Quote:
Originally Posted by kalee20
The 40μA signal current will therefore give rise to an emitter voltage variation of 1mV - the same as the input voltage to the stage!
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This is correct, but by omission it makes the very statement that I was trying to make.
Firstly though about the "rock solid" remark, hyperbole I agree, but the fact that the collector voltage of the lower device only moves about equal to the small signal input voltage is pretty "rock solid" compared to what it would be if there was the usual impedance in the collector (R or L etc) which allows its voltage to move 10 to 100 times more, so by comparison to that, I can say it is sort rock solid, but not zero though.
Secondly, in your analysis, you will note that your calculations did not have to refer to the gm of the upper transistor, just saying it was equal to the lower one.
You only needed its input impedance (25 Ohms) derived from its collector current, which ignoring base current, is its emitter current and depends on the lower device's collector current.
And then you calculated that the 40uA of signal current (generated by the lower transistor's gm) creates the emitter voltage variation in the upper transistor.
But this completely neglects the fact that nearly all (99%) at least, of the upper transistor's signal collector current (where the output signal of cascode is derived from) is from the lower transistor's signal collector current and perhaps only 1/100 being the upper transistor's base current, contributing to the lower transistor's collector current.
This means you could double the upper transistor's gm (or hfe) and the gain of the cascode circuit remains the same (all that this does is lower the upper transistors base current).
But the gain of the cascode circuit is directly linearly proportional to the gm of the lower device, which is the point I was making about where the gain in the cascode circuit originates, rather than saying the lower device has a "fixed gain of 1" which it only appears to have across the input and output terminals of the lower device.