AN-934: 60 dB Wide Dynamic Range, Low Frequency AGC Circuit Using a Single VGA (Rev. 0) PDF

AN-934
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
60 dB Wide Dynamic Range, Low Frequency AGC Circuit Using a Single VGA
by James Staley
INTRODUCTION
Controllable Gain Element
Low frequency automatic gain control (AGC) circuits are used
in audio and power equipment for applications such as sensitive
microphone preamplifiers (preamps) and regulators. An AGC
circuit, a closed-loop feedback system, is shown in Figure 1. The
loop consists of a controllable gain element, a detector, a stable
reference and a comparison circuit.
A VGA is a special type of amplifier, which controls its gain by
electronic means instead of by a set of fixed resistors, as is the
case with the classic op amp circuit. VGAs are the familiar and
preferred solution for automatic gain control circuits in a
variety of communications applications.
VGAs operate at frequencies from hundreds of kilohertz up to
hundreds of megahertz. An ideal VGA performs as a linear
amplifier, without introducing distortion or otherwise
corrupting the desired signal.
CONTROLLABLE
GAIN
ELEMENT
OUTPUT
When a VGA is used, the gain element is an amplifier combined
with electronic volume control. In this example, the controllable
gain element is further reduced to an electronic potentiometer
and a fixed gain amplifier and it adjusts the loop gain by
attenuating the input signal, without contributing significant
distortion. The other fundamental elements of the loop are the
detector, a stable reference, and a summing circuit that senses
the state of the loop, compares it to the stable reference, and
adjusts the output accordingly.
GAIN CONTROL
VOLTAGE
–
DETECTOR
06975-001
+
REFERENCE
Figure 1. AGC Circuit Using a Variable Gain Amplifier
This application note describes a low frequency AGC circuit
using a wide dynamic range AD8336 variable gain amplifier
(VGA) as the gain control element, an AD736 rms-to-dc
converter as the detector, a low cost rail-to-rail AD8551 op amp,
and an ADP3339 LDO as the reference. Because of its wide
controllable gain range and circuit flexibility, the AD8336 is
featured in this application note.
INPP
4
INPN
5
PWRA
2
A functional block diagram of the AD8336 is shown in Figure 2.
PRAO
VGAI
8
9
ATTENUATOR
–60dB TO 0dB
PrA
AD8336
34dB
1
VOUT
GAIN CONTROL
INTERFACE
BIAS
10
13
3
11
12
VNEG
VPOS
VCOM
GPOS
GNEG
Figure 2. AD8336 Functional Block Diagram
Rev. 0 | Page 1 of 4
06975-002
INPUT
AN-934
TABLE OF CONTENTS
Introduction ...................................................................................... 1
AGC Circuit Design Example......................................................3
Circuit Design ................................................................................... 3
AGC Circuit Operation ................................................................4
Rev. 0 | Page 2 of 4
AN-934
70
CIRCUIT DESIGN
60
An audio AGC requires the following features:
50
Wide dynamic range, which is the ability to amplify very
low level signals and very large signals.
FOR PREAMP GAIN = 26dB
40
•
Amplification with low distortion over the entire operating range.
•
A means of adjusting the minimum and maximum
gain limits.
GAIN (dB)
GAIN INTERPOLATOR
FIXED GAIN
AMPLIFIER
–6dB
FOR PREAMP GAIN = 12dB
FOR PREAMP GAIN = 6dB
–20
–30
–0.7
–0.5
–0.3
–0.1
0.1
VG (V)
0.3
0.5
0.7
Figure 4. Gain Characteristics of the AD8336
for Various Operating Conditions
AGC CIRCUIT DESIGN EXAMPLE
Signal Voltage levels
The range of signal voltages to be controlled, the supply voltages
and input and output voltage levels, all highly interactive factors,
influence the topology of an AGC circuit. In this example, the
goal is to fully exploit the full 60 dB gain control range of the
AD8336. First, assume power supply voltages of ±5 V.
60dB
10 SECTION LADDER NETWORK
06975-003
–6dB
10
0
Gm
VGA
IN
20
–10
The AD8336 described in this application note uses the Analog
Devices, Inc. exclusive X-AMP® architecture, consisting of a
ladder network with multiple taps spaced in equi-resistive
increments, and accessed by an array of differential amplifiers.
See Figure 3.
OUTPUT
30
USEABLE GAIN RANGE
OF AD8336
06975-004
•
GAIN CHARACTERISTICS
COMPOSITE GAIN
VGA STAGE GAIN
Figure 3. AD8336 Ladder Network
This circuit architecture offers several important advantages:
•
A passive resistor ladder network performs the gain control
function, introducing no distortion.
