Quote:
Originally Posted by Argus25
Yes. I guess I was just struggling with the notion that a voltage gain figure for the lower valve on its own (its dynamic plate to dynamic grid voltage) could even be specified or have any meaning in the Cascode configuration, since the plate was held to a near fixed potential by the upper valve. Of course in transistor Cascode circuits the collector of the lower transistor has almost perfect clamping to a fixed potential and better than in the valve case.
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Hopefully we've resolved this - and it could be that Horowitz and Hill has misled you a bit Hugo - but the transistor cascode has the 'join' no more fixed than the valve cascode.
The lower device does not have zero voltage gain, it has unity gain, assuming the gm's of the two devices are equal.
Quote:
Originally Posted by G8HQP Dave
As I keep saying, the lower device has voltage gain if you use a big enough anode resistor. Just to pick some numbers at random, assume gm of 5mA/V and mu of 20. An anode resistor of 20k means a voltage gain of around 100. 20 of that comes from the upper valve, and 5 of that comes from the lower valve, approximately. 10mV in would become 50mV at the 'join' and then 1V at the output. What would you call an amplifying stage with 10mV in and 50mV out (or 50mV in and 1V out)? I would call it a voltage amplifier.
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That's a particularly useful illustration, Dave!
Quote:
Originally Posted by Argus25
Yes I agree and the valves are not ideal transconductance amplifiers either and that is indicated by the presence of the plate resistances in the equations (and of the valves) which alter the the voltages seen on the actual anodes and affect the gain calculations accordingly.
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Valves (triodes) aren't particularly ideal as transconductance devices, no. BJT's are better, from the point of view of the output. But don't forget that BJT's have a rather lousy input characteristic, as well as the necessary voltage drive they suck significant base current as a by-product. Triodes don't. If you look as a figure of merit, Rout/Rin, valves win hands down!