Electronic Devices and Circuits Questions and Answers – MOSFET in Small Signal Operation

This set of Tricky Electronic Devices and Circuits Questions and Answers focuses on “MOSFET in Small Signal Operation”.

1. An NMOS technology has μ_nC_ox = 50 μA/V² 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
View Answer

Answer: b
Explanation:

2. Consider an NMOS transistor having k_n= 2 mA/V². 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
View Answer

Answer: a
Explanation:

3. 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%
View Answer

Answer: c
Explanation:

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
View Answer

Answer: d
Explanation:

5. 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. 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
View Answer

6. 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. 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
View Answer

Answer: d
Explanation:

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. What W/L ratio is required if μ_nC_ox = 200 μA/V²?
a) 1.23
b) 1.23
c) 1.43
d) 1.53
View Answer

Answer: c
Explanation:

8. For a 0.8-μm CMOS fabrication process: V_tn= 0.8 V, V_tp = −0.9 V, μ_nC_ox = 90 μA/V², μ_pC_ox = 30 μA/V², Cox = 1.9 fF/μm2, V_A (n-channel devices) = 8L (μm), and |V_A| (p-channel devices) = 12L (μm). 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
View Answer

Answer: a
Explanation:

9. For a 0.8-μm CMOS fabrication process: V_tn= 0.8 V, V_tp = −0.9 V, μ_nC_ox = 90 μA/V², μ_pC_ox = 30 μA/V², Cox = 1.9 fF/μm2, V_A (n-channel devices) = 8L (μm), and |V_A| (p-channel devices) = 12L (μm). 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
View Answer

Answer: c
Explanation:

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
View Answer

Answer: b
Explanation:
NMOS case

PMOS case

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