# MOSFET in Small Signal Operation MCQ’s

This set of Electronic Devices & Circuits Multiple Choice Questions & Answers (MCQs) focuses on “MOSFET in Small Signal Operation”.

1. Consider an NMOS transistor having k_{n}= 2 mA/V^{2}. Let the transistor be biased at V_{OV} = 1V. For operation in saturation, what dc bias current I_{D} results? If a +0.1-V signal is superimposed on V_{GS}, find the corresponding increment in collector current by evaluating the total collector current I_{D} and subtracting the dc bias current I_{D}.

a) I_{D} = 1mA and Increment = 0.21 mA

b) I_{D} = 1mA and Increment = 0.42 mA

c) I_{D} = 2mA and Increment = 0.21 mA

d) I_{D} = 2mA and Increment = 0.42 mA

2. We know I_{D} =1/2 k_{n} (V_{GS} + v_{gs} – V_{t})^{2}. Let the signal v_{gs} be a sine wave with amplitude V_{gs}, and substitute v_{gs} = V_{gs} sin ω t in Eq.(5.43). Using the trigonometric identity show that the ratio of the signal at frequency 2ω to that at frequency ω , expressed as a percentage (known as the second-harmonic distortion) is

a) V_{gs}/V_{ov} x 100%

b) 1/2V_{gs}/V_{ov} x 100%

c) 1/4V_{gs}/V_{ov} x 100%

d) 1/8V_{gs}/V_{ov} x 100%

3. An NMOS technology has μ_{n}C_{ox} = 50 μA/V^{2} and V_{t} = 0.7 V. For a transistor with L = 1μm, find the value of W that results in g_{m} 1mA/V at I_{D} = 0.5 mA.

a) 10 μm

b) 20 μm

c) 30 μm

d) 40 μm

4. If in a particular application V_{gs} is 10 mV, find the minimum overdrive voltage at which the transistor should be operated so that the second-harmonic distortion is kept to less than 1%.

a) 1V

b) 0.75V

c) 0.5V

d) 0.25V

(Q.5-Q.7) An NMOS amplifier is to be designed to provide a 0.50-V peak output signal across a 50-kΩ load that can be used as a drain resistor.

5. Using a dc supply of 3 V, what values of I_{D} and V_{OV} would you choose?

a) 0.34 mA and 0.35 V respectively

b) 0.34 mA and 0.69 V respectively

c) 0.034 mA and 0.35 V respectively

d) 0.034 mA and 0.69 V respectively

6. If a gain of at least 5 V/V is needed, what value of g_{m} is required?

a) 0.1 mA/V

b) 0.2 mA/V

c) 0.4 mA/V

d) 0.8 mA/V

7. What W/L ratio is required if μ_{n}C_{ox} = 200 μA/V^{2}?

a) 1.23

b) 1.23

c) 1.43

d) 1.53

(Q.8-Q.9) For a 0.8-μm CMOS fabrication process: V_{tn}= 0.8 V, V_{tp} = −0.9 V, μ_{n}C_{ox} = 90 μA/V^{2}, μ_{p}C_{ox }= 30 μA/V^{2}, Cox = 1.9 fF/μm2, V_{A} (n-channel devices) = 8L (μm), and |V_{A}| (p-channel devices) = 12L (μm).

8. Find the small-signal model parameters (g_{m}, r_{o} and g_{mb}) for a PMOS transistor having W/L = 20 μm/2 μm and operating at I_{D} = 100 μA and |V_{SB}| = 1V.

a) g_{m}= 0.24mA/V, r_{o}= 240 kΩ, g_{mb} = 0.024 mA/V

b) g_{m}= 0.24mA/V, r_{o}= 120 kΩ, g_{mb} = 0.048 mA/V

c) g_{m}= 0.24mA/V, r_{o}=240 kΩ, g_{mb} = 0.048 mA/V

d) g_{m}= 0.12mA/V, r_{o}= 240 kΩ, g_{mb} = 0.048 mA/V

9. Find the small-signal model parameters (g_{m}, r_{o} and g_{mb}) for an NMOS transistor having W/L = 20 μm/2 μm and operating at I_{D} = 100 μA and |V_{SB}| = 1V.

a) g_{m}= 0.42mA/V, r_{o}= 160 kΩ, g_{mb} = 0.084 mA/V

b) g_{m}= 0.21mA/V, r_{o}= 160 kΩ, g_{mb}= 0.042 mA/V

c) g_{m}= 0.42mA/V, r_{o}= 80 kΩ, g_{mb} = 0.042 mA/V

d) g_{m}= 0.24mA/V, r_{o}= 80 kΩ, g_{mb} = 0.084 mA/V

10. The overdrive voltage at which each device must be operating is

a) NMOS = 0.83V and PMOS = 0.48V

b) NMOS = 0.48V and PMOS = 0.83V

c) NMOS = 0.24V and PMOS = 0.41V

d) NMOS = 0.41V and PMOS = 0.24V

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