past questions for electronic devices PDF

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Basic Electronic Devices and Circuits Tutorial Sheet -7 (Feedback in Amplifiers) 1. The voltage gain of an amplifier without feedback is 60 dB. It decreases to 40 dB when negative feedback is applied. Calculate the feedback factor. [Ans. 0.009] 2. An amplifier without feedback has a voltage gain of 500. Determine its voltage gain when a negative feedback with β = 0.1 is applied to it. [Ans. 9.8] 3. The gain of an amplifier is 200. When negative feedback is applied, its gain decreases to 10. Determine the fraction of the output that is fed back to the input. [Ans. 0.095] 4. To an amplifier with a gain of 60 dB, a negative feedback of β = 0.006 is applied. What will be the percentage change in the overall gain of the feedback amplifier if the internal gain of the amplifier reduces by (a) 3 %, (b) 20 % ? [Ans. (a) 0.43 %; (b) 3.43 %] 5. An amplifier has a voltage gain of 1000 and input resistance of 1 kΩ. If a negative feedback of β = 0.01 is applied to it, what will be the input resistance of the feedback amplifier ? [Ans. 11 kΩ] 6. An amplifier without any feedback gives an output of 5 V when a signal of 0.01 V is applied to its input. This circuit is now connected to a negative feedback circuit such that 20 % of the output voltage is fed back to the input. If we want the same output as earlier, how much voltage is required to be given at the input ? [Ans. 1.01 V] 7.

A two-stage amplifier has a gain of 80 dB. To improve its performance, a negative feedback of β = 0.02 is applied to it. (a) How much gain will the amplifier have now ? (b) If, due to the replacement of the transistors, the gain of the basic amplifier reduces by 50 %, what will be the percentage reduction in the gain of the feedback amplifier ? [Ans. (a) 49.75; (b) 0.5%]

8. The output resistance of an amplifier with gain 200 is 10 kΩ. If a negative feedback with a feedback factor of 0.2 is introduced into the circuit, what would be the output resistance of the feedback amplifier ? If the feedback factor is changed to 0.02, what will be the output resistance of the feedback amplifier ? [Ans. 244 Ω, 2 kΩ] 9. A negative feedback amplifier has an internal gain of 1000 and a feedback factor of 0.04. If the input resistance of this circuit is 12 kΩ, what would have been the input resistance of the amplifier if the feedback were not introduced ? [Ans. 293 Ω] 10. An amplifier having a gain of 100 is producing 12 % harmonic distortion in its output. With a view to improve the performance of the amplifier, a negative feedback is introduced so that the harmonic distortion is limited to only 2 %. Determine how much feedback is needed to meet the requirement. [Ans. 5 %]...