•
The gain element is a fixed gain op amp. Because the gain
of the op amp remains unchanged, the application benefits
from constant bandwidth, distortion, and overload performance optimized over a wide range of operating
conditions.
The AD8336 features a wide gain range (60 dB) and extended
supply voltage, capable of operating with power supplies up to
±15 V. It features an uncommitted preamplifier and permits
inverting, noninverting, or differential input configurations.
The preamplifier and VGA sections are completely independent, and the VGA can be used as a standalone element if no
preamp is needed. The gain control inputs are fully differential.
Figure 4 shows the gain characteristic for the VGA, for two
values of preamplifier gain.
With a known power supply voltage, the stabilized output
voltage is established. Because saturation in either the
preamplifier or the 34 dB fixed gain stage limits the available
output swing to about 7 V p-p, a nominal maximum swing of
5 V p-p is easily attainable. With a preamp output-voltage swing
of 5 V p-p and the X-AMP attenuator set for −26 dB (0.05×),
the output voltage is 250 mV p-p. If the preamp gain is set at
−1× (unity inverting gain (equivalent to a noise gain of 2×),
the maximum input voltage is 5 V p-p. Finally, with a gain
range of 60 dB, the minimum input voltage is 5 mV p-p. The
AGC circuit operates with an input voltage range of 60 dB
(5 mV p-p to 5 V p-p), with a fixed output voltage of 250 mV p-p.
Control Voltage Levels
The differential gain control input of the AD8336 performs any
level shifting required for the available control voltages considerably simplifying the gain control drive circuitry. In this
example, the GNEG input (Pin 12) is biased at 0.75 V and the
gain range voltage at GPOS is 1.5 V.
Detector
The detector is an AD736 rms-to-dc converter and provides an
accurate dc control voltage directly proportional to the rms
value of the output signal. The output of the AD736 drives the
inverting input of an op amp connected for very high dc gain
for accurate loop control.
Rev. 0 | Page 3 of 4
AN-934
Comparison Circuit
Table 1.
The AD8551 is a single-supply rail-to-rail op amp with a very
low offset voltage. The voltage applied to the noninverting input
is the reference voltage, and establishes the rms value of the
output. The voltage to be compared is the detector voltage from
the rms–to-dc converter. When the comparison input falls
below the reference the comparison output voltages increases to
restore the output to its nominal level.
EIN
(V rms)
0.001
0.002
0.003
0.005
0.01
0.1
1
1.5
1.8
2
AGC CIRCUIT OPERATION
Table 1 lists the data for the AGC control for six frequencies
from 1 mV to 2 V rms input. Refer to Figure 6 for a plot
showing the flat output level vs. input for the typical audio
frequency range of 20 Hz to 20 kHz. The output level is flat
over the 2 mV rms to 2 V rms range.
1kΩ
8
9
PrA
0dB
5
0dB
10µF
100Ω
ATTENUATOR
–60dB TO 0dB
AD8336
100 Hz
130
255
250
250
250
250
250
250
249
256
EOUT (mV p-p)
1 kHz 5 kHz
136
135
253
253
251
253
250
251
250
251
250
251
250
251
250
251
250
250
261
266
10 kHz
140
260
257
256
255
254
254
254
254
266
GAIN RANGE:
–26 TO +34dB
(0.05× TO 50×)
4
INPUT 1kΩ
20 Hz
125
245
251
250
250
250
250
250
250
250
34dB
1
NP
OUTPUT
4.7nF
0.1µF
2
1
GAIN CONTROL
INTERFACE
BIAS
2
10
+5V
13
–5V
ADP3339AKC-1.5
0.1µF
3
IN
2
1
OUT
GND
11
1MΩ
2.2µF
+5V
3
10kΩ
4
–5V
1.5V
COM
VIN
+VS
AD736
CF
OUTPUT
–VS
CAV
8
7
+5V
6
5
0.022µF
1kΩ
0.1µF
3
12
CC
0.75V
7
AD8551
2
3
4
06975-005
88mV
1kΩ
TAB
10kΩ
6
9.09kΩ
OUT
33µF
+5V
1kΩ
Figure 5. AGC Circuit Using the AD8336
300
290
OUTPUT (mV p-p)
280
270
260
250
240
230
220
200
0.001
0.01
0.1
INPUT (V rms)
5kHz
10kHz
20kHz
1
Figure 6. AGC Performance at Various Frequencies
©2007 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN06975-0-11/07(0)
Rev. 0 | Page 4 of 4
06975-006
20Hz
100Hz
1kHz
210
10
20 kHz
140
265
258
258
255
254
254
254
254
266
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