Quote:
Originally Posted by Argus25
... 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.
This means you could double the upper transistor's gm (or hfe) and I the gain of the cascode circuit remains the same (all that this does is lower the upper transistors base current).
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That's true, but based on the assumptions I stated, the gm's will be the same (to within 1% as the emitter currents are equal to within 1%, the hfe being 100 and both devices at room temperature).
The only way to double the gm of a transistor, that I know of, is to double the operating current (impossible in a series-connected cascode), or run the transistor at double the absolute temperature. You can't choose another type with a different gm (unlike valves). A cascode comprised of a BC107 and a 2N3055 will still display equal gm's and have the same signal voltage on the collector of the lower device as on the input. It's not a massive signal, but it's not rock-steady either!