AD811SQ/883B AD [Analog Devices], AD811SQ/883B Datasheet - Page 12

• Manufacturer Part Number: AD811SQ/883B
• Description: High Performance Video Op Amp
• Manufacturer: AD [Analog Devices]

AD811
An 80 MHz Voltage-Controlled Amplifier Circuit
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, V
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
+12 dB, (a numerical gain of four), when V
all transfer function of the VCA is:
which reduces to V
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
V
V
IN
G
, the overall gain ranges from –70 dB, for V
+
–
V
OUT
OUT
R1 100
R2 100
= 4 (X1 – X2)(Y1 – Y2)
= 4 V
G
. The AD834 outputs are in the
G
V
IN
X2
Y1
8
1
using the labeling conven-
Y2
249
X1 +V
2
7
AD834
Figure 40. An 80 MHz Voltage-Controlled Amplifier
R3
U1
–V
6
3
*R8 = R9 = 511
S
S
G
= +1 V. The over-
W1
W2
4
5
or 75
= 1.27k
G
= 0 dB to
FOR X4 GAIN
to 2 V.
182
182
FOR X10 GAIN
R4
R5
–12–
The gain can be increased to 20 dB ( 10) by raising R8 and R9
to 1.27 k , 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 gain-control input voltage, V
tive ground-referenced voltage, or fully differential, depending
on the user’s choice of connections at Pins 7 and 8. A positive
value of V
ing the sign of V
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.
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.
294
294
R6
R7
G
results in an overall noninverting response. Revers-
G
R9*
R8*
simply causes the sign of the overall response
AD811
U3
0.1 F
0.1 F
C1
C2
G
, may be a positive or nega-
FB
FB
R
L
V
+12V
–12V
OUT
REV. D