Simple model of a differential-input, single-ended-output MOS gain stage:

 

 

Evaluation of the output resistance

Based on the small-signal equivalent circuit:

 

Output resistance:

 

 

 

After using approximations and supposing the currents ix4 and ix3 cancel each other:

 

Low-frequency gain:

 

 

Related Reading

Gregorian, R., Temes G. C., Analog MOS integrated circuits for signal processing, Wiley, 1986.

Johns, D. A., Martin, K., Analog integrated circuit design, Wiley, 1997.

Razavi, B., RF Microelectronics, Prentice Hall, 1998.