Category: Microwave Engineering Objective Questions

250+ TOP MCQs on IMPATT and BARITT Diodes and Answers

Microwave Engineering Multiple Choice Questions on “IMPATT and BARITT Diodes”.

1. The material used to fabricate IMPATT diodes is GaAs since they have the highest efficiency in all aspects.
A. true
B. false
Answer: B
Clarification: IMPATT diodes can be fabricated using silicon, germanium, GaAs or indium phosphide. Out of these materials, GaAs have highest efficiency, low noise and high operating frequencies. But GaAs has a major disadvantage of complex fabrication process and higher cost. So, GaAs are not preferred over silicon and germanium.

2. When a reverse bias voltage exceeding the breakdown voltage is applied to an IMPATT diode, it results in:
A. avalanche multiplication
B. break down of depletion region
C. high reverse saturation current
D. none of the mentioned
Answer: A
Clarification: A reverse bias voltage exceeding the breakdown voltage is applied to an IMPATT diode, a high electric field appears across the n+ p junction. This high field imparts sufficient energy to the holes and also to valence electrons to raise themselves to the conduction band. This results in avalanche multiplication of electron hole pair.

3. To prevent an IMPATT diode from burning, a constant bias source is used to maintain _______ at safe limit.
A. average current
B. average voltage
C. average bias voltage
D. average resistance
Answer: A
Clarification: Avalanche multiplication is a cumulative process resulting in rapid increase of carrier density. To prevent the diode from burning due to this increased carrier density, a constant bias source is used to maintain average current at safe limit.

4. The number of semiconductor layers in IMPATT diode is:
A. two
B. three
C. four
D. none of the mentioned
Answer: C
Clarification: IMPATT diode consists of 4 layers according to the construction. It consists of a p+ region and n+ layers at the two ends. In between these layers, a p type layer and an intrinsic region is sandwiched.

5. The resonant frequency of an IMPATT diode is given by:
A. Vd/2l
B. Vd/l
C. Vd/2πl
D. Vdd/4πl
Answer: A
Clarification: The resonant frequency of an IMPATT diode is given by the expression Vd/2l. Here VD is the carrier drift velocity; L is the length of the intrinsic region in the IMPATT diode.

6. If the length of the intrinsic region in IMPATT diode is 2 µm and the carrier drift velocity are 107 cm/s, then the drift time of the carrier is:
A. 10-11 seconds
B. 2×10-11 seconds
C. 2.5×10-11 seconds
D. none of the mentioned
Answer: B
Clarification: The drift time of the carrier is defined as the ratio of length of the intrinsic region to the carrier drift velocity. Substituting the given values in this relation, the drift time of the carrier is 2×10-11 seconds.

7. If the length of the intrinsic region in IMPATT diode is 2 µm and the carrier drift velocity are 107 cm/s, then the nominal frequency of the diode is:
A. 12 GHz
B. 25 GHz
C. 30 GHz
D. 24 GHz
Answer: B
Clarification: Nominal frequency is defined as the ratio of the carrier drift velocity to twice the length of the intrinsic region. Substituting the given values in the above equation, the nominal frequency is 25 GHz.

8. IMPATT diodes employ impact ionization technique which is a noisy mechanism of generating charge carriers.
A. true
B. false
Answer: A
Clarification: IMPATT devices employ impact ionization techniques which is too noisy. Hence in order to achieve low noise figure, impact ionization is avoided in BARITT diodes. The minority injection is provided by punch through of the intermediate region.

9. An essential requirement for the BARITT diode is that the intermediate drift region be completely filled to cause the punch through to occur.
A. true
B. false
Answer: B
Clarification: An essential requirement for the BARITT diode is that the intermediate drift region be completely filled to cause the punch through to the emitter-base junction without causing avalanche breakdown of the base collector junction.

10. If the RMS peak current in an IMPATT diode is 700 mA and if DC input power is 6 watt, with the load resistance being equal to 2.5 Ω, the efficiency of the diode is:
A. 10.1 %
B. 10.21 %
C. 12 %
D. 15.2 %
Answer: B
Clarification: Efficiency of IMPATT diode is defined as the ratio of output RMS power to the input DC power. Calculating the RMS output power from the given RMS current and substituting in the equation of efficiency, the efficiency is 10.21%.

