T H A T

C o r p o r a t i o n

COMPENSATION

BIAS CURRENT

PLIER

MULTI-

Vbe

Figure 1. 2150 Series Equivalent Circuit Diagram

TYP.

ITEM

MILLIMETERS

INCHES

20.32 MAX.

1.1 MIN.

0.25

2.54

1.27 MAX.

0.8 MAX.

0.043 MIN.

0.02

0.01

0.1

0.05 MAX.

0.02 MIN.

0.2 MAX.

0.11

.008

0.227 MAX.

0.058 MAX.

0.01

.004

.002

0.126

.02

0.043 MIN.

0.51 MIN.

5.08 MAX.

2.8

5.75 MAX.

0.25

+.10

3.2

1.1 MIN.

1.5 MAX.

.04

.004

0.5

_ .1

PIN 1

THAT

MODEL NO.

Figure 2. 2150 Series Physical Outline

IC Voltage-Controlled

Amplifiers

THAT

2151, 2150A, 2155

FEATURES

Wide Dynamic Range: >116 dB

Wide Gain Range: >130 dB

Exponential (dB) Gain Control

Low Distortion: (0.008% @ 0 dB
gain, 0.035% @15dB gain)

Wide Gain-Bandwidth: 6 MHz

Low Cost: $2.20 in '000s (2155)

Single In-Line Package

Dual Gain-Control Ports (pos/neg)

APPLICATIONS

Faders

Panners

Compressors

Expanders

Equalizers

Filters

Oscillators

Automation Systems

Description

The THAT 2150 Series integrated-circuit voltage-

controlled amplifiers (VCAs) are high-performance

current-in/current-out devices with two opposing-

polarity, voltage-sensitive control ports. Based on

dbx

technology, they offer wide-range exponential

control of gain and attenuation with low signal dis-

tortion. The parts are housed in a space-efficient,

plastic 8-pin single-in-line (SIP) package, and re-

quire minimal support circuitry. Fabricated in a

super low-noise process utilizing high h

, comple-

mentary NPN/PNP pairs, the 2150 Series VCAs

combine high gain-bandwidth product with low

noise, low distortion, and low offset to offer discrete

performance at IC prices. They are available in

three grades, selected for distortion, allowing the

user to optimize cost vs. performance.

dbx

is a registered trademark of Carillon Electronics Corporation

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

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

SPECIFICATIONS

Absolute-Maximum Ratings (T

= 25°C)

Positive Supply Voltage (V

)

+18 V

Negative Supply Voltage (V

)

-18 V

Supply Current (I

)

10 mA

Power Dissipation (P

) (T

= 75°C)

330 mW

Operating Temperature Range (T

)

-20 to +75°C

Storage Temperature Range (T

)

-40 to +125°C

Recommended Operating Conditions

2151

2150A

2155

Parameter

Symbol

Conditions

Min

Typ

Max

Min

Typ

Max

Min

Typ

Max

Units

Positive Supply Voltage

+12

+15

+12

+15

+12

+15

Negative Supply Voltage

-5

-12

-15

-5

-12

-15

-5

-12

-15

Bias Current

SET

-V

= 24 V

2.4

Signal Current

OUT

SET

= 2.4 mA

175

750

175

750

125

550

Arms

Electrical Characteristics

2151

2150A

2155

Parameter

Symbol

Conditions

Min

Typ

Max

Min

Typ

Max

Min

Typ

Max

Units

Supply Current

No Signal

2.4

Equiv. Input Bias Current

No Signal

Input Offset Voltage

OFF(IN)

No Signal

+10

Output Offset Voltage

OFF(OUT)

out

=20 k

0 dB gain

+15 dB gain

+40 dB gain

Gain Cell Idling Current

IDLE

Gain-Control Constant

=25°C (T

CHIP

35°C)

-60 dB < gain < +40 dB

C+

/Gain (dB)

Pins 2 & 4 (Fig. 14)

6.0

6.1

6.2

6.0

6.1

6.2

6.0

6.1

6.2

mV/dB

C-

/Gain (dB)

Pin 3

-6.0

-6.1

-6.2

-6.0

-6.1

-6.2

-6.0

-6.1

-6.2

mV/dB

Gain-control TempCo

CHIP

Ref T

CHIP

= 27°C

+0.33

%/°C

Gain-Control Linearity

-60 to +40 dB gain

0.5

Off Isolation (Fig. 14)

C+

=-360mV, E

C-

=+360mV

110

115

110

115

110

115

Output Noise

n(OUT)

20 Hz-20 kHz

out

= 20k

0 dB gain

-98

-97

-98

-96

-98

-96

dBV

+15 dB gain

-88

-86

-88

-86

-88

-86

dBV

1. All specifications subject to change without notice.

2. Unless otherwise noted, TA=25°C, VCC = +15V, VEE= -15V. Test circuit is as shown in Figure 3. SYMADJ is ad-
justed for minimum THD @ Vin=1 V, 1 kHz, 0 dB gain.

