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THAT Corporation Design Note 108 Single Chip Automatic Gain Control The circuits within this application note feature THAT4301 Analog Engine® to provide the essential elements of voltage-controlled amplifier (VCA) and rms-level detector (RMS). Since writing this note, THAT has introduced several new models of Analog Engines, as well as new VCAs. With minor modifications, these newer ICs are generally applicable to the designs shown herein, and may offer advantages in performance, cost, power consumption, etc., depending on the design requirements. As well, a standalone RMS is available to complement our standalone VCAs. We encourage readers to consider the following alternatives in addition to the 4301: • Low supply voltage and power consumption: 4320 • Low cost, supply voltage, and power consumption: 4315 • Low cost and power consumption: 4305 • High-performance (VCA only): 2180-series, 2181-series • Dual (VCA only): 2162 • RMS (standalone): 2252 For more information about making these substitutions, please contact THAT Corporation's technical support group at [email protected]. 45 Sumner St, Milford, MA 01757-1656 USA; www.thatcorp.com; [email protected] Copyright © 2000 - 2010 by THAT Corporation; All rights reserved. Document 600151 Revision 01 THAT Corporation Design Note 108 Single Chip Automatic Gain Control AGCs, as a rule, maintain a constant output signal level while the input signal level varies. This circuit is essentially a limiter operated most of the time above its threshold. Gain control is accomplished by connecting the output of the RMS detector to the negative control port of the VCA. In a feed forward topology, the gain is then reduced by the same amount that the input level increases, keeping the output level constant. The RMS detector, which senses the level of the input signal, is set for a 0db reference level of -10dBu. This level is essentially arbitrary, and allows for convenient component values and signal levels. The timing current is determined by the equation: It = 15V 2M , and the 0dB input reference current is related to It by Iin 0dB = 1.13 % I t . If we set Vin0dB = -10dBu as the reference input signal voltage, then we can calculate the input reference current as −10 Iin 0dB = 0.775 % 10 20 l 0.245 V rms . Using this result, the value of the RMS-Detector’s input voltage-to-current resistor becomes R1 = Vin 0dB Iin 0dB = 0.245 8.5A l 28.8k This setting will result in the RMS detector having zero volts out when the input signal level is -10dBu ( corresponding to unity gain in the VCA). The threshold amplifier has been modified to have an offset injected through R5 that sets the nominal threshold to -40 dBu, and VR2 allows this level to be adjusted by ±10dBu. The threshold amplifiers output, at the junction of D1, R2 and R6, is zero volts when the input signal is below the threshold level, but decreases so as to reduce the gain of the VCA to just compensate for the increase in the input signal the input signal once the input exceeds threshold. VR4 allows the user to set the nominal gain. The remainder of the circuit functions essentially the same as the compressor/limiter mentioned in the THAT4301's datasheet. See the datasheet for details. Copyright © 2000-2010 by THAT Corporation All rights reserved. 45 Sumner St, Milford, MA 01757 USA; www.thatcorp.com Design Note 108 Page 2 of 3 Doc. 600151 Rev. 01 Copyright © 2000-2010 by THAT Corporation All rights reserved. Input 45 Sumner St, Milford, MA 01757 USA; www.thatcorp.com 2 1 IT IN V- 4301P CT R5 379k C4 10u R13 4k99 20 19 U1D cw R15 1M43 V- -20 dB Gain VR4 10k V+ R6 24k3 -13.2 mV/dB D1 1N4148 +20 dB 4301P OA1 18 1N4148 D2 10n R2 10k0 C7 C5 17 8 V- 7 ol conlttar ge vo THAT4301P U1C OA2 U1A OA3 12 THAT4301P 11 V+ IN VCA OUT 13 VGND 10 EC- 9 16 47n MY R14 12K4 6 R11 20k0 C3 47p NPO 5% R10 20k0 C6 22u A simple AGC circuit using the THAT4301 5 OUT 4 RMS U1B -30 dBu R7 2M00 V- R1 47u 28k7 C1 -50 dBu V+ R4 AGC VR2 Threshold 10k cw 1M1 0dB reference at -10dBu 47u C2 V+ 15 EC+ 14 SYM R9 VR1 Symmetry 50k cw 300k 5% R8 V51R 5% V+ R12 Output 100R 5% THAT Corporation Design Note 108 Single Chip Automatic Gain Control Design Note 108 Page 3 of 3 Doc. 600151 Rev. 01