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CS amplifier with resisitive load
Introduction and Theory

Shown in Fig. (a), the basic CS stage is similar to the common-emitter topology, with the input applied to the gate and the output sensed at the drain. For small signals, M1 converts the input voltage variations to proportional drain current changes, and RD transforms the drain currents to the output voltage. If channel-length modulation is neglected, the small-signal model in Fig. (b) yields vin = v1 and vout = -gmv1RD. That is,