# Electronic Devices and Circuits Questions and Answers – The FET Small-Signal Model

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

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 VGS 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

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 VGS vs ID was 0.002. What is the value ofgm?
a) 1
b) 2
c) 0.002
d) 0

Explanation: gm = change in drain current/ change in gate to source voltage gm = slope of VGS vs ID gm = 0.002.

4. Which of the following is an expression for gm0?
a) gm0 = IDSS/Vp
b) gm0 = 2IDSS/|Vp|
c) gm0 = IDSS/5Vp
d) gm0 = IDSS/2Vp

Explanation: gm0 is the value of gm when VGS=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 gm for FET with IDSS=10mA, Vp=-2V, VGS=5V?
a) 10mS
b) 20mS
c) 1mS
d) 0

Explanation: gm0=2IDSS/|Vp| gm0=2×10mA/2V gm0=10mS.
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6. Find the value of gm for FET with IDSS=8mA, Vp=4V, VGS=-0.5V?
a) 1mS
b) 2mS
c) 3mS
d) 3.5mS

Explanation:

7. A FET has IDSS=4ID and gm0 = 10mS then gm = _________________________
a) 10mS
b) 20mS
c) 5mS
d) 14mS

Explanation:

8. Determine the value of output impedance for JFET, if the value of gm =1mS?
a) 1Kohm
b) 0
c) 100Kohm
d) 5Kohm

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 gm VGS stand for?
a) A pure resistor
b) Voltage controlled current source
c) Current controlled current source
d) Voltage controlled voltage source

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 gm VGS represents a current source which can be controlled by the input voltage VGS.

10. Given yfs = 3.6mS and yos = 0.02mS, determine r0?
a) 100Kohm
b) 50Mohm
c) 50Kohm
d) 20Kohm

Explanation: r0=1/yos
It is independent of yfs. => r0=1/20mS
r0=50Kohm.

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