250+ TOP MCQs on The FET Small-Signal Model and Answers

Electronic Devices and Circuits Multiple Choice Questions on “The FET Small-Signal Model”.

1. What is trans-conductance?
A. Ratio of change in drain current to change in collector current
B. Ratio of change in drain current to change in gate to source voltage
C. Ratio of change in collector current to change in drain current
D. Ratio of change in collector current to change in gate to source voltage

Answer: B
Clarification: The change in drain current which is resulted due to change in gate to source voltage in a FET is measured by trans-conductance. This is termed as trans because it provides relationship between input and output quantity.

2. For a FET, graph is drawn by taking voltage V_GS in X axis and drain current in Y axis, if the value of X changes from 10 to 20 results in change in value of Y axis from 2 to 3. What is the value of trans conductance?
A. 1
B. 2
C. 0.1
D. 0.01

Answer: C
Clarification: Trans-conductance= change in drain current/ change in gate to source voltage
trans conductance=3-2/20-10=1/10
trans conductance=0.1.

3. The slope obtained in V_GS vs I_D was 0.002. What is the value ofg_m?
A. 1
B. 2
C. 0.002
D. 0

Answer: C
Clarification: g_m = change in drain current/ change in gate to source voltage g_m = slope of V_GS vs I_D g_m = 0.002.

4. Which of the following is an expression for g_m0?
A. g_m0 = I_DSS/V_p
B. g_m0 = 2I_DSS/|V_p|
C. g_m0 = I_DSS/5V_p
D. g_m0 = I_DSS/2V_p

Answer: B
Clarification: g_m0 is the value of g_m when V_GS=0, we have seen that trans conductance is the ratio of drain current to change in gate to source voltage, but this is an exceptional case, here the transistor will be working in cut off region.

5. Find the maximum value of g_m for FET with I_DSS=10mA, V_p=-2V, V_GS=5V?
A. 10mS
B. 20mS
C. 1mS
D. 0

Answer: A
Clarification: g_m0=2I_DSS/|V_p| g_m0=2×10mA/2V g_m0=10mS.

6. Find the value of g_m for FET with I_DSS=8mA, V_p=4V, V_GS=-0.5V?
A. 1mS
B. 2mS
C. 3mS
D. 3.5mS

7. A FET has I_DSS=4I_D and g_m0 = 10mS then g_m = _________________________
A. 10mS
B. 20mS
C. 5mS
D. 14mS

8. Determine the value of output impedance for JFET, if the value of g_m =1mS?
A. 1Kohm
B. 0
C. 100Kohm
D. 5Kohm

Answer: A
Clarification: Output impedance=inverse of trans conductance
Output impedance=1/1mS
Output impedance=1Kohm.

9. In a small signal equivalent model of an FET, What does g_m V_GS stand for?
A. A pure resistor
B. Voltage controlled current source
C. Current controlled current source
D. Voltage controlled voltage source

Answer: B
Clarification: For FET, the voltage is applied across the gate and source to control the drain current, hence while writing small signal model of an FET, on the output side g_m V_GS represents a current source which can be controlled by the input voltage V_GS.

10. Given y_fs = 3.6mS and y_os = 0.02mS, determine r₀?
A. 100Kohm
B. 50Mohm
C. 50Kohm
D. 20Kohm

Answer: C
Clarification: r₀=1/y_os
It is independent of y_fs. => r₀=1/20mS
r₀=50Kohm.