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FET Capacitance Effects

Field Effect Transistor circuits where this equivalent circuit is useful are assumed to be operating at frequencies where the reactance of the device capacitance is such that they do not significantly affect the circuit response. Such as, the equivalent has limited usefulness.





The FET demonstrates capacitive effects. These are labeled as CDS CGS CGD and are depicted on the equivalent circuit as follows:



Measurements of these capacitances are often difficult. Manufactures may specify the capacitances as Ciss Crss and Coss.


Where:

CDS = Ciss - Crss

CDS = Coss - Crss

CDS = Crss


The Miller Effect is the apparent modification of capacitance effect by a factor of the voltage gain between the capacitor terminals. The Miller Effect will modify the apparent capacitance between gate and drain (CGD) of the drain loaded amplifier:

CGD` = CGD(1 + gm ZL)

CGD` = CGD(1 - AVGD)

The Miler Effect also modifies the apparent capacitance from gate to source in the source follower and drain loaded amplifier with un-bypassed resistance in the source lead.



Cin = (GCS(1 - AVGS)) + (CGD(1 - AVGD))




AVGS is the voltage gain gate-to-source. This is zero (AVGS = 0) if the source is AC ground. AVGD is the voltage gain, gate-to-drain (-gm ZL in the drain loaded amplifier). At high frequencies the effects of Cin and Co are apparent. In a circuit test situation the scope probe capacitance, or other measuring instrument will appear in parallel and add to Cin or Co. The input gate circuit RC will establish one high frequency break-point. The output circuit RC will produce another.

The input circuit RC is the capacitance Cin and the R seen by the capacitance.

Req = RX / RG

C = Cin


On the output side, the R seen by the capacitance, Co is RL in parallel with rd.

The frequency break points established by each are calculated by the common relationship:

f = 1 / 2piRC

The accumulative high frequency effects can be determined by converting the input and output RC times to rise-time, then finding the overall circuit rise-time and converting this result to frequency response.

tr1,2 = 2.2RC

tr = 1.1 sqrt(tr12+tr22)

f2 = 0.35 / tr

Or by combining the interactive effects of the input pole and output pole when doing the Bode Plot of each.

Engineering key words: capacitor, field effect transistor, FET, equivalent device, measurement, specification sheets, spec sheet, manufacture, drain, gate, source, DGS, amplifier, amp, resistance, impedance, inductance, miller effect, load, voltage, current, susceptance, leads, break point, frequency, over driven.


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