port, with an opposite sense of control from that at

pin 3. To use this port, both pins must be driven with

the control voltage, while a small differential voltage is

accommodated between the two pins. (Figure 14,

Page 9, shows the typical connection.) Either pin 3, or

pins 2 and 4, or both ports together may be used for

gain control. Mathematically, this relationship is as fol-

lows:

= 10

EC

+−EC−

0.122

Gain

=

0.0061

, where Gain is the gain in decibels.

Control Port Source Impedance

The control ports (pins 2 through 4) are connected

directly to the bases of the logging and/or antilogging

transistors. As was implied in the earlier discussion on

Logging and Antilogging (Page 4) the accuracy of the

The base current in the transistors will follow the col-

lector currents, of course. Since the collector currents

are signal-related, the base currents will also be signal-

related. Should the source impedance of the control

voltage(s) be large, the signal-related base currents will

cause signal-related voltages to appear at the control

ports, which will interfere with precise logging and

antilogging, in turn causing distortion.

The 2150 Series VCAs are designed to be operated

with zero source impedance at pins 2 and 3, and a 50

Ω

source impedance at pin 4. (Pin 4 is intended for con-

nection to the symmetry control, hence the higher de-

sign-center source impedance.) One can estimate the

distortion caused by a specific, non-zero source imped-

ance by determining the base voltage modulation due

to signal current based on a core-transistor

β of ap-

proximately 300 (NPN) or 100 (PNP), and converting the

resulting decibel gain modulation to a percentage.

Even 100

Ω can spoil the good performance of these

parts at high signal levels.

DC Input Signals

Any dc currents in the feedback loop of the internal

op amp will show up as dc terms in the output signal,

and will be modulated by gain commands. Input bias

currents will cause a dc current to flow in the feedback

loop provided by the input side of the core. For this

reason, input bias currents in the internal op amp

must be kept very low. The bias current compensation

at the input stage provides excellent cancellation of the

bias current required by the input differential ampli-

fier. Of course, this good performance can be negated

by a dc current supplied from outside the VCA. To pre-

vent such dc terms, ac input coupling is strongly rec-

ommended. A plot of typical output offset voltage ver-

sus gain for the circuit of Figure 3 is shown in

Figure 9. (The LF351’s offset was adjusted to 0 V for

this plot.)

Current Programming

The size of the current source at the bottom of the

core (Figure 6, Page 4) is programmed externally via

divides into two portions: approximately 400

µA is used

for internal biasing, and the rest is available for the

therefore be 400

µA larger than the total of the peak

input and output signal currents.

Note that the output impedance of the internal op-

amp is approximately 2 k

Ω, and under peak demands,

must be supplied through this impedance, lowering the

voltage available to drive the core. For more informa-

tion, see the Power Supplies section on Page 8.

Headroom

Maximum signal currents are also limited by the

logarithmic characteristics of the core transistors. In

the 2150 Series, these devices are specially con-

structed to conform to an ideal log-linear curve over a

wide range of currents, but they reach their limit at ap-

proximately 1 mA. The symptom of failing log confor-

mance is increasing distortion with increasing current

levels. The onset of distortion is gradual at low current

levels, and then more rapid as current becomes high.

Figures 10 through 12 show distortion versus signal

level for the three parts in the 2150 Series for -15 dB,

0 dB, and +15 dB gain. The acceptable distortion will

determine the maximum signal level for a particular

design.

Figure 9. DC Offset Vs. Gain, After Symmetry

Adjustment

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

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

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2150 Series IC VCAs