Question

The output resistance of FET when operating in pinch off at a current 10 mA and λ = 0.05 V^-1 is given as
1. 450 Ω 
2. 2 kΩ 
3. 200 Ω 
4. 2 Ω 

Answer 1

Correct Answer - Option 2 : 2 kΩ 

Concept:

The output Resistance of FET is defined as

\({r_o} = \frac{1}{{\lambda {I_D}}}\)

Where λ = channel length modulator parameter

I_D = drain current

Calculation:

Given

λ = 0.05 V^-1

I_D = 10 mA Important Point:

\({r_o} = \frac{1}{{\lambda {I_D}}} = \frac{1}{{0.05 \times 10 \times {{10}^{ - 3}}}} = 2K{\rm{\Omega }}\)

NOTE: 

If MOSFET does not have a channel length modulator then ⇒ λ = 0

\({r_{ds}} = \frac{1}{{\lambda {I_{DS}}}} = \infty \) (of P resistance)