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Output Capacitance of a Source Follower Amplifier

The input capacitance of a source follower configuration shown in Figure 1 is defined as the sum of the gate to drain capacitance, and gate to source capacitance multiplied by, 1 minus the gate to source gain, so:

Cin = CGD + CGS(1 - AGS)

Where: AGS = RS / RS + (1/gm) if the drain resistance is much greater then the source resistance as is often the case (rd >> RS).

Therefore, the output capacitance seen looking toward the output voltage Vo terminal, presents another problem we will analyze. Letís begin by looking at the equivalent source follower circuit in Figure 2.

The equivalent circuit in Figure 2 can be further reduced into a more useful form, thus resulting in Figure 3, a simplified source follower configuration.

The effective impedance of the gate to source capacitance, CGS branch as seen looking in at the output voltage terminal, Vo is:

ZC,GS = Vo / iC,GS, where iC,GS = Vo - Vin / (1 / j ω CGS)

Sense Vo = AVin is the source follower gain:

iC,GS = (Vo - V0 / A) / (1 / j ω CGS)

iC,GS = Vo(A - 1 / A) j ω CGS


ZC,GS = Vo / iC,GS

ZC,GS = 1 / ((A - 1) / A) j ω CGS

ZC,GS = A / j ω CGS (A - 1)

Where, Z is equal to 1 / j ω C so the apparent capacitance of the CGS branch equates to,

CGS = (A - 1) / A

Therefore, the derived equation for the source follower amplifier configuration is,

Cout = CDS + CGS (A - 1 / A)

When, A = RS / (RS + 1 / gm) if rd >> RS

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