Design Ideas: January 19, 1995

Low-cost audio VCA has high performance

Mike Sims,
Lectrosonics Inc, Rio Rancho, NM

The inherent matching between the two transconductance amplifiers in IC1 (Fig. 1) facilitates a voltage-controlled amplifier (VCA) that offers high performance and low cost. The circuit's maximum input voltage is +20 dBu (dBu=dB referred to 775 mV rms). THD measures less than 0.015%; noise, -70 dBu; and control-voltage feedthrough, -70 dB.

Matched transconductance amplifiers enable an inexpensive voltage-controlled amplifier whose gain is linearly proportional to the control voltage.

The signal path is simple. IC 1A's negative-feedback configuration forces the current from IC1A 's pin 5 to equal the ac-input current through R1 . IC1A 's pin 5 also connects to IC1B's pin 13, causing a virtually identical current to flow from IC1B 's pin 12. IC2A converts this current to an output voltage, VOUT .

The output op amp limits the circuit's maximum output voltage. (A dual FET-input op amp, such as the TL072, works well for IC2 .) C1 sets the circuit's 3-dB bandwidth at about 40 kHz.

Varying the control current into IC1B relative to fixed current flowing through IC1A 's R2 controls the circuit's gain. R2 sets the control current in IC1A to

ICONT A =(15-2VD )/R2,

which is approximately 265 µA. The 2VD factor arises because the control-port voltage is two diode drops above the negative rail.

IC2B and associated components form a linear voltage-to-current converter that feeds the control port of IC1B . The control current for IC1B is

ICONT B=VCONT /R3 .

The overall circuit gain is

VOUT/VIN,=-(R4/R1) × (I CONTB /ICONTA ).

A 5V control signal generates unity gain. The gain is linearly proportional to the control voltage.

R2 , in conjunction with R1 , limits the circuit's maximum input voltage to

V INMAX=2 × ICONTA×R1 ,

or a peak input voltage of around 16V.

Use R5 to trim the circuit for minimum THD. Trimming for minimum THD simultaneously achieves minimum control-voltage feedthrough. Interchanging pins 13 and 14 of IC1B realizes a noninverting VCA. (DI #1604)


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