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AD827AQ

High Speed, Low Power Dual Op Amp

Analog Devices 

AD827

A TWO-CHIP VOLTAGE-CONTROLLED AMPLIFIER

(VCA) WITH EXPONENTIAL RESPONSE

Voltage-controlled amplifiers are often used as building blocks

in automatic gain control systems. Figure 25 shows a two-chip

VCA built using the AD827 and the AD539, a dual, current-

output multiplier. As configured, the circuit has its two

Figure 25. A Wide Range Voltage-Controlled

Amplifier Circuit

multipliers connected in series. They could also be placed in

parallel with an increase in bandwidth and a reduction in gain.

The gain of the circuit is controlled by VX, which can range

from 0 to 3 V dc. Measurements show that this circuit easily

supplies 2 V p-p into a 100 Ω load while operating from ± 5 V

supplies. The overall bandwidth of the circuit is approximately

7 MHz with 0.5 dB of peaking.

Each half of the AD827 serves as an I/V converter and converts

the output current of one of the two multipliers in the AD539

into an output voltage. Each of the AD539’s two multipliers

contains two internal 6 kΩ feedback resistors; one is connected

between the CH1 output and Z1, the other between the CH1

output and W1. Likewise, in the CH2 multiplier, one of the

feedback resistors is connected between CH2 and Z2 and the

other is connected between CH2 and Z2. In Figure 25, Z1 and

W1 are tied together, as are Z2 and W2, providing a 3 kΩ

feedback resistor for the op amp. The 2 pF capacitors connected

between the AD539’s W1 and CH1 and W2 and CH2 pins are

in parallel with the feedback resistors and thus reduce peaking

in the VCA’s frequency response. Increasing the values of C3

and C4 can further reduce the peaking at the expense of

reduced bandwidth. The 1.25 mA full-scale output current of

the AD539 and the 3 kΩ feedback resistor set the full-scale

output voltage of each multiplier at 3.25 V p-p.

Current limiting in the AD827 (typically 30 mA) limits the out-

put voltage in this application to about 3 V p-p across a 100 Ω

load. Driving a 50 Ω reverse-terminated load divides this value

by two, limiting the maximum signal delivered to a 50 Ω load to

about 1.5 V p-p, which suffices for video signal levels. The

dynamic range of this circuit is approximately 55 dB and is

primarily limited by feedthrough at low input levels and by the

maximum output voltage at high levels.

Guidelines for Grounding and Bypassing

When designing practical high frequency circuits using the AD827,

some special precautions are in order. Both short interconnection

leads and a large ground plane are needed whenever possible to

provide low resistance, low inductance circuit paths. One should

remember to minimize the effects of capacitive coupling

between circuits. Furthermore, IC sockets should be avoided.

Feedback resistors should be of a low enough value that the

time constant formed with stray circuit capacitances at the

amplifier summing junction will not limit circuit performance.

As a rule of thumb, use feedback resistor values that are less

than 5 kΩ. If a larger resistor value is necessary, a small (<10 pF)

feedback capacitor in parallel with the feedback resistor may be

used. The use of 0.1 µF ceramic disc capacitors is recommended

for bypassing the op amp’s power supply leads.

–8–

REV. C

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