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Old 6th Jan 2019, 2:33 am   #30
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Puzzling audio circuitry

The lower valve is indeed operating as a transconductance machine. In a normal single-valve amplifier stage the voltage gain is set by the transconductance (modified by any undecoupled cathode resistor) multiplied by the resistive load applied to the anode (accounting for the valve's own Ra)

The grid swings with the input, the anode swings with the output so it's fair to say that voltage gain can be seen at the valve. The amount of that gain is a function of not just the valve but also of the cathode and anode impedances it is embedded in.

It is the movement of the anode voltage versus the movement of the grid voltage which forms the Miller effect, and so this gain, seen at the three electrodes of a single valve which is the right gain to use in considering Miller-related.

Mu is the gain a valve will naturally give (at low frequencies) unless the valve is constrained by external impedances.

In the cascode the lower valve is heavily constrained indeed. Its anode thinks it's looking into a cathode follower backwards and that has a Z pretty similar to the valve's own reciprocal-Gm. -1 gain anode swing/grid swing still leaves some Miller effect, and in calculation of the pole frequency, Zs, Cga and this gain are needed. We might disagree over nomenclature, but it's a necessary parameter whatever we call it.

Done with transistors we can make the top stage Zin much lower and really push the pole up that last octave.

Like any circuit, it can be viewed in several ways. Each is right and each illuminates different aspects.

David
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