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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 VI 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»

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.


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