When channel length modulation effect lambda is equal to zer

When channel length modulation effect (lambda) is equal to zero, do we ignore r_o or gm_b ?

Solution

when channel-length modulation ( = 0), the output resistance is infinite. so we cant ignore the value of r_o

The channel-length modulation parameter usually is taken to be inversely proportional to MOSFET channel length L, we ignore gmb .

gmb = 1/r_o = 0