[AMPS] Cathode Impedance Measurements

[Peter Chadwick]

In Class A, a g-g amp has an input impedance of 1/gm. What happens in Class
AB or B is another matter. Since the gain is approximately the ratio of
cathode volts to plate volts (because plate current is common to both
circuits) I surmise the input impedance can be determined from   Pin/(Ek
squared). That means settling down with the tube curves and working out the
volts and amps over the cycle. 

Using the resistors is fine if you know the value to shoot for - as Jon did.
If you don't, then the whole thing is probably more miss than hit. It's also
complicated because you're relying on the tank circuit Q to keep it free
wheeling, while for best linearity, you really want to make the feed
impedance very high - see Pappenfus et al.

73

Peter G3RZP


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