LMH6505 Datasheet, PDF (17/20 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear-in-dB, Variable Gain Amplifier

Application Information (Continued)
FIGURE 9. Automatic Gain Control Circuit #1
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Signal frequencies must not reach the gain control port of the
LMH6505, or the output signal will be distorted (modulated
by itself). A fast settling AGC needs additional filtering be-
yond the integrator stage to block signal frequencies. This is
provided in Figure 9 by a simple R-C filter (R₁₀ and C₃);
better distortion performance can be achieved with a more
complex filter. These filters should be scaled with the input
signal frequency. Loops with slower response time, which
means longer integration time constants, may not need the
R₁₀ – C₃ filter.
Checking the loop stability can be done by monitoring the V_G
voltage while applying a step change in input signal ampli-
tude. Changing the input signal amplitude can be easily
done with an arbitrary waveform generator.
AUTOMATIC GAIN CONTROL (AGC) #2
Figure 10 illustrates an automatic gain control circuit that
employs two LMH6505. In this circuit, U1 receives the input
signal and produces an output signal of constant amplitude.
U2 is configured to provide negative feedback. U2 generates
a rectified gain control signal that works against an adjust-
able bias level which may be set by the potentiometer and
R_B. C_I integrates the bias and negative feedback. The result-
ant gain control signal is applied to the U1 gain control input
V_G. The bias adjustment allows the U1 output to be set at an
arbitrary level less than the maximum output specification of
the amplifier. Rectification is accomplished in U2 by driving
both the amplifier input and the gain control input with the U1
output signal. The voltage divider that is formed by R₁ and
R₂, sets the rectifier gain.
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