Want to know the principle of gain control by AGC circuit feedback

rxjsn

Want to know the principle of gain control by AGC circuit feedback [Copy link]

I am currently learning to use AD603 to build an AGC circuit, but I still don't understand how to calculate the final control output result. According to my understanding, automatic gain control should be independent of the input, but after consulting the chip manual, the official reference circuit, and some solutions on the Internet, I still can't understand. According to the formula 40VG+10=20lg (Vo/Vi), VG is Vo after rectification, filtering, and then passing through the inverting integration circuit to the negative feedback end. How to calculate the final result is still related to Vi. After I built the simulation circuit, I found that my calculation results and simulation results are consistent, so I want to understand how to achieve gain control and set the output amplitude.

gmchen

That formula is the gain calculation formula of AD603. Since it is gain, it is of course related to the input.

gmchen

Automatic gain control has nothing to do with input? This statement is wrong! Which automatic control system has nothing to do with input? The so-called automatic control must be controlled for a certain input (disturbance can also be regarded as a kind of input)

gmchen

AD603 constitutes the AGC circuit. AD603 is a variable gain amplifier. The signal is amplified by this amplifier, and then the amplified signal is detected to obtain the peak value (or average value, or effective value, as needed) of the output signal. The voltage obtained by this detection is fed back to the control end of AD603 to change the amplification factor of AD603 and control the amplitude of the output voltage within a certain agreed range.

gmchen

The index requirements of the AGC circuit include:

1. Maximum gain. When the input signal is very small (does not reach the control level), the amplifier has maximum gain

2. Start-up level. When the input signal (or output signal) reaches the start-up level, the AGC amplifier enters the start-up state. At this time, the gain of the amplifier begins to decrease, and the output amplitude basically no longer changes with the input amplitude.

3. Control range. The control range of AGC is not infinite. It starts from the start control level and ends at the end level. After exceeding the end level, the amplifier works at the lowest gain (the gain will not decrease any further). When using a single AD603, this control range is 42dB.

gmchen

The most confusing part of designing AGC circuits (especially for beginners) is how to calculate the control level.

This calculation is related to the feedback part of the detection, but the relationship with AD603 becomes very simple.

However, the specific calculation is related to the circuit, and different circuits have different calculation methods.

rxjsn

gmchen posted on 2024-4-15 20:31 When designing an AGC circuit, the most confusing thing (especially for newcomers) is how to calculate the control level. This calculation is related to the detection feedback part and the AD6...

So that's it. Thank you for your explanation. Then can I understand that if I want to control the voltage of different input amplitudes to output the same result, I need to design the starting control level of the feedback circuit and finally achieve the design goal?

gmchen

I have a post you can refer to.

https://bbs.eeworld.com.cn/thread-643656-1-1.html

gmchen

There is also a post with an example of a real design

https://bbs.eeworld.com.cn/thread-1221860-1-1.html

syb_8787

The AGC (Automatic Gain Control) circuit uses the AD603 as its core variable gain amplifier to automatically adjust the signal amplification factor through a closed-loop feedback mechanism to ensure that the amplitude of the output signal remains within a predetermined, stable range. The following is a detailed rewrite of the process:

In the AGC circuit, the signal is first sent to the AD603 variable gain amplifier. As the core component of the system, AD603 can dynamically adjust its gain according to the change of the control voltage, thereby achieving precise control of the input signal amplitude.

The amplified signal then enters a detection stage, which is responsible for extracting the amplitude of the amplified signal. Depending on the application requirements, you can choose to extract the peak value, average value, or effective value of the signal as the control basis. For example, if you need to respond quickly to instantaneous changes in the signal amplitude, you may choose peak detection; if you need to focus on the overall level of the signal, average value or effective value detection is more appropriate.

The voltage value obtained by detection is then used as a feedback signal and fed back to the control terminal of AD603. This feedback process constitutes the closed-loop control mechanism of the AGC circuit. According to the comparison result between the feedback voltage and the preset target value, the system changes the amplitude of the output signal by adjusting the gain of AD603 until the amplitude of the output signal stabilizes within the predetermined range of variation.

In this way, the AGC circuit can automatically adapt to changes in the input signal amplitude, ensure the stability of the output signal, and avoid distortion or loss of the signal due to being too strong or too weak. This is of great significance for improving the performance of communication, measurement, audio processing and other systems.