop gain would be extremely high, perhaps 2,500... And with such enormous gain comes lots of distortions Hmm, thinking about it, if you cut down the input signal so the output voltage is reasonable and the load impedance is highish (say 680k) compared with the 6k8 you had before, then I think you would get low distortion (with or without an unbypassed emitter resistor, although the latter would be better). The high impedance (a CCS in parallel with something like 500k-1Mohm plus test equipment, or into a good transformer, means the percentage change in collector current would be very low, and this is what is needed to reduce distortion. Signal across the base-emitter junction varies with the log of the percentage change in collector current (as a first approximation, assuming we are biasing the transistor sensibly), and the distortion component of that signal drops very quickly as the signal decreases. So, if you still have the circuit laying around to test a modification, I think it is worth giving an active load for the 2N3904 a try - I think you could be pleasantly surprised with the results. Mark. |
#162
Posted to rec.audio.tubes
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Distortion in amplifiers / distortion in 2N3904 vs tubes.
Mark Aitchison wrote: Patrick Turner wrote: If we had a pure CCS load to the 2N3904 collector, its open loop gain would be extremely high, perhaps 2,500... And with such enormous gain comes lots of distortions Hmm, thinking about it, if you cut down the input signal so the output voltage is reasonable and the load impedance is highish (say 680k) compared with the 6k8 you had before, then I think Apparently not ! you would get low distortion (with or without an unbypassed emitter resistor, although the latter would be better). The high impedance (a CCS in parallel with something like 500k-1Mohm plus test equipment, or into a good transformer, means the percentage change in collector current would be very low, and this is what is needed to reduce distortion. Signal across the base-emitter junction varies with the log of the percentage change in collector current (as a first approximation, assuming we are biasing the transistor sensibly), and the distortion component of that signal drops very quickly as the signal decreases. So, if you still have the circuit laying around to test a modification, I think it is worth giving an active load for the 2N3904 a try - I think you could be pleasantly surprised with the results. Did you study at the University of Toytown ? Graham |
#163
Posted to rec.audio.tubes
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Distortion in amplifiers / distortion in 2N3904 vs tubes.
I wrote:
very low, and this is what is needed to reduce distortion. Signal across the base-emitter junction varies with the log of the percentage change in collector current (as a first approximation,... Oops! Not quite right, the way I wrote it. "Signal" is not the best word to use there. I mean: the base-emitter signal reduces with *percentage change* in collector current (which can be small for voltage outputs of decent size) as a first approximation, and the distortion in that small signal is very small. Even for larger signals, where the change in base voltage is not well enough approximated on a straight-line graph but from deltas on a log curve between input voltage and output current, arranging for the fractional change in output current to be low for a good output voltage is still good news because the base voltage is still shrinking as the load impedance increases, just not linearly, and the fraction of the small signal that is distorted shrinks quickly. And where I talk about a distortion component in the base voltage this only makes sense in the case of emitter degeneration (or other nfb); I often think of the input voltage being distorted with respect to the output as being another equally valid way of thinking about the output being distorted with respect to the input - maybe my brain thinks this way due to a misspent youth listening to Spike Milligan; I probably shouldn't write in these terms, as it probably confuses still-sane readers. But thinking in terms of distortion in the base signal is valuable because it relates nicely to the base junction I-V equation (after differentiating and using a bit of mathematics). Mark. |
#164
Posted to rec.audio.tubes
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Distortion in amplifiers / distortion in 2N3904 vs tubes.
Mark Aitchison wrote: Patrick Turner wrote: If we had a pure CCS load to the 2N3904 collector, its open loop gain would be extremely high, perhaps 2,500... And with such enormous gain comes lots of distortions Hmm, thinking about it, if you cut down the input signal so the output voltage is reasonable and the load impedance is highish (say 680k) compared with the 6k8 you had before, then I think you would get low distortion (with or without an unbypassed emitter resistor, although the latter would be better). The high impedance (a CCS in parallel with something like 500k-1Mohm plus test equipment, or into a good transformer, means the percentage change in collector current would be very low, and this is what is needed to reduce distortion. Signal across the base-emitter junction varies with the log of the percentage change in collector current (as a first approximation, assuming we are biasing the transistor sensibly), and the distortion component of that signal drops very quickly as the signal decreases. So, if you still have the circuit laying around to test a modification, I think it is worth giving an active load for the 2N3904 a try - I think you could be pleasantly surprised with the results. Mark. Assemble a circuit and test the measured results. That's the only way to really find out what is fact about such things. Bu my expereince is that were anyone to have a gain of say 2,000 from a single bjt, its utterly useless because of the noise, distortion, poor BW and high Rout, so a shirtload of shunt voltage NFB needs to be used. Where the collector load is a high ohm value, ie, CCS, massive dc displacement oocurs as well, and current FB doesn't work because there is such low current change. So voltage NFB has to be used. Setting up a darlington pair gain common emitter stage and with darlington pair in a µ-follower topology like a tube circuit will give you easily gain = 1,000, and a shunt FB network from the top device emitters to base input of the bottom gain devices to reduce gain to say 20 will result in again reduction of 50 times, and if the open loop THD was 5% at 10Vrms without NFB then expect 0.1% with NFB. Do the work to find out!!!! Analyse it all!! Understand all of what you are doing!! Forget trying to use a transformer in the collector circuit of a bjt. Now why did I say that? Patrick Turner. |
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