Unsymmetric differential amplifier

[Aravind_tucson]

Could somebody please explain how the gain of a MOS differential amplifier changes with unsymmetrical input transistors.
Unsymmetric as in different W/L ratios. Usually both the input transistors have the same sizing.

 

[kshuang]

The unsymmetric match input pair has difference gain. For example, two-stage OPA output gain is unsymmetric. Also, ouputs has HD/DC and SR unsymmetric issue. But I think unsymmetric design case 'ONLY' suit fully-differential structure.

B.R.
Jerry[/youtube]

 

[acbalbason]

hi!

unsymmetric input diff pairs cause mismatch to your diff stage. if you notice you have a "tail" to control the current flowing through both pairs, ideally you have identical sizes so same currents flow through them. since current is proportional to W/L in a mosfet, diff W/L ratios mean different current through your transistors. usually gain is equal to gmRo where gm is transconductance and Ro is inversely proportional to current Ids. when you change W/L, you change current and you change Ro hence the gain also changes. usually gain is inversely proportional to current.

- al