Quote:
Originally Posted by Radio Wrangler
The lower triode is giving no significant voltage gain. It still gives plenty of current gain and power gain.
|
David,
It may just be semantics, please correct this if it is wrong:
A valve in itself is not a voltage amplifier, its a transconductance device. For it to be defined as having any
voltage gain or voltage amplification, it requires that the plate current variations be converted to a voltage (in a load resistance or impedance).
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, does not mean that the lower valve has no overall voltage gain in Cascode (considering the circuit as a whole), it just means at that point in the circuit, on the anode of the lower valve, where the voltage has been stabilized by the upper valve, no significant voltage variations are seen, say with the scope.
But the lower valve is still doing its usual job as a transconductance amplifier, and the voltage variations that are seen in the load resistor (in the anode of the upper valve) are due to plate current variations generated by the lower valve. The output signal from the cascode configuration is always taken from the load resistor (or some load impedance) of the upper valve, not the plate of the lower valve of course.
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. It makes more sense to specify the voltage gain from the input to output of the complete circuit , and the contributions from each valve in it, lower valve -gmR, upper valve 1.
Do you agree with this ?