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close this bookRadio and Electronics
View the document(introduction...)
Open this folder and view contents1. INTRODUCTION
Open this folder and view contents2. PRINCIPLES OF RADIO COMMUNICATION UNICATION
Open this folder and view contents3. TRANSDUCERS
Open this folder and view contents4. RADIOWAVES
Open this folder and view contents5. MODULATION OF RADIOWAVES
Open this folder and view contents6. RECEPTION OF RADIOSIGNALS (AM - TYPE)
Open this folder and view contents7. COMPONENTS OF MODERN RADIO RECEIVERS
Open this folder and view contents8. PASSIVE COMPONENTS
Open this folder and view contents9. ACTIVE COMPONENTS -1- DIODES
Open this folder and view contents10. BLOCKS OF RADIOS / -1- / POWER SUPPLIES
Open this folder and view contents11. ACTIVE COMPONENTS -2- / TRANSISTORS
Open this folder and view contents12. AMPLIFIERS
Open this folder and view contents13. CLASS B AMPLIFIERS
View the document14. DETECTOR OR DEMODULATOR
View the document15. AGC-AUTOMATIC GAIN CONTROL
View the document16. IF-AMPLIFIERS
View the document17. FEEDBACK
View the document18. OSCILLATORS
View the document19. FREQUENCY CHANGERS MIXERSTAGE
View the document20. DECOUPLING CIRCUITS
View the document21. MATCHING OF AMPLIFIERSTAGES
View the document22. COUPLING OF AMPLIFIERSTAGES
Open this folder and view contents23. RADIO SERVICING
View the document24. 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