Question

If Vth is .45V, what is the output voltage for the following C.S. stage?

(a) 4.7 V

(b) 3.9 V

(c) 2.1 V

(d) 3.5 V

I had been asked this question during an internship interview.

The query is from MOSFET Amplifier with CS Configuration topic in division FET Amplifiers of Analog Circuits

Answer 1

The correct choice is (a) 4.7 V

Explanation: Firstly, we know that the voltage gain from the gate to the source is gm * R4. We observe that Vg is Vin * [(R2 || R3) / {R1 || R2 || R3}, where Vin is 1v and the values of R1, R2 and R3 are provided, which is roughly equal to .55V. This .55V is getting amplified by a factor of gm*R4. Now, Vout is related to Id, the drain current, as Vdd – Id*R4. Again, we know that gm = 2Id/Vgs-Vth. So, we have 2 equations as follows:

i. Vout = gm*R4 = 2*Id/(Vgs – Vth)*R4

ii. Vout = Vdd – Id * R4

We have the values of all the parameters. Solving for Vout will yield Vout as 4.7V.