This set of Microwave Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Power Amplifiers”.
1. _______________ are used in the final stages of radar and radio transmitters to increase the radiated power level.
a) Power amplifiers
Explanation: Power amplifiers are used in the final stages of radar and radio transmitters to increase the radiated power level. Output of power amplifiers are in the range of 100-500 mW.
2. Important factors to be considered for power amplifier design are:
c) Thermal effect
d) All of the mentioned
Explanation: As per the application requirement and considering various aspects of an amplifier like efficiency, gain, thermal efficiency and inter modulation distortion, amplifiers need to be designed.
3. Amplifier efficiency is the ratio of RF output power to DC input power. This parameter determines the performance of an amplifier.
Explanation: Power amplifier is the primary consumer of DC power in most hand-held wireless devices, so amplifier efficiency is an important consideration. Amplifier efficiency is the ratio of RF output power to DC input power.
4. Gain of power amplifiers __________ with increase in operating frequency.
c) Increases exponentially
d) Decreases exponentially
Explanation: Silicon bipolar junction transistor amplifiers in the cellular telephone band of 800-900 MHz band have power added efficiencies of about 80%. But this efficiency drops quickly with increase in the operating frequency.
5. ___________ amplifiers are linear circuits, where the transistor is biased to conduct over the entire range of the input signal cycle.
a) Class A amplifiers
b) Class B amplifiers
c) Class C amplifiers
d) None of the mentioned
Explanation: Class A amplifiers are linear circuits, where the transistor is biased to conduct over the entire range of the input signal cycle. Because of this, class A amplifiers theoretically have a maximum efficiency of 50%.
6. A class B amplifier consists of _______ transistors in order to conduct the input signal over the entire cycle.
Explanation: Class B amplifier is biased to conduct only during one-half of the input signal cycle. 2 complementary transistors are operated in a class B push pull amplifier to provide amplification over the entire cycle.
7. Power amplifiers in the increasing order of efficiency is:
a) Class A, B, C
b) Class C, A, B
c) Class B, A, C
d) Efficiency of all the 3 amplifiers is the same
Explanation: Class A amplifiers have an efficiency of about 50%. Class B amplifiers have an efficiency of about 78%, class C amplifiers can achieve efficiencies up to 100%. In the increasing order of efficiency, C > B> a)
8. Behavior of a transistor in power amplifiers is unpredictable at all input signal levels.
Explanation: A transistor behaves linearly for signal powers below 1dB compression point and so, the small –signal scattering parameters should not depend either on the input power level or the output termination impedance.
9. If the output power of an amplifier is 10 V, and the input power supplied to the amplifier is 0.229 V given that the DC voltage used is 38.5 V, efficiency of the power amplifier is:
Explanation: Efficiency of a power amplifier is (Pout- Pin)/ PDc Substituting the given values in the above expression, efficiency of the power amplifier is 25%.
10. If a power amplifier has an output power of 10 W, and an amplifier gain of 16.4 dB, then the input drive power is:
a) 400 mW
b) 225 mW
c) 229 mW
d) 240 mW
Explanation: Input drive power required to get an output of 10 W is Pout (dBm)- G (dB). G is the gain of the amplifier. Substituting the given values in the above equation, 229 mW.
Sanfoundry Global Education & Learning Series – Microwave Engineering.
To practice all areas of Microwave Engineering, here is complete set of 1000+ Multiple Choice Questions and Answers.