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CS Amplifier with Active Load :

A MOSFET can operate as a resistor if its gate and drain terminals are shorted. This type of configuration is called as “diode connected” resistor as shown in Figure below.

In CMOS technology it is difficult to fabricate resistors with tightly controlled values of physical size. Hence the load resistor R_D is replaced by the MOS transistor.

To find the load resistance of the diode connected NMOS load consider the circuit shown in
Figure below and its corresponding small signal equivalent circuit.

By applying KVL,.

We have, V_GS = V_G - V_S = - V_x

V_BS = V_B - V_S = - V_x [ Body is connected to ground]

By applying KCL at Node A,

We get,

I_x + g_m V_GS + g_mb V _BS + = 0

i.e. I_x = g_m V_x + g_mb V _x + = V_x

=

i.e. = = || ro

\ The load resistance of NMOS diode connected load is given by || ro

For PMOS diode connected load consider Figure shown below, which shows the circuit to calculate load resistance and its small signal equivalent circuit.

Now, we have V_GS = V_x

\ By applying KCL,

We get, I_x = g_m V_GS + = g_m V _x +

I_x = V_x

= = = || ro

\ The load resistance of PMOS diode connected load is given as || ro.

Now consider the CS amplifier with diode connected load shown in figure below.

Here the gain of the amplifier is given by replacing the R_D with the corresponding load resistance of NMOS and PMOS diode connected loads.

\ For NMOS diode connected load.

A_n = - g_m1

For PMOS diode connected load,

A_n = - g_m1