fore, capacitive coupling is almost mandatory for qual-

ity audio applications. Choose a capacitor which will

give acceptable low frequency performance for the ap-

plication.

Multiple signals may be summed by multiple resis-

tors, just as with an inverting op amp configuration. In

such a case, a single coupling capacitor may be located

next to pin 1 rather than multiple capacitors at the

driven ends of the summing resistors. However, take

care that the capacitor does not act as an antenna for

stray signals.

Output

The output pin (pin 8) is intended to be connected to

a virtual ground node, so that current flowing in it may

be converted to a voltage (see Figures 3, 14, & 15).

Choose the external op amp for good audio perfor-

mance. The feedback resistor should be chosen based

on the desired current-to-voltage conversion constant.

Since the input resistor determines the voltage-to-cur-

for an inverting op amp will determine the overall volt-

age gain when the VCA IC is set for 0 dB current gain.

Since the VCA performs best at settings near unity

gain, use the input and feedback resistors to provide

design-center gain or loss, if necessary.

A small feedback capacitor around the output op

amp is necessary to cancel the output capacitance of

the VCA. Without it, this capacitance will destabilize

most op amps. The capacitance at pin 8 is typically

30 pf.

Power Supplies

The positive supply is connected directly to pin 7.

No special bypassing is necessary, but it is good prac-

tice to include a small (~1

µf) electrolytic close to the

VCA IC on the PCB. Performance is not particularly de-

pendent on supply voltage. The lowest permissible sup-

ply voltage is determined by the sum of the input and

through the resistor at the top of the core transistors

(see Figure 1) while still allowing enough voltage swing

to bias the internal op amp and the core transistors

themselves. This resistor is approximately 2 k

Ω. Re-

ducing signal currents may help accommodate low

supply voltages.

The highest permissible supply voltage is fixed by

the process characteristics and internal power con-

sumption. +15 V is the nominal limit.

The negative supply terminal is intended to be con-

nected to a resistive current source, which determines

the current available for the core. As mentioned before,

this source must supply the sum of the input and out-

put signal currents, plus the bias to run the rest of the

IC. The minimum value for this current is 430

µA over

the sum of the required signal currents. 2.4 mA is rec-

ommended for most pro audio applications where

+15 V supplies are common and headroom is import-

ant.

Higher bias levels are of limited value, partly be-

cause the resistor mentioned in the positive supply dis-

cussion must supply all the current devoted to the

core, and partly because the core transistors become

ineffective at logging and antilogging at currents over

1 mA.

Since pin 5 is intended as a current supply, not a

voltage supply, bypassing at pin 5 is not necessary.

Pin 6 is used as a ground reference for the VCA. The

non-inverting input of the internal op amp is con-

nected here, as are various portions of the internal bias

network. It may not be used as an additional input pin.

Voltage Control

The primary voltage-control pin is pin 3. This point

controls gain inversely with applied voltage: positive

voltage causes loss, negative voltage causes gain. As

described on Page 6, the current gain of the VCA is

unity when pin 3 is at 0 V with respect to pins 2 and 4,

and varies with voltage at approximately -6.1 mV/dB,

at room temperature.

Page 4), the gain is sensitive to temperature, in propor-

tion to the amount of gain or loss commanded. The

constant of proportionality is 0.33% of the decibel gain

commanded, per degree Celsius, referenced to 27

°C

(300

°K). This means that at 0 dB gain, there is no

change in gain with temperature. However, at -122 mV,

the gain will be +20 dB at room temperature, but will

be 20.66 dB at a temperature 10˚C lower. The formula

is:

Gain

=

(0.0061) (1+0.0033) ∆T

,

∆T is the difference between

the actual temperature and room temperature (25˚C).

For most audio applications, this change with tem-

perature is of little consequence. However, if necessary,

it may be compensated by a resistor which varies its

value by .33%/˚C. Such parts are available from RCD

Components, Inc, 3301 Bedford St., Manchester, NH,

USA [(603) 669-0054], and KOA/Speer Electronics, PO

Box 547, Bradford, PA, 16701 USA [(814)362-5536].

When pin 3 is used for voltage control, pin 2 is con-

nected to ground and pin 4 is used to apply a small

THAT Corporation; 734 Forest Street; Marlborough, Massachusetts 01752; USA

Tel: (508) 229-2500; Fax: (508) 229-2590; Web: http://www.thatcorp.com

Page 8

2150 Series IC VCAs