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Preamplifier with automatic gain control (AGC)
In some cases, the developer requires preliminary signal amplification with its automatic control (AGC). Such solutions are required to stabilize the operation of the final power amplifier, or to ensure a stable amplitude of the control voltage. Unlike limiters and compressors, AGC does not distort the signal even at large input values. The principle of its operation is based on adjusting the gain depending on the input signal amplitude.
The circuit presented here can operate stably in the range from 10 Hz to 250 kHz in frequency, and from 10 mV to 5 V in amplitude of the input signal. Moreover, if the input signal changes from 100 mV to 5 V (50 times), then the output signal changes only 1.7 times, and the circuit does not introduce distortion into it.
Fig.1. Schematic diagram of a pre-amplifier with automatic gain control on the KA2220 chip
The device (Fig. 1) is based on the KA2220 microcircuit in a SIP package (or its analogues: TA7137P, LA3210). In the documentation for this microcircuit [1], the AGC mode is presented somewhat confusingly, and it is quite difficult to assemble an amplifier according to the circuit shown there. In fact, its design is quite simple: it consists of an operational amplifier with two inputs - direct and inverse, and a gain regulator. AGC here is achieved by the fact that the signal from the output of the amplifier is detected using the C6-D1D2-R5C7 chain, after which it is fed to this regulator. R1, and the internal adjustable resistance, form a voltage divider, which, in fact, regulates the gain. C4 reduces the likelihood of self-excitation at some frequencies. The R3R4C3 chain and capacitor C2 are needed to create the necessary modes for the correct functioning of the microcircuit. Resistor R2 creates optimal gain feedback.
The input signal is supplied to the IN connector, and the balanced output signal is removed from the OUT connector. It should be noted here that if you need a unipolar output signal, with a midpoint equal to half the power, then it can be taken directly from pin 8 of the DA1 chip.
Below are the electronic parts and their possible replacements (in parentheses):
  • DA1 -- microcircuit KA2220 (TA7137P, LA3210);
  • C1, C4, C7 -- any ceramic capacitors;
  • C2, C3, C5, C6 - electrolytic capacitors for a voltage of 6.3 volts or more;
  • D1, D2 -- any low-power Schottky diodes, for example BAT42 or BAT43.
You can use any resistors here, at your discretion.
Scheme setup
A correctly assembled circuit does not require configuration. If deeper adjustment of the AGC is necessary, then the resistance of R1 can be increased to 5 kOhm.
Printed circuit board
Although the circuit turns out to be quite simple and the layout of the printed circuit board is not difficult to do yourself, nevertheless, Below we provide documentation for the production of a professional printed circuit board.
Production version: PCB (open)
The production option provides a set of documentation for manufacturing a printed circuit board in production: GERBER file for PCB, BOM file of the specification of components and a schematic diagram showing the values of the elements. All this allows you to immediately order a PCB, for example, here, and then quickly assemble it.
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Materials used
  1. Equalizer amplifier with ALC. Datasheet KA2220 [PDF].