voltage-control circuitry. The 2150 Series VCAs act like

double-balanced multipliers: when no signal is present

at the signal input, noise at the control input is re-

jected. So, when measuring noise (in the absence of

signal — as most everyone does), even very noisy con-

trol circuitry often goes unnoticed. However, noise at

the control port of these parts will cause noise modula-

tion of the signal. This can become significant if care

is not taken to drive the control ports with quiet sig-

nals.

The 2150 Series VCAs have a small amount of in-

herent noise modulation because of its class AB bias-

ing

scheme,

where

the

shot noise

in

the

core

transistors reaches a minimum with no signal, and in-

creases with the square root of the instantaneous sig-

nal current. However, in an optimum circuit, the noise

floor rises only to -94 dBV with a 50

µA signal at unity

gain — 4 dB of noise modulation. By contrast, if a

unity-gain connected, inverting 5534 opamp is used to

directly drive the control port, the noise floor will rise

to 92 dBV — 6 dB of noise modulation.

To avoid excessive noise, one must take care to use

quiet electronics throughout the control-voltage cir-

cuitry. One useful technique is to process control volt-

ages at a multiple of the eventual control constant

(e.g., 61 mV/dB — ten times higher than the VCA re-

quires), and then attenuate the control signal just be-

fore the final drive amplifier. With careful attention to

impedance levels, relatively noisy op amps may be

used for all but the final stage.

Closing Thoughts

The design and application of Voltage-Controlled

Amplifiers has traditionally been partly black art, in-

volving as much magic as science. We hope that the

foregoing discussion will help to de-mystify the subject.

THAT Corporation welcomes comments, questions

and suggestions regarding these devices, their design

and application. Please feel free to contact us with your

thoughts.

Ec+

Ec+

Ec-

OUT

-IN

V-

GND

V+

-

+

-

+

VCA

Series

2150

1k

1k

Rsym

Ec+

10u

LF351

47p

20k

50k

ADJ

SYM

150k

51

5.1k

20k

+15V

-15V

-15V

+15V

OUTPUT

INPUT

1

5

6

2

4

8

3

7

2150

Series

VCA

240k

300k (2155)

390k (2150A)

470k (2151)

Figure 15. Using Both Control Ports (Differential Drive)

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

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

Page 10

2150 Series IC VCAs