$3.$ The MOS differential pairs with active load are implemented as shown in below. All NMOS and PMOS are symmetric $(40$pt$).$

Figure $3$

$1)$ Estimate the voltage gain of both circuit $($a$)$ and circuit $($b$)(20$pt$).$

$2)$ Discuss the advantage and disadvantage of circuit $($b$)$ compared to circuit $($a$)(20$pt$).$