AD811

REV. D

–12–

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

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 V

all transfer function of the VCA is:

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.

The gain can be increased to 20 dB (

×10) by raising R8 and R9

to 1.27 k

Ω, 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.

tive ground-referenced voltage, or fully differential, depending

on the user’s choice of connections at Pins 7 and 8. A positive

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.

12

3

4

87

6

5

X2

W1

Y1

Y2

W2

U1

AD834

U3

AD811

R4

182

R5

182

R1 100

R2 100

R3

249

R6

294

R7

294

R9*

R8*

C1

0.1 F

FB

C2

0.1 F

–12V

FB

+12V

*R8 = R9 = 511

FOR X4 GAIN

= 1.27k

FOR X10 GAIN

+

–

Figure 40. An 80 MHz Voltage-Controlled Amplifier