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Originally Posted by Argus25
So I don't agree that in cascode " the upper valve greatly reduces the voltage gain of the lower valve"
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The voltage gain of the lower valve in a cascode is typically around unity, because its anode load is the cathode of the upper valve.
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but I do agree the voltage gain of the SRPP will be higher because of the higher impedance to signal voltage in the plate circuit created by the upper tube and the point from where the signal is extracted.
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The voltage gain of an SRPP can be higher or lower than the voltage gain of a cascode. This is because the SRPP gain is approximately mu, while the cascode voltage gain is approximately transconductance x anode load.
What I said was that the voltage gain of the lower valve in an SRPP is higher than the lower valve in a cascode. The ratio between the two is approximately mu.
The cascode is grounded cathode followed by grounded grid. The SRPP is grounded cathode followed by cathode follower, with bootstrapping. Hence they are quite different circuits. The problem is that when used in series form and drawn in the usual way they can look superficially similar at first glance.