# Dominant Pole of source follower

1. ## Dominant Pole of source follower

Why is the dominant pole equal to gm/(CL + Cgs) ?

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2. ## Re: Dominant Pole of source follower

P=1/(2*pi*RC)

The output resistance is 1/gm, the output capacitance is CL+Cgs.

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3. ## Re: Dominant Pole of source follower

In source follower the Cgs is between non-inverting nodes, so capacitive current from the source through this capacitor is negligible, the Cgd is an overlap capacitance, which is also small. The driving resistance, RS has to be low, than the second pole will be at high frequency (≈1/(RS*Cgd)).
Usually the load capacitance is relevant at source follower, which adds a pole to the circuit by the output resistance of the circuit, which is 1/gm. So the dominant pole is ≈gm/CL.

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4. ## Re: Dominant Pole of source follower

The following is the transfer function derivation of source follower as well as dominant pole approximation. The approximated dominant pole is gm/(CL+Cgs)

5. ## Re: Dominant Pole of source follower

Yes. Approximated.

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