11. If the critical field in a Gunn diode oscillator is 3.2 KV/cm and effective length is 20 microns, then the critical voltage is:
A. 3.2 V
B. 6.4 V
C. 2.4 V
D. 6.5 V
Answer: B
Clarification: Critical voltage of a Gunn diode oscillator is given by the expression lEc where l is the effective length and Ec is the critical field. Substituting the given values in the above equation, critical voltage is 6.4 volts.


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250+ TOP MCQs on Microwave Oscillators and Answers

Microwave Engineering Multiple Choice Questions on “Microwave Oscillators”.

1. In microwave oscillators, negative resistance transistors and diodes are used in order to generate oscillations in the circuit.
A. True
B. False
Answer: A
Clarification: In microwave oscillator, for a current to flow in the circuit the negative impedance of the device must be matched with positive impedance. This results in current being non-zero and generates oscillation.

2. Any device with negative impedance as its characteristic property can be called:
A. Energy source
B. Energy sink
C. Oscillator
D. None of the mentioned
Answer: A
Clarification: A positive resistance implies energy dissipation while a negative resistance implies an energy source. The negative resistance device used in the microwave oscillator, thus acts as a source. The condition Xin+ XL=0 controls the frequency of oscillation. Xin is the impedance of the negative resistance device.

3. In a microwave oscillator, a load of 50+50j is connected across a negative resistance device of impedance -50-50j. Steady state oscillation is not achieved in the oscillator.
A. True
B. False
Answer: B
Clarification: The condition for steady state oscillation in a microwave oscillator is Zin=-ZL. Since this condition is satisfied in the above case, steady state oscillation is achieved.

4. For achieving steady state oscillation, the condition to be satisfied in terms of reflection coefficients is:
A. Гin=ГL
B. Гin=-ГL
C. Гin=1/ГL
D. None of the mentioned
Answer: C
Clarification: The condition for steady state oscillation to be achieved in terms of reflection coefficient is Гin=1/ГL. Here Гin is the reflection coefficient towards the reflection coefficient device and ГL is the reflection coefficient towards the load.

5. A one port oscillator uses a negative resistance diode having Гin=0.9575+j0.8034 (Z0=50Ω) at its desired frequency point. Then the input impedance of the diode is:
A. -44+j123
B. 50+j100
C. -44+j145
D. None of the mentioned
Answer: A
Clarification: The input impedance of the diode given reflection coefficient and characteristic impedance is Z0 (1+Гin)/ (1-Гin). Substituting in the given equation, the input impedance is -44 +j123 Ω.

6. If the input impedance of a diode used in the microwave oscillator is 45-j23 Ω, then the load impedance is to achieve stable oscillation is:
A. 45-j23 Ω
B. -45+j23 Ω
C. 50 Ω
D. 23-j45 Ω
Answer: B
Clarification: The condition for stabilized oscillation is Zin=-ZL. According to this equation, the load impedance required for stabilized oscillation is – (45-j23) Ω. The load impedance is thus -45+j23 Ω.

7. To achieve stable oscillation, Zin + ZL=0 is the only necessary and sufficient condition to be satisfied by the microwave oscillator.
A. True
B. False
Answer: B
Clarification: The condition Zin + ZL=0 is only a necessary condition for stable oscillation and not sufficient. Stability requires that any perturbation in current or frequency is damped out, allowing the oscillator to return to its original state.

8. In transistor oscillators, the requirement of a negative resistance device is satisfied using a varactor diode.
A. True
B. False
Answer: B
Clarification: In a transistor oscillator, a negative resistance one port network is created by terminating a potentially unstable transistor with impedance designed to drive the device in an unstable region.

9. In transistor oscillators, FET and BJT are used. Instability is achieved by:
A. Giving a negative feedback
B. Giving a positive feedback
C. Using a tank circuit
D. None of the mentioned
Answer: B
Clarification: Oscillators require a device that has high instability. To achieve this condition, transistors are used with a positive feedback to increase instability.

10. In a transistor amplifier, if the input impedance is -84-j1.9 Ω, then the terminating impedance required to create enough instability is:
A. -84-j1.9 Ω
B. 28+j1.9 Ω
C. – (28+j1.9) Ω
D. None of the mentioned
Answer: B
Clarification: Relation between terminating impedance and input impedance is Zs=-Rin/3. Zs is the terminating impedance. Substituting in the given equation, the terminated impedance is 28+j1.9 Ω.


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