Figure (a) shows a MOS cascode amplifier loaded with an ideal constant-current source. The voltage gain realized can be found from the equivalent circuit in figure (b). Since the load is an ideal constant-current source, the load resistance is infinite. That is, the amplifier is operating with an open-circuit load, and the gain is

- (1)

Now, since Rs of Q2 is ro1, the output resistance Ro is given by the approximate expression

Substituting in equation (1), we get

For the case gm1 = gm2 = gm and ro1 = ro2 = ro,

Thus cascoding increases the gain magnitude from