Data Sheet AD811
Rev. G | Page 15 of 20
AN 80 MHZ VOLTAGE-CONTROLLED AMPLIFIER
CIRCUIT
The voltage-controlled amplifier (VCA) circuit of Figure 46 shows
the AD811 being used with the AD834, a 500 MHz, 4-quadrant
multiplier. The AD834 multiplies the signal input by the dc control
voltage, V
G
. The AD834 outputs are in the form of differential
currents from a pair of open collectors, ensuring that the full
bandwidth of the multiplier (which exceeds 500 MHz) is
available for certain applications. Here, the AD811 op amp
provides a buffered, single-ended, ground-referenced output.
Using feedback resistors R8 and R9 of 511 Ω, the overall gain
ranges from −70 dB for V
G
= 0 V to +12 V (a numerical gain of
+4) when V
G
= 1 V. The overall transfer function of the VCA is
V
OUT
= 4 (X1 − X2)(Y1 − Y2), which reduces to V
OUT
= 4 V
G
V
IN
using the labeling conventions shown in Figure 46. The circuit’s
−3 dB bandwidth of 80 MHz is maintained essentially constant—
that is, independent of gain. The response can be maintained
flat to within ±0.1 dB from dc to 40 MHz at full gain with the
addition of an optional capacitor of about 0.3 pF across the
feedback resistor R8. The circuit produces a full-scale output of
±4 V for a ±1 V input and can drive a reverse-terminated load
of 50 Ω or 75 Ω to ±2 V.
The gain can be increased to 20 dB (×10) by raising R8 and R9
to 1.27 kΩ, with a corresponding decrease in −3 dB bandwidth
to approximately 25 MHz. The maximum output voltage under
these conditions is increased to ±9 V using ±12 V supplies.
The gain-control input voltage, V
G
, may be a positive or negative
ground-referenced voltage, or fully differential, depending on
the choice of connections at Pin 7 and Pin 8. A positive value of
V
G
results in an overall noninverting response. Reversing the sign
of V
G
simply causes the sign of the overall response to invert. In
fact, although this circuit has been classified as a voltage-controlled
amplifier, it is also quite useful as a general-purpose, four-quadrant
multiplier, with good load driving capabilities and fully
symmetrical responses from the X and Y inputs.
The AD811 and AD834 can both be operated from power supply
voltages of ±5 V. While it is not necessary to power them from
the same supplies, the common-mode voltage at W1 and W2
must be biased within the common-mode range of the input
stage of the AD811. To achieve the lowest differential gain and
phase errors, it is recommended that the AD811 be operated
from power supply voltages of ±10 V or greater. This VCA
circuit operates from a ±12 V dual power supply.
Figure 46. An 80 MHz Voltage-Controlled Amplifier
X2
X1 +V
S
W1
Y1 Y2 W2
–V
S
U1
AD834
U3
AD811
R4
182
R5
182
R1 100
R2 100
R3
249
R6
294
R7
294
R9*
R8*
R
L
FB
C2
0.1F
C1
0.1F
–12V
V
OUT
FB
+12V
V
G
V
IN
*R8 = R9 = 511 FOR 4 GAIN
R8 = R9 = 1.27k FOR 10 GAIN
+
–
1234
8765
7
6
4
3
2
+
–
00866-E-047
