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.
|
If the transconductance gain is due to the lower device (which is what I have been saying too) The only thing that converts transconductance gain to a voltage gain is the load resistor, not the upper device, the voltage gain is -gm.R where the gm is the lower device's gm and R the load resistance.
So can you explain then, how the upper device can increase the voltage gain beyond that provided by the lower device's gm and the load resistance product, that is the part I'm having trouble understanding. I accept that measured across the lower device's terminals the gain looks like -1, but it is still the lower device's gm that does the amplification, its current merely passes through to the load (as its a series circuit), making it appear as though the large signal variations there are due to the upper device.