# Input impedance of grounded grid stage?

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#### 50AE

I'm trying to find info, but I'm stuck between two formulas:

1. Zin = Eg / Ip = Rp + Zload / µ + 1

in here: Grounded Grid Amplifiers

2. 1 / S (transconductance

May you please clarify what is more correct?

#### rayma

I'm trying to find info, but I'm stuck between two formulas.

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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#### jackinnj

I'm trying to find info, but I'm stuck between two formulas:

1. Zin = Eg / Ip = Rp + Zload / µ + 1

in here: Grounded Grid Amplifiers

2. 1 / S (transconductance

May you please clarify what is more correct?

Zin = Eg / Ip = Rp + Zload / µ + 1

Should be

Zin = Eg / Ip = (Rp + Zload) /( µ + 1)

#### Miles Prower

I'm trying to find info, but I'm stuck between two formulas:

1. Zin = Eg / Ip = Rp + Zload / µ + 1

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?

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.

#### 50AE

The exact explanation I needed! Many thanks!

#### Sch3mat1c

If you use the identity mu = Rp / gm, you will see that these are nearly equivalent.

#### DF96

One includes Rp, the other does not. In many cases this is a significant difference.

#### DF96

Hopefully the right one, as explained above in posts 2 and 3.

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