Questions and Problems

Author: N.H. Crowhurst

1. How do coupling capacitors and circuit self-capacitance affect the performance of feedback circuits? At a certain frequency, the phase shift due to coupling capacitors, of which there are four, is 45°, 55°, and 30° for the first three. How much phase shift can be allowed in the fourth capacitor at this frequency, for what is nominally negative feedback to become positive?

2. What is a Nyquist diagram? If the basic quantity plotted on the polar diagram is loop gain, how can the diagram be used to show the response of the feedback factor and the overall response shaping?

3. What is gain margin? Explain how an amplifier that is stable with feedback connected can become unstable if its gain is increased, or if the fed-back amount is increased,

4. What is a cathode follower? Explain its action in terms of feedback action. A certain tube, using a plate load resistance of 100,000 ohms, has an a-c resistance of 60,000 ohms and gives an amplification of 48, Rearranging this as a cathode follower; calculate (a) the input impedance with a grid return resistor of 220,000 ohms and (b) the output source resistance (a-c).

5. How can positive feedback be used with negative feedback to get certain improvements? State the specific precautions necessary in adopting this method.

6. Positive feedback is increased just to the point where oscillation commences, without negative feedback. Overall negative feedback that reduces gain by 14 db when the positive feedback is not connected is also used. By how much will the gain change when the positive feedback is now connected?

7. Discuss the relative merits of different positions for a gain control or volume control that lead to choice of an ideal, stating clearly the objections to other positions.

8. Describe the action of a special type of tube that is used to provide automatic volume control electronically. Show how the same basic arrangement can be modified to act as a volume limiter, rather than as a compressor.

9. What is the function of a tone conrtol? Discuss simple circuits that will provide the basic variations required,

10. Why is equalization necessary? Name various locations where an equalizer is required. In what way does an equalizer differ from a tone control?