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