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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: Call for comments on tubes vs. transistors posted by theaudiohobby on July 15, 2006 at 21:59:29:
I'm not going to comment on the referenced articles, but the whole argument of "device type A vs device type B" makes no sense at all. What matters is the performance of the devices in actual circuits.A device which only has second-order distortion in its I-V characteristic (such as a JFET) can have quite complex high-order terms when used in a very simple circuit. In the case of the JFET, this happens from the apparently simple addition of a resistor from the source to AC ground. This is demonstrated by Boyk and Sussman. In that paper, they show the complex, high-order distortion products of a simple FET amplifier with a source resistor. This occurs in spite of the device characteristic containing only second-order terms.
OTOH, the BJT has lots of high-order distortion in its device I-V characteristic. But when an emitter resistor is used (analogous to the source resistor in the FET case) the situation changes. This is how Boyk and Sussman describe it.
"The spectrum of e, the BJT amplifier with feedback (figure 8) is like the spectrum without feedback except that all the distortion products are substantially attenuated. When we increase the feedback, as in circuit f, we see (figure 9) that the performance of the amplifier again improves dramatically, so that the principal distortion lines are suppressed below -100 dB and the next tier is suppressed below -180 dB. Thus, in the case of the BJT, feedback improves the nonlinear distortion of the amplifier.
Although the no-feedback BJT amplifier started out much worse than the no-feedback FET amplifier it is vastly improved by feedback; and because the transconductance of the BJT is much higher than the transconductance of the FET, we can use much more feedback and still obtain the same overall gain."
This is on page 17 of the paper. They then go on to show that the distortion of the BJT amp with emitter resistor is lower than that of the FET amp for the same gain and input level. They also go on to show vacuum tube circuits with cathode resistors having lots of higher-order distortion products.
One interesting observation related to their BJT analysis. There is a myth that feedback always increases higher-order harmonics relative to lower ones, regardless of configuration. This is true in the specific case of devices which exhibit only second-order distortion in their device characteristic (as demonstrated by Boyk and Sussman with FETs, and by Cheever). But Boyk and Sussman clearly show that assumption to be false in the case of BJTs.
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Follow Ups
- The fallacy of only considering device characteristics - andy_c 11:48:03 07/22/06 (2)
- Re: Thanks for these rather excellent comments and article -nt - theaudiohobby 12:23:05 07/23/06 (0)
- Excellent point, only the circuit performance actually counts. - Soundmind 03:33:41 07/23/06 (1)
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