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how to find input impedance of common gate configuration

srini91

what is the general way to find input impedance of common gate amplifier stage in simulation. I tried to provide input as IAC mag =1 , trying to find the impedance at input node (Vin), trying to make it as(Vin/In) ,its coming as a gaussian distribution with peak of 95 Mega V at 300 Mhz.  I m running a system around 500 MHz, I m sure the for common gate amplifier input impedance must be low, but its coming like this. I m not sure whether my method is right or wrong.  for low frequency its coming around 350 ohm.

how to calculate generally input and output impedance .

  • jeremyh

    Your method seems reasonable (assuming IDC=0). I've used it with good results in the past.  The issue may be related to the impedance of your gate-biasing network.  If it is high-impedance and Cgs is large, your input voltage may be coupling to the gate, causing the AC component of vgs to be ~0 at high frequencies. You can test this hypothesis by dropping a large bypass cap on the gate or by plotting dB20(VF("/gate")-VF("/source"))

  • Qusai

    Hello Mr. Srini,

    Can I please talk to you about the common gate? I am having a few issues with it

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