Monday, December 8, 2014

Previous GATE Questions on Power Amplifiers (1987 - Till Date)


1.       In case of class A amplifiers, the ratio of efficiency of transformer coupled amplifier to efficiency of a transformer less amplifier is
a.       2.0
b.      1.36
c.       1.0
d.      0.5
Answer:     A
   Solution :          


1.       A pick up signal is power amplified using a complementary symmetry push pull amplifier and fed to a 5 loud speaker as shown. 
The specifications of power transistors are as follows:
ICmax = Maximum collector current = 2 Amps and PCmax = Maximum power dissipation = 1 Watt.
Find the maximum power delivered to the loud speaker, if the maximum output voltage subject to OP-AMP voltage limitations is 15 volts.

If the transistors are mounted on a heat sink, so that PCmax changes to 3 Watts. What will be the new value of the maximum power delivered?
Answer:    (i) 3.08 Watts, circuit gets breakdown (ii) 3.08 Watts, circuit gets breakdown


1.       In a transistor push-pull amplifier
a.       there is no DC present at the output
b.      there is no distortion in the output
c.       there are no even harmonics in the output
d.      there are no odd harmonics in the output
Answer:    C


1.       A class-A transformer coupled transistor power amplifier is required to deliver a power output of 10 watts. The maximum power rating of the transistor should not be less than ______
      a.       5 watts
      b.      10 watts
      c.       20 watts
      d.      40 watts
Answer:    C
                Solution :

     2.       In order to reduce the harmonic distortion in an amplifier, its dynamic range has to be _________

 Ans :  Dynamic Range expressed in decibels (dB),  is the ratio of the highest signal level a circuit, component, or system can handle, to the lowest signal level it can handle. Several parameters can be used to define the highest-level signal, such as the 1-dB compression point as well as the TOi point.
The lowest level signal level is limited by output noise, while the highest level signal is limited most often by distortion.
The 1-dB compression point for an amplifier, for example, is where the linearity of the component begins to degrade. Under linear conditions, a 1-dB rise in input power will result in a 1-dB increase in output power. When the increase at the output is 1-dB less than the increase at the input, it is said to be the amplifier's 1-dB compression point.
High Dynamic Range Amplifiers have low noise figures, low gain, low distortion and high 1 dB compression points. This combination of performance characteristics provides the greatest range between the minimum detectable signal and the signal level that will start to compress the amplifier. So in order to reduce the harmonic distortion in an amplifier, its dynamic range has to be compressed to 1-dB compression point.

     3.       For the 2N338 transistor, the manufacturer specifies Pmax = 100 mW at 25oC, free air temperature and the maximum junction temperature Tjmax = 125oC. Its thermal resistance is ________
Answer:    1000 oC/W
                Solution :


1.       A power amplifier delivers 50W output at 50% efficiency. The ambient temperature is 25oC. If the maximum allowable junction temperature is 150oC, then the maximum thermal resistance øjc that can be tolerated is __________
Answer:    2.5 oC/W
               Solution :

1.       A distorted sinusoid has the amplitudes A1, A2, A3 … of the fundamental, second harmonic, third harmonic, …..respectively. The total harmonic distortion is 

Answer:    B


1.       Crossover distortion behavior is characteristic of
a.       Class A output stage
b.      Class B output stage
            c.       Class AB output stage
d.      Common base output stage
Answer:     B
                Solution :

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