A voltage amplifier is constructed using enhancement mode $MOSFETs$ labeled $M_1,M_2,M_3$ and $M_4$ in the figure below. $M_1,M_2$ and $M_4$ are n-channel MOSFETs and $M_3$ 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 ooutput voltages are $v_{in}$ and $V_{out}$ respectively and the dc power supply voltage is $V_{DD}$. 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
- $-g_{mn}/g_{mp}$
- $-g_{mn}(g_{mn}+g_{mp})^{-1}$
- $+g_{mn}/g_{mp}$
- $g_{mn}(g_{mn}+g_{mp})^{-1}$