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Input impedance of grounded grid stage?

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#2
50AE said:
I'm trying to find info, but I'm stuck between two formulas.
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With a plate resistor Rl, a cathode resistor Rk, a tube plate resistance Rp, and a tube voltage gain mu.

If Rk goes to ground: Rin = {(Rl + Rp) / (mu + 1)} in parallel with Rk

If Rk goes to the input: Rin = {(Rl + Rp) / (mu + 1)} + Rk
 
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#4
50AE said:
I'm trying to find info, but I'm stuck between two formulas:

1. Zin = Eg / Ip = Rp + Zload / µ + 1
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This formula works for triodes, and shows how impedances are reflected between the plate and cathode. Triodes are unique in that the plate current is dependent on vpk to a much greater extent than for other active devices. This provides an internal NFB mechanism that accounts for the low dynamic plate resistance, and the improved linearity -- both at the expense of voltage gain.

2. 1 / S (transconductance

May you please clarify what is more correct?
Thanks in advance.
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This formula applies with the most accuracy to transistors and pentodes The screen grid isolates the plate, making the plate current more or less independent of the vpk, increasing both the voltage gain and dynamic plate resistance. Transistors isolate the collector/drain in a similar manner, but without the need for that extra element.
 
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