This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses 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

View Answer

Explanation: 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

View Answer

Explanation: 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

View Answer

Explanation: 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}

View Answer

Explanation: 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

View Answer

Explanation: 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

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7. A FET has I_{DSS}=4I_{D} and g_{m0} = 10mS then g_{m} = _________________________

a) 10mS

b) 20mS

c) 5mS

d) 14mS

View Answer

8. Determine the value of output impedance for JFET, if the value of g_{m} =1mS?

a) 1Kohm

b) 0

c) 100Kohm

d) 5Kohm

View Answer

Explanation: 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

View Answer

Explanation: 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 r_{0}?

a) 100Kohm

b) 50Mohm

c) 50Kohm

d) 20Kohm

View Answer

Explanation: r

_{0}=1/y

_{os}

It is independent of y

_{fs.}=> r

_{0}=1/20mS

r

_{0}=50Kohm.

**Sanfoundry Global Education & Learning Series – Electronic Devices and Circuits.**

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