8-5
Automatic Gain Control
Figure 6 shows the HA-2557 configured in an Automatic Gain
Control or AGC application. The HA-2842 serves as an output I
to V converter using R
Z
which is trimmed to provide an
accurate 4V Fullscale conversion. Refer to
Voltage
Output
Conversion
for more details about this function. The HA-5127
low noise amplifier provides the gain control signal to the X
input. This control signal sets the peak output voltage of the
multiplier to match the preset reference level. The feedback
network around the HA-5127 provides a response time
adjustment. High frequency changes in the peak are rejected
as noise or the desired signal to be transmitted. These signals
do not indicate a change in the average peak value and
therefore no gain adjustment is needed. Lower frequency
changes in the peak value are given a gain of -1 for feedback to
the control input. At DC the circuit is an integrator automatically
compensating for offset and other constant error terms.
This multiplier has the advantage over other AGC circuits, in
that the signal bandwidth is not affected by the control signal
gain adjustment.
Voltage Output Conversion
The HA-2842 is an excellent choice to perform the output
current to voltage conversion as shown in Figure 7. The
combination of 400V/
μ
s slew rate and 80MHz Gain Band-
width product will maintain signal dynamics while providing a
full scale
±
4V output. The HA-2842 also provides a hefty out-
put drive capability of 100mA.
This voltage feedback amplifier takes advantage of the inter-
nal R
Z
resistor, trimmed to provide an accurate 4V fullscale
conversion. The parasitic capacitance at the negative input
of the HA-2842 must be compensated with a 3pF capacitor
from pin 2 to pin 6. This compensation will also insure that
the amp will see a noise gain of 2 at its crossover frequency,
the minimum required for stability with this device. The full
power bandwidth curve and large signal pulse response for
this circuit are shown in Figure 11 and Figure 12 respec-
tively. The fast slew rate of the HA-2842 results in a minimal
reduction of bandwidth for large signals.
Another choice for an I to V converter that takes better
advantage of the wide bandwidth of the HA-2557, is to use
the HA5023 Dual 100MHz current feedback amp. The opti-
mum bandwidth of a current feedback amp is obtained with a
fixed feedback resistor. Therefore scaling the I to V conver-
sion to a convenient value requires two stages. Fortunately
the HA5023 provides two wideband amplifiers in a single 8
pin Mini-DIP or SOIC package, while their current feedback
architecture provides signal gain with minimal reduction in
bandwidth. This circuit configuration is shown in Figure 8.
The optimum bandwidth is achieved in stage 1 with a 909
feedback resistor. This voltage is then gained up by the sec-
ond stage to provide a
±
4V Fullscale Voltage output with a
bandwidth in excess of 90MHz. The 10pF capacitor and the
additional 220
resistor improve gain flatness and reduce
gain peaking. The HA5023 also provides excellent Full
Power Bandwidth (-3dB at 80MHz for a 3.5V
P-P
signal). Typ-
ical curves for this application circuit are shown in Figures
13, 14, 15 and 16.
5k
10k
HA-5127
0.01
μ
F
10k
0.1
μ
F
1N914
5.6V
0.1
μ
F
+15V
20k
14
15
16
9
13
12
11
10
1
2
3
4
5
7
6
8
+15V
V
OUT
NC
NC
NC
-15V
NC
V
Y
V
X
REF
NC
NC
NC
X
0.01
μ
1.0
μ
3pF
I
OUT
1.0
μ
0.01
μ
2.5K
R
Z
+
-
HA-2842
+
-
Y
X
FIGURE 6. AUTOMATIC GAIN CONTROL
14
15
16
9
13
12
11
10
1
2
3
4
5
7
6
8
+15V
V
OUT
NC
NC
NC
-15V
NC
V
Y
V
X
REF
NC
NC
NC
X
0.01
μ
1.0
μ
+15V -15V
HA-2842
3pF
I
OUT
1.0
μ
0.01
μ
0.01
μ
1.0
μ
0.01
μ
1.0
μ
2.5K
2
3
6
R
Z
+
-
Y
X
FIGURE 7. VOLTAGE OUTPUT CONVERSION
HA-2557