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101.

What is the relation between Vb and Vin in the following circuit?(a) Vb > Vin by Vth(b) Vb = Vin(c) Vb < Vin by Vth(d) No relationThis question was posed to me in my homework.Query is from MOSFET Amplifier with CG Configuration topic in section FET Amplifiers of Analog Circuits

Answer»

Right choice is (a) VB > Vin by Vth

Easy explanation: To turn an NMOS on, we need to keep the voltage DIFFERENCE between the GATE and the source greater than the threshold voltage. Hence, the gate potential should greater than the source potential by Vth.
Discussion
102.

What is the total output impedance at node x?(a) R1 || ro1 || 2 * ro2 || ro2 || 2*ro4… || ron(b) R1 || 2 * ( ro1 || ro2 … || ron)(c) R1 || ro1 || ro2 … || ron(d) R1 + (ro1 || ro2 … || ron)This question was posed to me in quiz.My question is based upon MOSFET Amplifier with CG Configuration topic in portion FET Amplifiers of Analog Circuits

Answer» RIGHT answer is (c) R1 || ro1 || ro2 … || ron

The best EXPLANATION: We note that all the MOSFET’s are in parallel. The output impedance of the i^th MOSFET is given by ROI. Hence, the total impedance at node X is simply R1 || ro1 || ro2…|| ron. Note that R1 is parallel to each MOSFET. This is because, while dining the output impedance, we SET Vin and Vcc to zero. Note that R1 is connected from node X to ground, all the MOSFET’s are connected form node x to ground.
Discussion