Quote:
Originally Posted by G8HQP Dave
No, the transconductance gain of the circuit is attributable to the lower transistor/valve/FET. The voltage gain comes from both, except perhaps for a BJT cascode where all the voltage gain comes from the upper device.
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Regarding the upper device as a common base or common grid amplifier, it is true that this configuration is said to have a high voltage gain, low current gain (usually near unity). These remarks though relate to driving it with a voltage source and comparing the input & output voltages across the load.
In Cascode of course, the drive is primarily a current source (or sink) from the lower valve (or transistor) and the dynamic current signal (derived from the lower device's transconductance) is in no way "amplified" by the upper device.
The dynamic current signal is converted to a dynamic voltage in the load resistance in the usual way. Thinking about it this way, it is hard to attribute any "gain" to the upper device (in the application) even though measuring across its 3 terminals (as RW pointed out) it is behaving in the usual way as a transconductance amplifier, but its current is being controlled by the lower device.