AD811
An 80 MHz Voltage-Controlled Amplifier Circuit
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 about 25 MHz. The maximum output voltage under these
conditions will be increased to ±9 V using ±12 V supplies.
The voltage-controlled amplifier (VCA) circuit of Figure 40
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, VG. 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 ex-
ceeds 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 VG = 0 dB to
+12 dB, (a numerical gain of four), when VG = +1 V. The over-
all transfer function of the VCA is:
The gain-control input voltage, VG, may be a positive or nega-
tive ground-referenced voltage, or fully differential, depending
on the user’s choice of connections at Pins 7 and 8. A positive
value of VG results in an overall noninverting response. Revers-
ing the sign of VG 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 capa-
bilities and fully-symmetrical responses from X- and Y-inputs.
V
OUT = 4 (X1 – X2)(Y1 – Y2)
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
AD811’s input stage. To achieve the lowest differential gain and
phase errors, it is recommended that the AD811 be operated
from power supply voltages of ±10 volts or greater. This VCA
circuit is designed to operate from a ±12 volt dual power
supply.
which reduces to VOUT = 4 VG VIN using the labeling conven-
tions shown in Figure 40. The circuit’s –3 dB bandwidth of
80 MHz, is maintained essentially constant—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.
FB
+12V
C1
0.1F
R1 100⍀
R8*
+
V
G
–
R2 100⍀
8
7
6
5
R4
182⍀
R6
294⍀
X2
X1 +V
W1
S
U1
AD834
U3
AD811
V
OUT
–V
3
S
Y1 Y2
W2
4
R7
294⍀
R5
182⍀
R
L
1
2
V
IN
R9*
R3
249⍀
C2
0.1F
–12V
FB
*R8 = R9 = 511⍀ FOR X4 GAIN
= 1.27k⍀ FOR X10 GAIN
Figure 40. An 80 MHz Voltage-Controlled Amplifier
–12–
REV. D
ADI [ ADI ]
