Gain control (gain control) reflected waves from different levels of the ground to the ground successively, the amplitude difference can be selected more than one million times. In order to record or display them more uniformly on one record, the total magnification of the seismic amplifier needs to be changed over time. The method to achieve this effect is called "gain control".
The common ways are:
MGC manual gain control, manual gain control, AGC Automatic Gain Control Automatic gain control, which is currently used more, is an automatic control method that makes the gain of the amplifier circuit automatically adjust with the signal strength.
Automatic gain control
An automatic control method that automatically adjusts the gain of the amplifier circuit with the signal strength. The circuit that implements this function is referred to as the AGC ring. The AGC loop is a closed-loop electronic circuit. It is a negative feedback system. It can be divided into two parts: a gain controlled amplifier circuit and a control voltage forming circuit. The gain-controlled amplification circuit is located in the forward amplification path, and its gain changes with the control voltage. The basic components of the control voltage forming circuit are an AGC detector and a low-pass smoothing filter, and sometimes include components such as a gate circuit and a DC amplifier. The output signal u0 of the amplifier circuit is detected and filtered by a filter to remove low-frequency modulation components and noise, thereby generating a voltage uc for controlling the gain-controlled amplifier. When the input signal ui increases, u0 and uc also increase. Increasing uc decreases the gain of the amplifier circuit, so that the amount of change in the output signal is significantly smaller than the amount of change in the input signal, achieving the purpose of automatic gain control.
The control methods of the amplification circuit gain are:
① Change the DC working state of the transistor to change the current amplification factor β of the transistor.
② Insert electrically controlled attenuators between amplifier stages.
③ Use electronically controlled variable resistors as amplifier loads. The AGC circuit is widely used in various receivers, recorders and measuring instruments. It is often used to keep the output level of the system within a certain range, so it is also called automatic level control. When used in a speech amplifier or radio, it is called For automatic volume control.
There are two control methods of AGC: one is to increase the AGC voltage to reduce the gain, which is called forward AGC, and the other is to reduce the gain, which is called the reverse AGC. The AGC has strong control ability and needs large control power. The working point of the controlled amplifier has a large range of change and the impedance changes at both ends of the amplifier are large. The reverse AGC requires small control power and small control range.