REV. D

MAT04

–10–

VOLTAGE-CONTROLLED ATTENUATOR

The voltage-controlled attenuator (VCA) of Figure 9, widely

used in professional audio circles, can easily be implemented

using a MAT04. The excellent matching characteristics of the

MAT04 enables the VCA to have a distortion level of under

0.03% over a wide range of control voltages. The VCA accepts a

3 V RMS input and easily handles the full 20 Hz–20 kHz audio

bandwidth as shown in Figure 10. Noise level for the VCA is

more than 110 dB below maximum output.

In the voltage controlled attenuator, the input signal modulates

the stage current of each differential pair. Op amps A2 and A3

in conjunction with transistors Q5 and Q6 form voltage-to-current

converters that transform a single input voltage into differential

currents which form the stage currents of each differential pair.

The control voltage shifts the current between each side of the

two differential pairs, regulating the signal level reaching the

output stage which consists of op amp A1. Figure 11 shows the

increase in signal attenuation as the control voltage becomes

more negative.

The ideal transfer function for the voltage-controlled

attenuator is:

V

V

R

RR

kT

q

OUT

IN

CONTROL

/

exp (

)

=

+

+

























2

1

14

13

14

Where

k = Boltzman constant 1.38

T = temperature in

°K

q = electronic charge = 1.602

From the transfer function it can be seen that the maxi-

mum gain of the circuit is 2 (6 dB).

To ensure best performance, resistors R2 through R7 should

be 1% metal film resistors. Since capacitor C2 can see small

amounts of reverse bias when the control voltage is positive, it

may be prudent to use a nonpolarized tantalum capacitor.

Figure 9. Voltage-Controlled Attenuator