# Electronic Devices and Circuits Questions and Answers – MOSFET in Amplfier Design

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “MOSFET in Amplfier Design”.

1. Various measurements are made on an NMOS amplifier for which the drain resistor RD is 20 kΩ. First, DC measurements show the voltage across the drain resistor, VRD, to be 2 V and the gate-to-source bias voltage to be 1.2 V. Then, ac measurements with small signals show the voltage gain to be −10 V/V. What is the value of Vt for this transistor?
a) 0.7V
b) 0.8V
c) 0.9V
d) 1.0V

Explanation:

2. Various measurements are made on an NMOS amplifier for which the drain resistor RD is 20 kΩ. First, DC measurements show the voltage across the drain resistor, VRD, to be 2 V and the gate-to-source bias voltage to be 1.2 V. Then, ac measurements with small signals show the voltage gain to be −10 V/V. If the process transconductance parameter is 50μA/V2, what is the MOSFET’s W/L?
a) 25
b) 50
c) 75
d) 100

Explanation:

3. Consider the amplifier below for the case VDD = 5 V, RD = 24 kΩ, (W/L) = 1 mA/V2, and Vt = 1 V. If the amplifier is biased to operate with an overdrive voltage VOV of 0.5 V, find the incremental gain at the bias point.

a) -3 V/V
b) -6 V/V
c) -9 V/V
d) -12 V/V

Explanation:

4. Consider the amplifier below for the case VDD = 5 V, RD = 24 kΩ, (W/L) = 1 mA/V2, and Vt = 1 V. For amplifier biased to operate with an overdrive voltage of 0.5V, and disregarding the distortion caused by the MOSFET’s square-law characteristic, what is the largest amplitude of a sine-wave voltage signal that can be applied at the input while the transistor remains in saturation?

a) 1.61 V
b) 1.5 V
c) 0.11 V
d) 3.11 V

Explanation: Use the standard mathematical formula to obtain the result.

5. Consider the amplifier below for the case VDD = 5 V, RD = 24 kΩ, (W/L) = 1 mA/V2, and Vt = 1 V. For the input signal of 1.5V what is the value of the gain value obtained?

a) -12.24 V/V
b) -12.44 V/V
c) -12.64 V/V
d) -12.84 V/V

Explanation: The amplitude of the output voltage signal that results is approximately equal to Voq – VOB = 2 – 0.61 = 1.39v.
The gain implied by amplitude is
Gain = -1.39/0.11 = -12.64 V/V.

6. Which of the following is the fastest switching device?
a) JEFT
b) Triode
c) MOSFET
d) BJT

Explanation: MOSFET is the fastest switching device among the given four options.

7. Bias point is also referred by the name
a) DC Operating Point
b) Quiescent Point
c) None of the mentioned
d) All of the mentioned

Explanation: Bias point is called dc operating point as the MOSFET functions best at this point. Also since at the bias point no signal component is present it is called quiescent point (he reason why it is represented by the symbol ‘Q’)

8. Consider the amplifier circuit shown below. The transistor is specified to have Vt = 0.4 V, kn = 0.4 mA/V2, W/L = 10 and λ = 0. Also, let VDD = 1.8V, RD = 17.5kΩ, VGS = 0.6V and vgs = 0V. Find ID.

a) 0.08 mA
b) 0.16 mA
c) 0.4 mA
d) 0.8 mA

Explanation:

9. Consider the amplifier circuit shown below. The transistor is specified to have Vt = 0.4 V, kn = 0.4 mA/V2, W/L = 10 and λ = 0. Also, let VDD = 1.8V, RD = 17.5kΩ, VGS = 0.6V and vgs = 0V. Find VDS.

a) 0.1V
b) 0.2 V
c) 0.4 V
d) 0.8 V

Explanation:

10. Consider the amplifier circuit shown below. The transistor is specified to have Vt = 0.4 V, kn = 0.4 mA/V2, W/L = 10 and λ = 0. Also, let VDD = 1.8V, RD = 17.5kΩ, VGS = 0.6V and vgs = 0V. Find Av.

a) -12 V/V
b) -14 V/V
c) -16 V/V
d) -18 V/V

Solution: b
Explanation: Av = – kn Vov RD
= -0.4 * 10 * 0.2 * 17.5
= – 14.4v

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