عمل دارة حكم الربح الالي (Automatic Gain Control)

Chapter Eight:

Automatic Gain Governor Gain Adjuster
Automatic:

8-1 Automatic Gain Control:

The Automatic Gain Control (G) circuit is similar in its function to the Automatic Audio Controller (AVC) in a radio. It maintains the signal level at a certain fixed value in the screen valve despite the change in the intensity of the signal entering the antenna, especially when changing the receiving station, whose signal may be stronger or weaker than the signal of other stations, which requires manual resetting when receiving each station. If there is no AGC circuit. In the color television set, the AGC circuit maintains the contrast of the image despite the change in the intensity of the received signal - or the change of tuning from one channel to another. Also, the synchronization pulses will be better stable when their amplitude is of a stable value and the shape
(2-8) shows the block diagram of the AGC circuit and its effect on the medium and high frequency amplifier. Also, the AGC circuit prevents image overloading, when increasing the RF stage magnification in the voter or intermediate frequency amplifiers of the IF image and Figure - - (1) the effect of image overloading is shown.

The work of the AGC circuit depends on the amplitude voltage of the synchronization pulses, which is also related to the level of the received TV signal. Its effect is on the bias of the valve or transistor, which increases or decreases the amount of amplification - and the change is linear in the part of the properties curve close to the cut-off point.

8-2 Types of AGC Circuits Automatic Gain Controller:

1- Peak AGC circuit:

It depends on the peak of the synchronous pulses to tune the AGC voltage, where the peak voltages of the synchronous pulses are filtered and channeled to the RF amplifier circuit and to the IF image amplifier circuit. The AGC voltage is DC.

2 - AC circuit closed (Keyed) (reserved):

It is commonly used and is called a tabbed circuit. It depends on the generation of AGC voltage during the horizontal scanning time - and it has advantages that compete with the first type (peak), which are:

A - The speed of its sensitivity.
B - Less affected by interference.
​​​​3 - The noise-canceling (reserved) AGC circuit, which is the same as the previous circuit, with the addition of a circuit that eliminates or reduces as much as possible the interference and interference Figure (8) - (3) The automatic gain governor is designed so that it does not sense the signal strength except for a period Very short time (T), which reduces interference in this short period, and reserves the remaining time without passing any current. Thus, it is not affected by interference, no matter how strong it is, and the sensitivity period is located in the back threshold of the extinguishing pulse.

​​​​​4 - AGC circuit with delay (AGC) is used only for the RF amplifier, so that the AGC voltage does not start until the input signal has reached the minimum (500) microvolts, and this ensures that the amplification ratio is not reduced RF only after the input signal has increased beyond a certain value, and after a certain time. Whereas, the effect of AGC is direct and instantaneous on the IF amplification.

e - Amplified AGC circuit, which is used to increase the sensitivity of the AGC system - it also works to reverse the polarity of the AGC to be compatible with some circuits