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Automatic gain control, or AGC, is found in most modern receivers This popularity is due to the necessity of increasing the usable dynamic range of a receiver, since without AGC powerful incoming signals would immediately saturate the receiver and create massive distortion, while feeble signals would go virtually undetected by the demodulator Both scenarios would cause very poor BER in a digital system or unreadable and distorted signals in an analog system Bias-based AGC circuits exist thanks to a particular transistor characteristic: The gain of a transistor is increased when we raise the transistor s collector current and, inversely, decreasing the collector current will decrease the transistor s gain Indeed, we can easily increase the collector current by raising the forward bias at the transistor s base, since increasing the base current will increase the collector current, and thus the gain As shown in Fig 827, however, a point is soon reached in which this capability will not only level off, but the gain will actually start to decrease slowly with any increase in collector current The control of the base current is created by the DC bias voltage that is impressed at the base of the transistor by the AGC circuit itself In fact, many variable gain amplifiers will depend only on this AGC voltage for their entire DC base bias Because of this capability of a transistor to increase and decrease gain by an external circuit increasing or decreasing its own collector current, we see that there can be two methods of implementing AGC, reverse and forward AGC Reverse AGC is by far the most popular, and can be found in the IF sections of many radios Forward AGC may sometimes be designed into certain frontend RF amplifiers, but is undesirable for general applications since it wastes more collector current than reverse AGC, and has a much more gradual gain response
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FIGURE 827 A transistor s base-bias voltage versus gain for use in an AGC loop
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With these DC bias-controlled amplifiers care must be taken to confirm that severe distortion does not occur when the gain is varied by the AGC, since the transistor can easily be biased into a nonlinear part of its operation, especially critical if the input signal is of a high amplitude This issue is not nearly as much of a consideration when AGC amplifiers use voltage variable attenuators at their input, instead of employing AGC bias control, for this gain control function In fact, many of the newer AGC circuits will feed the RF or IF detected and amplified control voltage back to one or more variable attenuators, which are inserted before fixed-gain amplifier stages (see Sec 311 and Sec 84) The voltage needed to feed an AGC loop can be tapped off the last IF stage (Fig 828) or, in some receivers, after detection by the detector As shown in the figure, the IF signal is first tapped from the IF strip s output, RF amplified, rectified to DC, DC
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DETECTOR
OUT TO BB AMPS
IN FROM MIXER
1ST IF STAGE
LAST IF STAGE
AGC AMP
REVERSE AGC BIAS
AGC LPF
DIRECT CURRENT
DC AMP
RECTIFIED AC
FIGURE 828
A type of AGC loop using the tapped IF signal
Support Circuit Design
COUPLER
IF INPUT IF OUTPUT
IF AMP
GAIN=Rf /Ri
AGC AMP DETECTOR
D R1 68
OP-AMP
AGC AMP/ COMPARITOR
VREF
R2 5 Ri
FIGURE 829 A common AGC circuit in a receiver s IF chain
amplified, filtered to a steady DC, and sent to the base of the first, second, and even third IF amplifiers through a trace on the PCB board called an AGC bias line
832 Automatic Gain Control Design
A complete AGC circuit is shown in Fig 829, and can be designed in various ways However, the basics still do not change: The signal to be controlled must be sampled, detected, filtered, and placed into a variable gain amplifier in order to change the stage gain, and thus the entire receiver system s total gain; and is ultimately dependent on the input RF signal s strength Discussed below are all of the typical stages found in an AGC circuit
Sampling the Signal
The signal to be controlled can be tapped from the IF by one of two ways A large resistor that is much higher in value than the 50 of the IF can be exploited to remove a small portion of the signal for feeding the AGC detector, or a directional coupler can be employed to remove a small sample of the signal for AGC detection (see Sec 882)
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