For the circuit shown below the transistor parameter are

For the circuit shown below the transistor parameter are V_TN = 0.8 V and kn = 30 µ A/V². If output voltage is V₀ = 0.1 V, when input voltage is V_i = 4.2 V, the required transistor width-to-length ratio is:

Given that
V_TN = 0.8 V
k′_n = 30 µA/V²
V₀ = 0.1 V
V_i = 4.2 V
From fig. V_GS = V_i = 4.2 V

and I_D =	5 – V₀	=	5 – 0·1
	10 kΩ		10 × 10³

=	4·9	= 0.49 mA
	10 × 10³

Again I_D =	1	. k'_n		W			(V_GS–V_TN)₂
	2			L

= 0.49 × 10^–3 or 0.49 × 10^–3

=	1	× 30 × 10^–6.		W			. (4.2 – 0.8)²
	2			L

or		W		=	0·49 × 10³	= 2.825
		L			15 × (3·4)²

Hence alternative (D) is the correct choice.