Question

If both the MOSFET’s are identical, what is the voltage gain from V1 to node S?

(a) Vcc – 2R1  * µn Cox  * (W/L) * (V1-Vth)^2

(b) Vcc   R1  * \(\frac{1}{2}\)µn Cox   (W/L) * (V1-Vth)^2

(c) Vcc  – R1  * µn Cox   (W/L) * (V1-Vth)^2

(d) Vcc  – 4R1  * \(\frac{1}{2}\)µn Cox * (W/L) * (V1-Vth)^2

I have been asked this question during an interview.

Enquiry is from MOSFET Amplifier with CS Configuration in chapter FET Amplifiers of Analog Circuits

Answer 1

The correct answer is (c) Vcc  – R1  * µn Cox   (W/L) * (V1-Vth)^2

For explanation I would say: Since M1 and M2 receive the same bias voltage V1, the current generated by both the MOSFET’s are same i.e. \(\frac{1}{2}\)µn Cox   (W/L) * (V1-Vth)^2. Both the currents enter node S and hence the voltage at node S is Vcc   R1  * \(\frac{1}{2}\)µn Cox   (W/L) * (V1-Vth)^2.