# Electronic Devices and Circuits Questions and Answers – The Common Source Amplifier

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “The Common Source Amplifier”.

1. An amplifier is designed using fixed bias configuration, what is its output impedance (source Resistor is bypassed)?
a) RD+rd
b) RG
c) RD || rd
d) 0

Explanation: By using ac equivalent of FET, gm Vgs is made 0. On observing the circuit from output side, ZO=RD || rd, where RD is the drain resistance. In general, rd>>RD, ZO=RD.

2. An amplifier is designed using fixed bias configuration, what is its input impedance (source Resistor is un bypassed)?
a) RD+rd
b) RG
c) RD || rd
d) 0

Explanation: From the ac small signal model, the input impedance of FET is open circuit.
Gate resistance RG is in parallel with open circuit resulting in Zin=∞||RG=RG.

3. Which of the following is true about the common Source amplifier?
a) It has low input impedance
b) It has high output impedance
c) Infinite gain
d) Phase reversal voltage output

Explanation: If we consider a fixed bias based amplifier, the voltage gain is Av=-gm RD,
The negative sign indicates that the voltage waveform is phase reversed by 180 degrees.

4. A Self bias configuration contains RD=3.3, Rs=1 KΩ, RG=1MΩ and gm=1.5mS. Determine Av?
a) -2
b) 3
c) -4
d) 5

Explanation: Av=-gm RD/ (1+gm RS) because RS is given and it is not bypassed.
Av=-1.5×3.3KΩ/ (1+1.5×1)
Av=-2.

5. What is the reason for connecting a capacitor in parallel with Rs?
a) It blocks the noise
b) For ac signal it acts a short circuit resulting in grounding source terminal
c) It blocks the noise & for ac signal it acts a short circuit resulting in grounding source terminal
d) To increase impedance

Explanation: The capacitor connected across Source blocks all the DC noise signals, when AC input is applied, at high frequency the capacitive reactance will be almost zero resulting in short circuit making the source to be directly connected to ground.
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6. A sinusoidal signal has 1.5V at t=2S, If this signal is given to Common source amplifier with |Av|=5V, what will be its output at t=2S?
a) 7.5V
b) -7.5V
c) 0
d) 15V

Explanation: Input at t=2S is 1.5V, |Av|=5V=> output=5×1.5V=7.5V,
But amplifier produces 180 degree phase-shift => output=-7.5V.

7. For a self bias amplifier, RS=RD, the output will be_________________
a) Same as Input
b) Same as input but phase reversed
c) 0
d) Infinite

Explanation: Av=-gm RD/ (1+gm RS),
If RS=RD, Av=-gm RD/ (1+gm RD) => Av is nearly equal to -1 Av=Vo/Vi=> Vo=-Vi.

8. Which of the following statement is true about FET common source amplifier compared to BJT amplifier?
a) It has High input impedance
b) It has low input impedance
c) No input Voltage is needed
d) Input Voltage is needed

Explanation: FET due to its physical nature has very high input impedance; hence the voltage drop in input side will be very less making the entire signal to pass through the input port.

9. If gain is need to be stabilized, which of the following element is used for a Common source amplifier?
a) Capacitive
b) Inductive
c) Resistive
d) LC tank circuit

Explanation: The negative feedback is given through a resistor which stabilizes the gain.
Hence a resistor is connected across source terminal. Any temperature changes will not result in fluctuation of the output because of the negative feedback.

10. What is the input impedance of voltage divider configuration?
a) R1+R2
b) RG
c) d) 0

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

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