Differential-input, single-ended-output MOS gain stage

• Differential pair with a current mirror as an active load
  
• Typical first gain stage of a classical two-stage integrated operational amplifier
  
• Input differential stage realized using n-channel transistors
  
• Active current mirror load realized using p-channel transistors

Small signal analysis:

Ignoring transistor output impedances:

Positive small-signal current is defined as the current going into the drain of a transistor

Output voltage:

In the case of capacitive load:

z_out = r_out || 1/(sC_L)

Gain: