دارة agc ذات التأخير .. كتاب التلفزيون الملون والعادي

- 8 - 6 AGC circuit with delay: in it the voltage feeding the medium frequency amplifiers coming from the AGC circuit differs from the voltage feeding the IF amplifiers, see Figure (8) (7), and this difference is imposed by the fact that the RF amplifier must work even in cases of weak signal, which Its value reaches (500 microvolts), and this is better to give the largest possible signal to the mixer - as the mixer is the largest source of interference in the device - when the signal is large, the output of the mixer
RF until the signal-to-noise ratio is as large as possible - it is desirable that the gain of the RF amplifier be less than its maximum when the signal is weak. To achieve this, the AGC voltage is delayed from reaching an amplifier: the signal strength reaches (500 microvolts). At this point, the AGC voltage rises quickly to prevent overloading the mixer, as in Figure (8-10), which shows the required AGC voltage changes, which are directed to the RF and IF stages. It is noted that the effect of AGC on the IF stage is direct and instantaneous - while its effect on RF is delayed and then begins to rise sharply.
Forward and reverse AGC method:


In the forward AGC method (Fig. 12B), the gain of the transistor decreases when the bias (the base to the emitter) increases, as the curve shows - Figure (8-11). As the bias increases, and the direction of the saturation point, the gain of the transistor decreases. If the transistor is a type NPN and an AGC voltage applied to its base - increasing the positive voltage will decrease its amplification - as for the PNP transistor, when an AGC voltage is applied to its base, increasing the negative voltage will reduce its amplification. As for the reverse AGC method (Fig. It will reduce the transistor amplification as shown in the same previous curve. Each method has some advantages and disadvantages, which we summarize as follows.

The curve shows how the AGC voltage can reduce the gain of the transistor by increasing or decreasing the forward bias. The forward AGC method makes the capacitive resistance in the base-collection junction of the transistor large and has a larger changeover. This is one of the disadvantages of the forward method

Its features are
​The inlet and outlet impedance has a lower change when the AGC voltage reverse changes.


As for the reverse AGC method, it is that the area used is located in the non-linear part of the curve shown in the previous figure, and therefore any change in gain will not be in the same proportion as the change in the AGC voltage. As for the linear part of the curve used in the forward method of AGC - it is noted that any change in gain has the same proportion as the change in the bias voltage. In the reverse AGC way, a strong signal can lower the AGC voltage to such an extent that the amplifier is in the cut-off region. This leads to significant distortions - and to avoid these distortions a special circuit is used to determine the amount of voltage applied to the phase

The choice of the AGC method is affected by the type of transistors used in the amplifiers, as some TV sets use one of the two forward or reverse methods for
In controlling the RF and IF amplification. In some devices, the forward method is used for the RF or IF amplification stage and the reverse method for the other amplification stage, in order to ensure the stability and improvement of the circuit in the best way.