Radio and Electronics (DED Philippinen, 66 p.): 17. FEEDBACK

Radio and Electronics (DED Philippinen, 66 p.)

		(introduction...)
		1. INTRODUCTION
		2. PRINCIPLES OF RADIO COMMUNICATION UNICATION
		3. TRANSDUCERS
		4. RADIOWAVES
		5. MODULATION OF RADIOWAVES
		6. RECEPTION OF RADIOSIGNALS (AM - TYPE)
		7. COMPONENTS OF MODERN RADIO RECEIVERS
		8. PASSIVE COMPONENTS
		9. ACTIVE COMPONENTS -1- DIODES
		10. BLOCKS OF RADIOS / -1- / POWER SUPPLIES
		11. ACTIVE COMPONENTS -2- / TRANSISTORS
		12. AMPLIFIERS
		13. CLASS B AMPLIFIERS
		14. DETECTOR OR DEMODULATOR
		15. AGC-AUTOMATIC GAIN CONTROL
		16. IF-AMPLIFIERS
		17. FEEDBACK
		18. OSCILLATORS
		19. FREQUENCY CHANGERS MIXERSTAGE
		20. DECOUPLING CIRCUITS
		21. MATCHING OF AMPLIFIERSTAGES
		22. COUPLING OF AMPLIFIERSTAGES
		23. RADIO SERVICING
		24. THE USE OF THE OSCILLOSCOPE

17. FEEDBACK

GENERALLY feedback is a process used in amplifier technology to feed a portion of the output signal back to the input.

Kinds of feedback:

In principle there are two possibilities of feeding back.

1. to add the output portion to the input signal.
This is called POSITIVE FEEDBACK.
This kind of feedback always produces increasing changes, because any change of the input signal is amplified and a part of the output signal is fed back to the input signal.
Sooner or later the upper or lower border of amplification will be reached which means a sudden change of the output signal again and this means - the process starts again in opposite direction. Consequence: OSCILLATIONS.
2. to subtract the output portion from the input signal.
This is called NEGATIVE FEEDBACK.
This type of feeding back obviously reduces the amplification. This fact sounds at first as if it is a big disadvantage (for example it can cause the necessity of another amplifier stage).
But on the other hand it increases the STABILITY OF AMPLIFICATION and it decreases distortion. One possibility to produce negative feedback is already explained in chapter 12.4. It is the emitter-resistor in a class A amplifier which reduces the amplification but stabilizes its function.
Beside the already mentioned advantages we find with negative feedback an INCREASE OF INPUT IMPEDANCE OF AMPLIFIERS. A fact which is very important in connection with the problem of MATCHING.

The effects of feedback will depend very strong on the method of connection whether it is a series or a parallel connection. The parallel method of connection is not used in radio receivers. The methods of application of negative feedback are illustrated in the following diagrams:

VOLTAGE-SERIES FEEDBACK

fig. 194

Results:

- reduction of gain and of distortion
- increase of input impedance
- decrease of output impedance
- improved frequency response
- stabilized gain

CURRENT-SERIES FEEDBACK

fig. 195

Actually current feedback is reading back a voltage which is depending on the current flowing in the output circuit.

Results:

- reduction of gain
- reduction of distortion
- improved frequency response
- increase of input impedance