Home > Analog CMOS Design > CMOS Opamp > Two Stage Op amp

Two Stage Op-amp :

In some applications the gain provided by single stage op-amps is not sufficient. In such circuits two stage op-amps are used. The first state of the op-amp provides the required large gain and second stage gives the large voltage swings. Figure below shows the simple two state op-amp circuit.

The first stage of the op-amp is a differential pair consists of transistors m₁, m₂, m₃ and m₄. The output V_a and V_y are the differential output 6 which are given by,

V_x = g_m2 (r₀₂ || r₀₄) V _in and

V_y = g_m1 (r₀₁ || r₀₃) V _in

These voltages V_x and V_y are then further amplified by the 2^nd stage which a is consists of the transistors m ₅, m₇ and m₆, m₈. The output voltage at the 2^nd stage is given by,

V_out1 = g_m5 (r₀₅ || r₀₇) V _y and

V_out2 = g_m6 (r₀₆ || r₀₈) V _x.

\ The total gain of the op-amp is the product of the gains of the stages which is given as,

A_v = g _m1 g _m5 (r₀₁ || r ₀₃) (r₀₅ || r₀₇)

or It can be also given as,

A_v = g _m2 g _m6 (r₀₂ || r ₀₄) (r₀₆ || r₀₈)