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A CG amplifier is specified to have Cgs = 4 pF, Cgd = 0.2 pF, CL = 2 pF, gm = 5 mA/V, Rsig = 1 kΩ, and RL = 10 kΩ. Neglecting the effects of ro, find the low-frequency gain Vo/Vsig, the frequencies of the poles fP1 and fP2, and hence an estimate of the 3-dB frequency fH.

A CG amplifier is specified to have Cgs = 4 pF, Cgd = 0.2 pF, CL = 2 pF, gm = 5 mA/V, Rsig = 1 kΩ, and RL = 10 kΩ. Neglecting the effects of ro, find the low-frequency gain Vo/Vsig, the frequencies of the poles fP1 and fP2, and hence an estimate of the 3-dB frequency fH.

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A CG amplifier is specified to have Cgs = 4 pF, Cgd = 0.2 pF, CL = 2 pF, gm = 5 mA/V, Rsig = 1 kΩ, and RL = 10 kΩ. Neglecting the effects of ro, find the low-frequency gain Vo/Vsig, the frequencies of the poles fP1 and fP2, and hence an estimate of the 3-dB frequency fH.

Explanation & Steps

First of all draw the small signal model of the CG amplifier circuit to determine voltage gain vo/vsig. Channel-length modulation and body bias effect is to be neglected while drawing small-signal model.

Next, apply open-circuit time constant method by calculating resistance Rgs seen by Cgs and resistance Rgd  seen by (CL + Cgd). Pole frequencies fp1, fp2 and 3-db high frequency, fH can be determined using Rgs, Cgs, Rgd, CL and Cgd.

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