voltage amplifier is constructed using enhancement mode MOSFETs labeled M1, M2, M3 and M4 in the figure below. M1, M2 and M4 are n-channel MOSFETs

Q. A voltage amplifier is constructed using enhancement mode MOSFETs labeled M1, M2, M3 and M4 in the figure below. M1, M2 and M4 are n-channel MOSFETs and M3 is a p-channel MOSFET. All MOSFETs operate in saturation mode and channel length modulation can be ignored. The low frequency, small signal input and output voltages are 𝑣_𝑖𝑛 and 𝑣_𝑜𝑢𝑡respectively and the dc power supply voltage is VDD . All n-channel MOSFETs have identical transconductance 𝑔_𝑚𝑛 while the p-channel MOSFET has transconductance 𝑔_𝑚𝑝. The expressions for the low frequency small signal voltage gain 𝑣_𝑜𝑢𝑡/𝑣_𝑖𝑛 is

Ans: +gmn/gmp

Sol:

M1, M2, M3, M4 are in saturation mode

M1, M2, M4 → NMOS

M3 → PMOS

By using small signal analysis

We can replace M1 and M3 by the internal impedance because drain and source terminal are short circuited

Now replace MOSFET to small signal model