AD603
3.0
2.5
2.0
1.5
GAIN ERROR – dB
THEORY OF THE AD603
A Low Noise AGC Amplifier
1.0
0.5
0.0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–0.1 0.0
0.1
0.2
0.3
0.4
0.5
V
C
0.6
0.7
0.8 0.9
1.0 1.1
Figure 15 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, here provided 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 pins 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.
0
0.2
0.4
V
C
0.6
0.8
1.0
1.2
Figure 13. Gain Error for Cascaded Stages–Low Ripple
Mode
90
85
80
IS/N RATIO – dB
75
70
65
60
55
50
–0.2
Figure 14. ISNR vs. Control Voltage–Low Ripple Mode
A half-wave detector is used, based on Q1 and R8. The current
into capacitor C
AV
is just the difference between the collector
current of Q2 (biased to be 300
µA
at 300 K, 27°C) and the col-
lector current of Q1, which increases with the amplitude of the
10V
THIS CAPACITOR SETS
AGC TIME CONSTANT
V
AGC
C7
0.1 F
C1
0.1 F
J1
RT
100
1
10V
R1
2.49k
+ C3
100 F
2
C4
0.1 F
R2
2.49k
10V
R13
2.49k
C8
0.1 F
C2
0.1 F
10V
R3
2.49k
+ C5
100 F
2
C6
0.1 F
R4
2.49k
10V
R14
2.49k
C
AV
0.1 F
R9
1.54k
Q2
2N3906
R10
1.24k
C11
0.1 F
R11
3.83k
Q1
2N3904
R8
806
5V
R12
4.99k
C9
0.1 F
A1
AD603
A2
AD603
J2
C10
0.1 F
AGC LINE
R5
5.49k
5.5V
NOTES
1
R PROVIDES A 50 INPUT IMPEDANCE
T
2
C3 AND C5 ARE TANTALUM
1V OFFSET FOR
SEQUENTIAL GAIN
R6
1.05k
6.5V
R7
3.48k
10V
Figure 15. A Low Noise AGC Amplifier
REV. C
–9–
AD [ ANALOG DEVICES ]
