Consider the fully differential amplifier shown below. The differential output of the amplifier is given by Vout $=$ Vop$-$Vom.

The transistors do not have channel$-$length modulation, i$.$e$.,$ ro is infinite. Both transistors are identical and have transconductance of gm$.$ Assume that there is no flicker noise. Each transistor has a noise current PSD given by $8$ kTgm and each resistor has a noise current PSD of $4k\frac{T}{R}.$

Calculate the total voltage noise PSD at the output.

$16$kTgm$+8$kTRD

$16$kTgm$+8$kT$/$RD

$16$kT$/$gm $+8kTR\frac{D}{g}{m}^2$

$16$kT$/$gm$+8$kT$/$RD