why the parameters of this opamp are set so strangely?

[huashizng]

a typical two stage opamp with NMOS input pairs, which is used as a buffer follower to increase the bandgap voltage 1.2v to 4.1v. there is a strange thing. the rail current applid to the differential pairs is only 1uA. the W/L ratio and m value of the nmos pairs is 20/1 and 4 respectively. the active load PMOS's parameters are: W/L=4/2,m=2.
the second stage is common source structure(PMOS), the size: W/L=20/1,m=8. the current flow the second stage PMOS is 8uA.

why are those parameters of the opamp so strange? what's the aim of the opamp?

whether it is for decreasing the input offset voltage? if it is, how to decrease?

 

[JT]

Hi,

What is strange about the opamp ?

It is very small in area (input pair and active load), and therefore will have a very large offset. (I estimate approx. 10..20 mV 1 sigma)

The offset is amplified by the ampifier, so you will see approx 30... 60mV sigma on the output. (which will also cause a lot of drift)

If you really need low offset, you should make the input pair and current mirror roughly 100 times bigger in area. (to have 10 times less offset).

 

[sunking]

it is for low offset+low noise

 

[bill_sun]

yes,increasing the L of the load (PMOS) is propitious to decreasing the noise.the second stage is common source PMOS providing a high PSRR.

 

[hunk]

en, guy you are good!