Negative feedback configration around variable-mu stages?

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Ribbledox

Active member
Joined
Oct 15, 2004
Messages
33
Location
London
Hello everybody!

Still working on paper...

I just have a short question.

Regarding the waveform distortion in variable-mu compressors. I have read that negative feedback configurations were not pracitcal around variable-mu circuits, and thus one had to rely on the push-pull configuration.

I would be glad if someone could give me a hint to why this was the case?

Anyone? :oops:

Cheers!

R :cool:
 
Why so? And why is NFB better suitable around variable-resistance stages? And what about modern VCA's?

Thanks!

R
 
Feedback stabilizes the gain of an amplifier stage, which is not what you want if you are deliberately diddling with the gain of this amplifier stage.

A variable resistance compressor doesn't have an audio feedback path around it, either. In this case, the individual amplifiers have feedback to linearize them, but the resistance stage is not in a feedback loop, at least not at audio frequencies. Obviously the control voltage is another feedback loop but that's not going to linearize any audio amplification.

-Dale
 
But what about the feedback applied between the drain and gate of a JFET, in order to linearize the resistance curves?

- Evans, 'Designing with Field Effect transistors':

"By applying a part of the positive-going drain voltage to the gate, the gate-source voltage is made more positive (or less negative) and the thickness of the source end of the channel is increased. This compensates for the positive going drain-gate voltage.?

Thanks

/R
 
Basic circuit theory.

One way to look at it: distortion means the gain varies over a cycle. To reduce distortion, you eliminate reasons why gain changes. But in a limiter you WANT gain to change. And there isn't any simple way to reduce the gain-change over a waveform that doesn't also make average gain-change almost negligible.

NFB sets an upper limit on the gain of a variable-gain amplifier. If the raw amp gain varies from 1,000 to 2,000 (2:1 variation), you can use 100:1 feedback and the gain will only vary from 90 to 95 (1.05:1 variation). You can use assorted tubes and get the same gain every time.

However a limiter needs a gain that DOES drop, not a fixed gain. You could apply feedback, and it would linearize the no-GR distortion, but you are giving-up some of your total GR range. And the benefit would be lost as soon as gain does drop, which may be where your distortion is worst.

The most fancy GE tube limiter seems to have a couple extra triodes feeding-back to the input stage. As far as I can tell they are "open circuit" (no effect) at zero and low GR, only passing feedback at the highest GR.

> what about the feedback applied between the drain and gate of a JFET, in order to linearize the resistance curves?

Yeah, that does work. I think it is specific to variable-resistance FETs. I've never seen anything like that on tubes. I think it mostly just cancels another hidden "feedback effect" inside the bulk of the FET. We think about "Gate-Source voltage", but at these low levels the pinch-point inside the FET is not referenced to the drain but to some point between drain and source. The voltage-divider sets the gate voltage to be more-like what is happening inside the channel.

There are other topologies where feedback can be applied. A current-steering layout gives two outputs: one gets louder while the other gets softer. The sum of the two is constant. You can sum them and leak it back to the input as negative feedback. This won't fix all flaws though.
 

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