This set of Microwave Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Varactor Diodes”.
1. Varactor diode is a semiconductor diode in which the _________ can be varied as a function of reverse voltage of the diode.
a) Junction resistance
b) Junction capacitance
c) Junction impedance
d) None of the mentioned
View Answer
Explanation: Varactors (variable-capacitor) have non-linearity of capacitance which is fast enough to follow microwaves. Varactor diode is a semiconductor diode in which the junction capacitance can be varied as a function of reverse voltage of the diode.
2. Any semiconductor diode has a junction capacitance varying with reverse bias. If such a diode has microwave characteristics, it is called:
a) IMPATT diode
b) TRAPITT diode
c) SKOTTKY diode
d) None of the mentioned
View Answer
Explanation: Any semiconductor diode has a junction capacitance varying with reverse bias. If such a diode has microwave characteristics, it is called varactor diode. With the reverse bias, the junction is depleted of mobile carriers resulting in a capacitance that is the diode behaves as a capacitance with the junction acting as dielectric between two conducting plates.
3. The width of depletion region of a varactor diode ________with increase in reverse bias voltage.
a) Increases
b) Decreases
c) Remains constant
d) None of the mentioned
View Answer
Explanation: The width of the depletion region goes on increasing with increase in reverse bias voltage of the varactor diode. As the width of depletion region is an inverse function for capacitance, as the width increases, capacitance decreases.
4. Diffused junction mesa silicon diodes are widely used at microwave frequencies.
a) True
b) False
View Answer
Explanation: Diffused junction mesa silicon diodes are widely used at microwave frequencies. They are capable handling large powers and large break down voltages. They have relative independence of ambient temperature and low noise.
5. Varactors made of ______ have higher frequency range of operation compared to silicon fabricated varactor diodes.
a) Germanium
b) GaAs
c) GaN
d) None of the mentioned
View Answer
Explanation: Varactor diodes made of silicon have frequency range of operation of 25 GHz. Varactor diodes made of gallium arsenide operate in the frequency range of 90 GHz. Varactors of gallium arsenide also have better at low temperature.
6. Varactor diodes are operated in _________ region to achieve maximum efficiency possible.
a) Cutoff region
b) Saturation region
c) Reverse saturation region
d) Active region
View Answer
Explanation: Varactors are used between the reverse saturation point and a point just above the avalanche region. The capacitance variation and the reverse voltage swing are limited to between the operating regions mentioned above.
7. The cutoff frequency for operation of a varactor diode at a specific bias is given by:
a) 1/2πRSCjv
b) 1/2πCSRjv
c) 1/2π√LC
d) None of the mentioned
View Answer
Explanation: Cutoff frequency for a certain bias voltage applied is given by 1/2πRSCjv. Here, Rs is the wafer resistance and Cjv is the junction capacitance measured in the varactor diode for a given specific bias voltage V.
8. ___________ is an amplifier constructed using a device whose reactance is varied to produce amplification.
a) Travelling wave tube
b) Parametric amplifier
c) Common emitter
d) Klystron amplifier
View Answer
Explanation: Parametric amplifier is an amplifier constructed using a device whose reactance is varied to produce amplification. Varactor diode is the most widely used element in a parametric amplifier.
9. Parametric amplifier is a ________ amplifier.
a) Low noise
b) High gain
c) Low gain
d) High noise
View Answer
Explanation: Parametric amplifiers are constructed using varactor diodes. Since they do not involve any resistance, they result in low noise levels. There will be no thermal noise, as the active element used involved is reactive and not resistive.
10. Parametric amplifiers find their application in long range RADAR and satellite ground stations.
a) True
b) False
View Answer
Explanation: Due to the advantage of low noise amplification, parametric amplifiers are used in applications when noise levels are high at the receiving end but the amplification of noise must not occur. Such applications include long range RADARS satellite ground stations, radio telescopes to name a few.
11. Gain of a parametric amplifier in terms of the frequencies involved in their operation is:
a) (fP – fS)/fS
b) fS/ (fP – fS)
c) fP/fS
d) None of the mentioned
View Answer
Explanation: Gain of a parametric amplifier in terms of the frequencies involved in their operation is (fP – fS)/fS. Here fP is the pump frequency, fS is the signal frequency and fi is the idler frequency.
Sanfoundry Global Education & Learning Series – Microwave Engineering.
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