Antennas and Microwave Engineering MCQs | Page - 9

 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.

 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.

 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.

 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.

 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%.

 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.

 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

 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.

 efficiency of a power amplifier is (pout- pin)/ pdc substituting the given values in the above expression, efficiency of the power amplifier is 25%.

 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.