Question

If we have 3 resistors of 1k, 2k and 3k, how should they be used amongst R1, R2 and R3 to get the maximum gate voltage in the following C.S. stage?

(a) R1 = 1k, R2 = 2k, R3 = 3k

(b) R1 = 3k, R2 = 2k, R3 = 1k

(c) R1 = 2k, R2 = 1k, R3 = 3k

(d) R1 = 3k, R2 = 1k, R3 = 2k

I got this question in exam.

The origin of the question is MOSFET Amplifier with CS Configuration in section FET Amplifiers of Analog Circuits

Answer 1

Right answer is (a) R1 = 1k, R2 = 2k, R3 = 3k

The best explanation: The Thevenin resistance seen by V1 is R1 + (R2 || R3). The gate voltage is a result of V1 going through a potential divider of R1 and (R2 || R3) where the voltage across (R2 || R3) is essentially the gate voltage ie V1 * [(R2 || R3) / { R1 || R2 || R3) }]. Hence, we need to maximize (R2 || R3) which is possible if R2=2k and R3=3k. This implies R1 has to be equal to 1k.