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Output Capacitance of a Drain Loaded Amplifier

The output capacitance of the amplifier in Figure 1 is more then the drain to source capacitance (CDS). Looking in at the output terminals of this circuit, it is also possible to see the gate to drain capacitance (CGD). Sense there is gain from the input voltage, Vin to the output voltage, Vo across the terminals of CGD, it is expected its effects will be modified by the gain. To determine the apparent value of CGD as seen looking in the output (Vo) terminal, analyze the circuit of Figure 2.




The effective impedance and therefore the apparent reactance or capacitance of CGD can be determined in the following manner:

ZC,GD = Vo / iC,GD

Seen from the output terminal, the voltage if Vo and the current through the CGD branch is iGD. However, the gate to drain current (iGD) is a result of the voltage across CGD which is actually the output voltage minus the input voltage (Vo - Vin).


iC,GD = (Vo - Vin) / (1 / jωCGD)

iC,GD = (A Vin - Vin) jωCGD

Vo = A Vin

ZC,GD = A Vin / Vin(A - 1) jωCGD

ZC,GD = 1 / jωCGD ((A - 1) / A)

ZC,GD is the effective impedance of CGD as seen from the circuit output terminal. Therefore, the apparent gate to drain capacitance CGD of the branch must be:

CGD(A - 1 / A), where: A = -gmZL

So the output capacitance Cout of the amplifier in Figure 1 is:

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



Engineering key words: FET amplifier, drain, gate, source, load, capacitor, output cap, terminals, circuit analysis, voltage source, impedance, gain, Av, lead, measurement points, internal capacitance, amp, spec sheet, manufacture.


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