gain of 2-satge compared to cascode opamps?

[vishnukanchi]

Hi All,

How do I prove that the gain of a 2-stage op-amp is more than a telescopic cascode opamp?. Here, I have ron=rop and Gmn = Gmp.

For telescopic cascode opamp,

Rout = gmro1r02 // gmro1ro2 = gmro^2/2.

Gm = gm

Gain = gm^2 * ro^2 /2


For 2-stage

gm (ro1//ro2) * gm (ro1//ro2) = gm ^2 * ro^2/4.

Here, I am getting the 2-stage gain to be less, but I believe I am making some mistake here.

Please let me know the answer to this. I know this is a very basic query.

Thanks and Regards,
Vishnu

 

[akgupta]

Hi vishnu,
Your analysis seems to be correct, with the assumptions that ro remains same.
But ro strongly depends on VDS. So for a cascode amplifier when the output swings the gain will fall off drastically, compared to two-stage case. Even without swing in most of the new technologies you will find two-stage gain to be larger than cascode, because of VDS difference.

regards,
amit