Subject: CMOS VLSI Design

Topic: MOS Operational Amplifiers

Difficulty: Medium

2- Stage Opamp:

i) In some application, gain and output swing of cascade op-amp is not adequate, we can use 2- stage op-amps to achieve the same.

ii) The basic block diagram of two stage op-amp is given by:

iii) 1st stage differential provides high gain. But still we will not get maximum output swings. So to achieve maximum swings, we use high swing stage -> CS stage.

iv) Simple 2-stage amplifier:

Gain of two stage op-amp is given by,

$A_v=A_{v1}\,A_{v2}$
where, $A_{v1}$=Gain of differential amp
$ \hspace{1cm} A_{v2}$=Gain of CS stage

$A_{v1}=\frac{V_{o1}}{V_{in}}=-gm_1(r_{o2}||r_{o4}) $

$ A_{v2}=\frac{V_{o1}}{V_{in}}=-gm_5(r_{o5} || r_{o6}) $

$\therefore A_v=gm_1\,gm_5\,(r_{o2}||r_{o4})(r_{o5} || r_{o6})$