The TDA520x, TDA521x, TDA522x, TDA7200, TDA7210 and TDA7210V receivers provide an AGC (Automatic Gain Control) circuit that can be used in the active mode or in the inactive low gain mode to extend the dynamic range of the receiver. The circuit diagram of the actual multiplier circuit as illus-trated in Figure 3 makes it easier to determine the multipli-cation constant, M. This change results. Automatic Gain Control (AGC) was implemented in first radios for the reason of fading propagation (defined as slow variations in the amplitude of the received signals) which required continuing adjustments in the receiver's gain in order to maintain a relative constant output signal. An AGC circuit, a closed-loop feedback system, is shown in Figure 1. Since the mixer output stage has a fixed bias current of 300uA. the present inventionis a circuit directed towards ensuring a constant RF output level in optical receivers that are suitable for use in the communications system of FIG.
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