Circuit Design: Automatic Gain Control (AGC) circuits Theory and design
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Many attempts have been made to fully describe an AGC system in terms of control system theory, from pseudo linear approximations to multivariable systems. Each model has its advantages and disadvantages, first order models are easy to analyze and...
Many attempts have been made to fully describe an AGC system in terms of control system theory, from pseudo linear approximations to multivariable systems. Each model has its advantages and disadvantages, first order models are easy to analyze and understand but sometimes the final results show a high degree of inaccuracy when they are compared with practical results. On the other hand, non-linear and multivariable systems show a relative high degree of accuracy but the theory and physical implementation of the system can become really tedious. From a practical point of view, the most general description of an AGC system is presented in figure 1. The input signal is amplified by a variable gain amplifier (VGA), whose gain is controlled by an external signal V C . The output from the VGA can be further amplified by a second stage to generate and adequate level of V O . Some the output signal’s parameters, such as amplitude, carrier frequency, index of modulation or frequency, are sensed by the detector; any undesired component is filtered out and the remaining signal is compared with a reference signal. The result of the comparison is used to generate the control voltage (V C ) and adjust the gain of the VGA.
Since an AGC is essentially a negative feedback system, the system can be described in terms of its transfer function. The idealized transfer function for an AGC system is illustrated in figure 2. For low input signals the AGC is disabled and the output is a linear function of the input, when the output reaches a threshold value (V1) the AGC becomes operative and maintains a constant output level until it reaches a second threshold value (V2). At this point, the AGC becomes inoperative again; this is usually done in order to prevent stability problems at high levels of gain.
Many of the parameters of the AGC loop depend on the type of modulation used inside the system. If any kind of amplitude modulation (AM) is present, the AGC should not respond to any change in amplitude modulation or distortion will occur. Thus the bandwidth of the AGC must be limited to a value lower than the lowest modulating frequency. For systems where frequency or pulse modulation is used, the system requirements are not that stringent.
As mentioned before, an AGC system is considered a nonlinear systems and it is very hard to find solutions for the nonlinear equation that arise during the analysis. Nevertheless, there are two models that describe the system’s behaviour with a good degree of accuracy and are relatively easy to implement when the small signal transfer equations of the main blocks are known (which is usually the case).
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