SSM2164

REV. 0

–8–

but higher noise, and the opposite is true for less current. The

increased noise is due to higher current noise in the gain core

transistors as their operating current is increased. THD has the

opposite relationship to collector current. The lower distortion

is due to the decrease in the gain core transistors’ emitter

impedance as their operating current increases.

This classical tradeoff between THD and noise in VCAs is

usually expressed as the choice of using a VCA in either Class A

or Class AB mode. Class AB operation refers to running a VCA

with less current in the gain core, resulting in lower noise but

higher distortion. More current in the core corresponds to

Class A performance with its lower THD but higher noise.

Figures 11 and 12 show the THD and noise performance of the

SSM2164 as the bias current is adjusted. Notice the two

characteristics have an inverse characteristic.

The quiescent current in the core is set by adding a single

resistor from the positive supply to the MODE pin. As the

simplified schematic shows, the potential at the MODE pin is

one diode drop above the ground pin. Thus, the formula for the

MODE current is:

I

(V

+) − 0.6V

R

B

With

±15 V supplies, an R

current of 1.9 mA. Leaving the MODE pin open sets the

SSM2164 in Class AB with 30

µA of current in the gain core.

Basic VCA Configuration

Figure 24 shows the basic application circuit for the SSM2164.

Each of the four channels is configured identically. A 30 k

Ω

resistor converts the input voltage to an input current for the

VCA. Additionally, a 500

Ω resistor in series with a 560 pF

capacitor must be added from each input to ground to ensure

stable operation. The output current pin should be maintained

at a virtual ground using an external amplifier. In this case the

OP482 quad JFET input amplifier is used. Its high slew rate,

wide bandwidth, and low power make it an excellent choice for

the current-to-voltage converter stage. A 30 k

Ω feedback

resistor is chosen to match the input resistor, giving unity gain

for a 0.0 V control voltage. The 100 pF capacitors ensure

stability and reduce high frequency noise. They can be

increased to reduce the low pass cutoff frequency for further

noise reduction.

For this example, the control voltage is developed using a

100 k

Ω potentiometer connected between +5 V and ground.

This configuration results in attenuation only. To produce both

gain and attenuation, the potentiometer should be connected

between a positive and negative voltage. The control input has

an impedance of 5 k

Ω. Because of this, any resistance in series

with V

If precise control of

the gain and attenuation is required, a buffered control voltage

should be used.

Notice that a capacitor is connected from the control input to

ground. Because the control port is connected directly to the

gain core transistors, any noise on the V

output noise of the VCA. Filtering the control voltage ensures

that a minimal amount of noise is introduced into the VCA,

allowing its full performance to be realized. In general, the

largest possible capacitor value should be used to set the filter at

a low cutoff frequency. The main exception to this is in

dynamic processing applications, where faster attack or decay

times may be needed.

Figure 24. Basic Quad VCA Configuration

Low Cost, Four-Channel Mixer

The four VCAs in a single package can be configured to create a

simple four-channel mixer as shown in Figure 25. The inputs

and control ports are configured the same as for the basic VCA,

but the outputs are summed into a single output amplifier. The

OP176 is an excellent amplifier for audio applications because

of its low noise and distortion and high output current drive.

The amount of signal from each input to the common output

can be independently controlled using up to 20 dB of gain or as

much as 100 dB of attenuation. Additional SSM2164s could be

added to increase the number of mixer channels by simply

summing their outputs into the same output amplifier. Another

possible configuration is to use a dual amplifier such as the

OP275 to create a stereo, two channel mixer with a single

SSM2164.

1µF

1µF

1µF

1µF

+5V

100k

30k

500

560pF

POWER SUPPLY

AND BIASING CIRCUITRY

VCA4

+5V

100k

30k

500

560pF

VCA3

+5V

100k

30k

500

560pF

VCA2

+5V

100k

30k

500

560pF

VCA1

98

16

1

V–

GND

V+

MODE

0.1µF

0.1µF

(OPEN CLASSAB)

+15V

–15V

1/4

OP482

1/4

OP482

1/4

OP482

1/4

OP482

30k

30k

30k

30k

100pF

100pF

100pF

100pF

3

2

6

7

4

5

13

12

11

10

14

15