Quote:
Originally Posted by Argus25
In the same way you can inject a signal into the screen grid of a pentode you can also do this with the grid of the upper valve in cascode which can make a handy "port" for some applications.
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In this case there might be a difference between the pentode and the cascode, though. By modulating the screen grid of a pentode (or for that matter its suppressor grid), the cathode current is hardly changed, but the anode current is, given that a variable amount is then diverted to the screen and leaves the valve by that route.
In the case of a cascode, modulating the upper valve grid cannot provide a variable alternative path for the electrons, as there is nowhere else for them to go. And I think it would need a fairly big voltage swing at the upper valve grid to produce (by cathode follower action) enough swing at the lower valve anode to have a material effect on the current through both valves.
I think that you could get there with a “triple” though, essentially a cathode coupled pair sitting on top of a third triode, with signal into the lower unit grid, and oscillator (or agc) into one of the upper unit grids, with the output taken from the anode of the other. In this case varying the signal or bias on the affected upper grid would vary the division of the current from the lower valve between the two upper valves.
Of course, transistor and fet cascodes are different in that respect, and the upper unit can be used to modulate the current flow, for such as oscillator injection, agc bias application.
Cheers,