LMH6504MAX National Semiconductor Corporation, LMH6504MAX Datasheet - Page 16

• Manufacturer Part Number: LMH6504MAX
• Description: Wideband, Low Power, Variable Gain Amplifier The LMH6504 is a Wideband DC Coupled Voltage Controlled Gain Stage Followed BY a High-speed Current Feedback op Amp Which CAN Directly Drive a Low Impedance Load. Gain Adjustment Range is 80 DB For up to 1
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
Signal frequencies must not reach the gain control port of the
LMH6504, 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 8 by a simple R-C filter (R
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 (longer
integration time constants) may not need the R
Checking the loop stability can be done by monitoring the V
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 9 illustrates an automatic gain control circuit that
employs two LMH6504’s. In this circuit, U1 receives the input
signal and produces an output signal of constant amplitude.
FIGURE 8. Automatic Gain Control Circuit #1
(Continued)
10
10
– C
and C
3
filter.
3
);
G
16
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
ant gain control signal is applied to the U1 gain control input
V
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 R
R
G
B
2
, sets the rectifier gain.
. C
. The bias adjustment allows the U1 output to be set at an
I
integrates the bias and negative feedback. The result-
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1
and