Questions and Problems

Author: N.H. Crowhurst

1. What simple facts about amplifiers led to the use of negative feedback to reduce distortion? If feedback reduces the gain by a ratio of 5:1, by how much should distortion be reduced?

2. What is the basic difference between positive and negative feedback? Show, with a sketch, the difference between loop gain and feedback factor,

3. What is the maximum loop gain that can be used with positive feedback without causing oscillation?

4. Explain the effect of (a) voltage feedback and (b) current feedback on amplifier output impedance. State clearly any condition that should be specified concerning the amount of feedback in figuring this effect.

5. Explain the effect of feedback, using (a) series injection and (b) shunt injection on input impedance in an amplifier.

6. The loop gain in an amplifier using negative feedback in 12 db. What is the feedback factor, in db?

7. Without feedback, an amplifier has an output source resistance that is 3 times its nominal load resistance. What will be the value when voltage feedback is used that reduces gain by 26 db without the load connected? Or with current feedback that reduces gain by 6 db with the output short-circuited ?

8. With the output load connected, a pentode output amplifier shows a reduction in gain of 20 db when voltage feedback is connected. What is the approximate source resistance (as a multiple or fraction of load impedance) with feedback connected? How could you evaluate the source resistance without feedback connected?

9. Without feedback, an amplifier shows 8% distortion unloaded, and 5% loaded, at maximum rated output voltage. Changing the output loading requires an input change of 8 db to maintain the voltage. Feedback is connected that reduces gain by 20 db in the loaded condition. Calculate: (a) the source resistance (as multiple or fraction of nominal load) without feedback; (b) the feedback, using the same circuit unloaded; (c) the source resistance with feedback; (d) distortion with feedback, unloaded; (e) distortion with feedback, loaded.

10. An amplifier uses overall feedback, voltage derived at the output, and series injected at the input. Without the load connected, removal of the feedback connection increases gain by 46 db. With it connected, the reduction is 26 db. An input resistor of 100,000 ohms is across the basic amplifier input (inside the feedback loop). Calculate: (a) the source resistance without feedback; (b) the source resistance with feedback;

(c) the input impedance with feedback, the output being unloaded; and

(d) the input impedance with feedback, the output being loaded.