

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:
C_{in} = C_{GD} + C_{GS}(1  A_{GS})
Where: A_{GS} = R_{S} / R_{S} + (1/g_{m}) if the drain resistance is much greater
then the source resistance as is often the case (r_{d} >> R_{S}).
Therefore, the output capacitance seen looking toward the output voltage V_{o} terminal, presents another
problem we will analyze. Let’s 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, C_{GS} branch as seen looking in at the output
voltage terminal, V_{o} is:
Z_{C,GS} = V_{o} / i_{C,GS}, where i_{C,GS} = V_{o} 
V_{in} / (1 / j ω C_{GS})
Sense V_{o} = AV_{in} is the source follower gain:
Where, Z is equal to 1 / j ω C so the apparent capacitance of the C_{GS} branch equates to,
C_{GS} = (A  1) / A
Therefore, the derived equation for the source follower amplifier configuration is,
C_{out} = C_{DS} + C_{GS} (A  1 / A)
When, A = R_{S} / (R_{S} + 1 / g_{m}) if r_{d} >> R_{S}
Engineering key words: Cout, FET, field effect transistor, output cap, source follower, amp, amplifier,
configuration, input, gate, source, drain, computation, calculated, gain, Av, resistance, real, imaginary,
scientific notation, omega, pi, alphanumeric, spec sheet, manufacture, amplifier, load, inductance, circuit
analysis, branch analysis, voltage, current, substance.
