Hi

I am trying to figure out how to calculate the first stage gain of the opamp shown in the lower portion of the attached picture. The opamp is similar to that of a folded cascode type but due to the presence of M10 and M11, I am not sure how to calculate the output resistance (and hence gain) of the first stage.

May I have some hints please? Thank you.

Best regards

Hi,

this topology has been published in
A compact power-efficient 3 V CMOS rail-to-rail input/output operational amplifier for VLSI cell libraries
Hogervorst, R.; Tero, J.P.; Eschauzier, R.G.H.; Huijsing, J.H.;
Solid-State Circuits, IEEE Journal of
Volume 29,  Issue 12,  Dec. 1994 Page(s):1505 - 1513

This topology provides class-AB operation of the output stage at a power-supply independent (and first-order process independent) quiescent current. Indeed, the use of a simple inverter in an integrated op-amp is usually prohibitive due to the large power consumption variations with process, temperature and supply.

As the amplifier output voltage is not affected actively, the amplifier gain is not modified. However, I am not exactly sure how M5 (added for symmetry?) in the other FC branch affects the amplifier behavior.

Hope this helps

Paul