# Electronic Devices and Circuits Questions and Answers – The Junction Field-Effect Transistor – 2

This set of Electronic Devices and Circuits Questions & Answers for Exams focuses on “The Junction Field-Effect Transistor – 2”.

1. Which of the following can be considered to be an advantage of FET amplifier as compared to BJT amplifier?
A – Higher input impedance
B – Good bias stability
C – Higher gain-bandwidth product
D – Lower noise figure
Select the correct answer using the codes given below
Codes:
a) A, B and C
b) A, B and D
c) B, C and D
d) A, C and D

Explanation: Advantage of FET over BJT are
i) Higher input impedance
ii) Good bias stability
iii) Lower noise figure.

2. Two identical FETs, each characterized by the parameters g_m and r_d are connected in parallel .The composite FET is then characterized by the parameters_____________
a) gm/2 and 2rd
b) gm/2 and 2rd
c) 2gm and rd/2
d) 2gm and rd/2

Explanation: By converting FET into ac equivalent circuits and connecting them in parallel,
gn = 2gm and rn = rd/2.

3. The pinch off voltage of JFET is 5v. What is its cut off voltage?
a) 2.5V
b) 3V
c) 4V
d) 5V

Explanation: Pinch off voltage =5V
At cut off the gate to source voltage of JFET is equal to pinch off voltage
Vgs = VP => Vgs(off) = 5V.

4. The action of JFET in its equivalent circuit can be represented as which of the following?
a) Current controlled current source
b) Current controlled voltage source
c) Voltage controlled current source
d) Voltage controlled Voltage source

Explanation: In JFET equivalent circuit, the output current is controlled by the gate to source voltage and hence we can say it is a Voltage controlled current source.

5. Which of the following is the main advantage of Self bias?
a) Eliminates the need of two power supply
b) Maximum stability
c) Minimum stability
d) Maximum & Minimum stability

Explanation: Self bias eliminates the need of 2 power supply by connecting gate resistance to the supply voltage.

6. FET amplifier does not obey the law of conservation of energy.
a) True
b) False

Explanation: Amplifier obeys the law of conservation of energy.

7. Which of the following is the necessary condition to design an amplifier?
a) Vce110 of RC
b) |Vce| ≤ 110 of RC
c) |Vce| ≤ 11000 of RC
d) |Vce| ≤ 1100 of RC

Explanation: In order to design an amplifier the time constant and RC and voltage across specified terminals.

8. At higher frequency, the capacitance of an amplifier circuit is mainly because of which capacitance?
a) Coupling capacitors
b) Stray capacitance
c) Resistors
d) Inductors

Explanation: During High frequency, the stray capacitance is the only source of capacitance in an amplifier circuit, the stray capacitance and junction capacitance.

9. Which of the following is the equation for stray capacitance frequency?
a) FH = 1/27R
b) FH = 1/R
c) FH = 54/27R
d) FH = 11/27R

Explanation: During high frequency the capacitance is due to combination of transmission capacitance and junction capacitance. The equation is given by FH = 1/27R.

10. What is the maximum value of gain of an amplifier?
a) 140dB
b) 130dB
c) 120db
d) 100dB

Explanation: The maximum gain of amplifier is 1000000. When we consider this in decibel scale, it gives 120dB => gain in dB = 20log10(100000) = 120dB.

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