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Originally Posted by Argus25
However, there is no voltage gain from the upper valve, as its cathode and grid are at fixed potentials.
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In a cascode almost all the voltage gain comes from the upper valve. It cathode is not fixed at all, as it is connected to the lower anode.
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There is voltage gain, provided from the lower valve's gm, because, despite the fact that the plate voltage of the lower valve is fixed, the plate current passes via the upper valve and the load resistance there.
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The anode of the lower valve is not fixed, although it cannot move very much. It moves just enough to drive the upper valve.
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A valve in itself is not a voltage amplifier, its a transconductance device.
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It is both. You use whichever model is a better fit to the actual situation you put the valve in. Put a low resistance in the anode and it acts like a transconductance device. Put a very high resistance in the anode and it acts like a voltage amplifier, although with significant output resistance.
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Then considering the lower valve in the Cascode configuration, the fact that if the dynamic plate voltage measured with changes in grid voltage is negligible,
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It is not negligible. It is significant. It is what drives the upper valve.
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no significant voltage variations are seen, say with the scope.
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Voltage variations here would be easily seen with a scope.
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It seems somewhat meaningless to specify a voltage gain across two points in a circuit, where one of the points, the plate of the lower valve, has had its potential stabilized.
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It hasn't had its potential stabilised. The cathode impedance of the upper valve is small, but not zero. It is roughly 1/gm + Rload/mu, so as the anode resistance is increased and overall cascode gain is increased you see the voltage at the 'join' increasing too.
The voltage gain of the lower valve in a cascode is approximately 1 + Rload/ra. In a typical circuit this could be in the region of 2. So yes you can specify it and calculate it. You can think of the upper valve as either simply passing on the current sent from the lower valve as a transconductance device, or amplifying the voltage at the 'join'. The first analysis gives gm x Rload as the answer. The second analysis gives mu + Rload x gm - but this ignores the degeneration caused by the anode impedance of the lower valve, which can be significant. Doing a full calculation either way will give the same answer, as it must.