# Microelectronics Questions and Answers – MOS Amplifiers – Common Source Stage – Set 2

This set of Microelectronics Multiple Choice Questions & Answers (MCQs) focuses on “MOS Amplifiers – Common Source Stage – Set 2”.

1. In presence of Channel Length Modulation, what is the voltage gain of the following circuit?

a) -gm1*(1/gm2 || (ro1 – ro2))
b) -gm1*(1/gm2 || 2*ro1 || ro2)
c) -gm1*(1/gm2 || ro1 || ro2)
d) -gm1*(1/gm2 || (ro1 + ro2))

Explanation: We note that the output resistance of this stage is ((1/gm2 || ro1 || ro2). The transconductance of this M1 is gm1. Not that the current (gm1*Vin) flowing through M1 will generate a voltage drop across this output resistance and we seek this voltage drop as the output voltage. We consider this transconductance because the input is given to M1 and not M2. Thus, the overall voltage gain is -gm1*((1/gm2 || ro1 || ro2) since the output is taken due to M1 behaving as a Common Source Stage.

2. In comparison to the diode-connected load, what does this circuit offer?

a) A higher gain
b) A lower gain
c) Equal gain
d) Unequal gain

Explanation: This circuit offers a higher gain. This is because the output impedance of this stage is higher than the diode-connected load Common Source Stage.

3. In comparison to the resistive load CS Stage, what does this circuit offer?

a) Error in design
b) Less capacitance
c) More chip size
d) Plausible chip size

Explanation: To get a greater gain from the resistive load CS stage, we use a higher impedance of increase the transconductance. Now, increasing the impedance leads to an increased area for the resistance in the semiconductor chip. However, the design of M2 allows us to use a lesser area in the bar. Though, we must note that the capacitance in the chip has increased and can lead to problems if the aspect ratio isn’t checked properly.

4. Using the following circuit, we increase the aspect ratio of M2 & find that the gain has increased. What is the backdrop?

a) The gain will decrease
b) Less Capacitance
c) Less resistance
d)Less Speed

Explanation: The speed of the circuit reduces. Increasing the aspect ratio leads to an increase in the value of Cox which in turn leads to a decrease in the speed of the circuit. This is because the presence of these capacitances decreases the transit frequency, which is approximately inversely proportional to the CGS, and thus the speed is said to be reduced.

5. We want to increase the intrinsic gain of the MOSFET by only increasing W. What happens to the output impedance for the device only?
a) It increases
b) It decreases
c) It remains same
d) There is no intrinsic gain

Explanation: Increasing W will increase the transconductance of the device. However, it will decrease the output impedance since it is inversely proportional to the drain current while the drain current has increased due to an increase in W. Note that the gain will actually decrease since the increase in transconductance is less compared to the increase in the current.

6. What happens to the output impedance of a CS stage if its degenerated by a resistor at the source?
a) It increases
b) It decreases
c) It remains the same
d) It depends on the design

Explanation: The output resistance of a CS stage will always increase if it is degenerated. This is observed from the expression of the output resistance for a degenerated common source stage.

7. What happens to the voltage gain of a CS stage if its degenerated by a resistor at the source?
a) It increases
b) It decreases
c) It remains the same
d) It depends on the aspect ratio

Explanation: The gain of the CS stage reduces when it is degenerated by a CS stage at the source. This can be understood there is a certain voltage drop at the source which reduces the output voltage.

8. What happens to the output resistance of the following circuit if only L is increased?

a) It increases
b) It decreases
c) It remains unchanged
d) It increases and then decreases

Explanation: The output resistance of the above circuit is (R3 || ro). ro is proportional to L/ID. If L increases, ro increases and the output resistance increases.

Sanfoundry Global Education & Learning Series – Microelectronics.

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