Download THAT Corporation Design Note 108

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