Quote:
Originally Posted by G8HQP Dave
In the cascode the upper valve greatly reduces the voltage gain of the lower valve; in the SRPP the upper valve raises somewhat the voltage gain of the lower valve. In some senses you could say that the SRPP is the opposite of a cascode!
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My understanding of cascode was that it was designed primary for RF circuits to circumvent the Miller effect. The main purpose was to pin the plate voltage at a fixed potential, thereby eliminating the effect of the Miller capacitance at the plate-grid circuit of the lower valve. This feature of course is not a great requirement for audio circuitry, so if a modified arrangement allows the plate voltage of the lower valve to move around with signal voltage, it is not a worry. In effect cascode behaves more or less exactly like a screen grid pentode.
Although, looking into the cathode of the upper valve is effectively a short circuit for signal voltages, the actual voltage amplification of cascode, the voltage gain A, primarily depends on the gm and the load resistance in the the anode of the upper valve and is A= -gm.R, provided u>>1 and and (u+2)rp>>R. (where rp is the plate resistance and u the amplification factor).So I don't agree that in cascode " the upper valve greatly reduces the voltage gain of the lower valve" because it is used with the load resistance, 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. So it is more of a Totem pole arrangement than cascode.