AD603-EB datasheet(18/20 Pages) AD

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AD603

Rev. G | Page 18 of 20

APPLICATIONS

A LOW NOISE AGC AMPLIFIER

Figure 47 shows the ease with which the AD603 can be

connected as an AGC amplifier. The circuit illustrates many of

the points previously discussed: It uses few parts, has linear-in-

dB gain, operates from a single supply, uses two cascaded

amplifiers in sequential gain mode for maximum S/N ratio, and

an external resistor programs each amplifier’s gain. It also uses a

simple temperature-compensated detector.

The circuit operates from a single 10 V supply. Resistors R1, R2,

R3, and R4 bias the common pins of A1 and A2 at 5 V. This pin

is a low impedance point and must have a low impedance path

to ground, provided here by the 100 µF tantalum capacitors and

the 0.1 µF ceramic capacitors.

The cascaded amplifiers operate in sequential gain. Here, the

offset voltage between the Pin 2 (GNEG) of A1 and A2 is 1.05 V

(42.14 dB × 25 mV/dB), provided by a voltage divider

consisting of resistors R5, R6, and R7. Using standard values, the

offset is not exact, but it is not critical for this application.

The gain of both A1 and A2 is programmed by resistors R13

and R14, respectively, to be about 42 dB; thus the maximum

gain of the circuit is twice that, or 84 dB. The gain control range

can be shifted up by as much as 20 dB by appropriate choices of

R13 and R14.

The circuit operates as follows. A1 and A2 are cascaded.

Capacitor C1 and the 100 Ω of resistance at the input of A1

form a time constant of 10 µs. C2 blocks the small dc offset

voltage at the output of A1 (which might otherwise saturate A2

at its maximum gain) and introduces a high-pass corner at

about 16 kHz, eliminating low frequency noise.

A half-wave detector is used, based on Q1 and R8. The current

into capacitor CAV is just the difference between the collector

current of Q2 (biased to be 300 µA at 300 K, 27°C) and the

collector current of Q1, which increases with the amplitude of

the output signal.

The automatic gain control voltage, VAGC, is the time integral of

this error current. In order for VAGC (and thus the gain) to

remain insensitive to short-term amplitude fluctuations in the

output signal, the rectified current in Q1 must, on average,

exactly balance the current in Q2. If the output of A2 is too

small to do this, VAGC will increase, causing the gain to increase,

until Q1 conducts sufficiently.

Consider the case where R8 is zero and the output voltage VOUT

is a square wave at, say, 455 kHz, which is well above the corner

frequency of the control loop.

0.1

µF

R1 3

2.49k

Ω

10V

5.5V

6.5V

2.49k

Ω

C32

100

µF

0.1

µF

2.49k

Ω

RT1

100

Ω

1.05k

Ω

5.49k

Ω

3.48k

Ω

10V

AGC LINE

CAV

0.1

µF

THIS CAPACITOR SETS

AGC TIME CONSTANT

VAGC

1.54k

Ω

806

Ω

2N3904

2N3906

R1 0

1.24k

Ω

R1 1

3.83k

Ω

R1 2

4.99k

Ω

C11

0.1

µF

0.1

µF

10V

C10

0.1

µF

10V

0.1

µF

0.1

µF

AD603

R1 4

2.49k

Ω

2.49k

Ω

C52

100

µF

0.1

µF

2.49k

Ω

10V

0.1

µF

AD603

1V OFFSET FOR

SEQUENTIAL GAIN

1 RT PR OVID ES A 5 0

Ω INPUT IMPEDANCE.

2 C 3 AN D C 5 A RE TA NTALUM.

Figure 47. A Low Noise AGC Amplifier

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