CS Amplifier with Current Source Load :

Figure below shows the circuit diagram of CS amplifier with current source load. In this a current source is made by using a PMOS transistor operated in saturation mode by using a Gate bias Vb. The small signal model of this circuit is also shown in Figure below


The equivalent circuit can be drawn as shown in figure below


By applying KVL, Vin = VGS1

By applying KCL at node A

gm1 VGS1 + + = 0

\ gm1 Vin = - Vout

\ = An = - gm1 (ro1 || ro2)

In order to derive the output impedance consider the circuit shown in Figure below.


\ By applying KCL at node A,

We get, gm1 VGS1 + + = Ix

Vx = Ix [ VGS1 = 0]

\ = Rout =

\ Rout = (ro1 || ro2)

\ Hence the output impedance of current source load of CS amplifier is ro 1 || ro2.

Additonal Information