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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

15. AGC-AUTOMATIC GAIN CONTROL

1. Purpose

This is the time when we can easily understand a special circuit which was not mentioned up to here. Its purpose is to cancel effects of the environment of a radio-receiver which can cause very intensive changes in reception of the wanted signal. Such a change of reception would cause a very strong change of the output-signal which means: a considerable change of sound volume. If there is not done anything about it, the receiver gives a reasonable sound at one moment a moment later the sound can be very weak or get unacceptable loud as a consequence of change of reception.

2. Function of the AGC-circuit

In order to stabilize the output signal we need an electric signal which presents the strength of the reception in order to use it for an automatical stabilization of the output-signal. The needed signal can be derived from the detector input signal. If we have a closer look to that signal we find, that this input signal has two components:

- the first part is the AF-component which normally does not change its amplitude during changes of reception strength.

- the second part is the dc-component - the distance of the AF-sinewave from the noughtline. This second part is the one which gives us an information about the reception strength (as represented in fig. 188).


fig. 188

The wanted dc-signal can be produced by means of an RC-combination across the volume-control-potentiometer.

The voltage desired can be measured across the capacitor of that RC-combination, it is called Automatic-Gain-Control-voltage (AGC). It will be fed back to the biasing of the first IF-stage. If the AGC-voltage is growing it will decrease the amplification in that 1. IF-stage.


fig. 189