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

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

Page 2

2150 Series IC VCAs

Electrical Characteristics (Cont'd.)

2151

2150A

2155

Parameter

Symbol

Conditions

Min

Typ

Max

Min

Typ

Max

Min

Typ

Max

Units

Total Harmonic Distortion

THD

+ I

OUT

= 180

A, 1 kHz

0 dB gain

0.004 0.02

0.005 0.03

15 dB gain

0.025 0.045

0.05

0.07

+ I

OUT

= 150

A, 1 kHz

0 dB gain

0.006 0.03

15 dB gain

0.05

0.07

Symmetry Control Voltage

SYM

= 0 dB, THD < 0.07%

-1.6

+1.6

-2

-2.5

+2.5

Gain at 0 V Control Voltage

= 0 mV

-0.1

0.0

+0.1

-0.15

0.0

+0.15

-0.2

0.0

+0.2

Ec+

Ec-

OUT

-IN

V-

GND

V+

Ec-

10u

LF351

47p

20k

50k

ADJ

SYM

150k

5.1k

20k

+15V

-15V

+15V

OUTPUT

Rsym

INPUT

300k (2155)

390k (2150A)

470k (2151)

Figure 3. Typical Application Circuit

Figure 4. Frequency Response Vs. Gain (2150A)

2150

Series

VCA

Figure 5. Noise (20kHz NBW) Vs. Gain (2150A)

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

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

Rev. 10/25/96

Page 3

Theory of Operation

The THAT 2150 Series VCAs are designed for high

performance in audio-frequency applications requiring

exponential gain control, low distortion, wide dynamic

range and low dc bias modulation. These parts control

gain by converting an input current signal to a bipolar

logged voltage, adding a dc control voltage, and re-con-

verting the summed voltage back to a current through

a bipolar antilog circuit.

Figure 6 presents a considerably simplified internal

circuit diagram of the IC. The ac input signal current

flows in pin 1, the input pin. The internal op amp

works to maintain pin 1 at a virtual ground potential

by driving the emitters of Q1 and (through the Voltage

Bias Generator) Q3. For positive input currents (I

de-

fined as flowing into pin 1), the op amp drives the emit-

ter of Q1 negative, turning off its collector current,

while simultaneously driving the emitter of Q3 nega-

tive, turning it on. The input signal current, therefore,

is forced to flow through Q3 and D3.

Logging & Antilogging

Because the voltage across a base-emitter junction

is logarithmic with collector current, the voltage from

the base of Q3 to the cathode of D3 is proportional to

the log of the positive input current. The voltage at the

cathodes of D3 and D4 is therefore proportional to the

log of the positive input currents plus the voltage at

pin 3, the negative control port. Mathematically,

where V

is the voltage at the junction of D3 and D4;

is the thermal voltage,

; I

is the collector cur-

rent of Q3; and I

is the reverse-saturation current of

Q3. It is assumed that D3 matches Q3 (and will be as-

sumed that they match Q4 and D4, as well).

In typical applications (see Figure 3, Page 3), pin 4

is connected to a voltage source at ground or nearly

ground potential. Pin 8 is connected to a virtual

ground (usually the inverting input of an op amp with

negative feedback around it). With pin 4 near ground,

and pin 8 at virtual ground, the voltage at the cathodes

of D3 and D4 will cause an exponentially-related cur-

rent to flow in D4 and Q4, and out via pin 8. A similar

equation governs this behavior:

Exponential Gain Control

The similarity between the two preceeding equations

begs further exploration. Accordingly:

Rearranging terms,

If pin 3 and pin 4 are at ground potential, the cur-

rent in Q4/D4 will precisely mirror that in Q3/D3.

When pin 3 is positive with respect to pin 4, the voltage

across the base-emitter junction of Q3 is higher than

that across the base-emitter junction of Q4, so the

Q4/D4 current remains proportional to, but less than,

the current in Q3/D3. In the same manner, a negative

voltage at pin 3 with respect to pin 4 causes the

Q4/D4 current to be proportional to, but greater than

that in Q3/D3.

The ratio of currents is exponential with the differ-

ence in the voltages E

C+

and E

, providing convenient

"deci-linear" control. Mathematically, this is:

, where A

is the current gain.

For pin 4 at or very near ground, at room tempera-

ture (25°C), allowing for a 10°C internal temperature

rise, and converting to a base of 10 for the exponential,

this reduces to:

(SYM)

V-

Ec+

OUT

Ec-

Generator

Bias

Voltage

Iin

Figure 6. Simplified Internal Circuit Diagram

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Page 4

2150 Series IC VCAs

0.122

When pin 3 is at O V, the current ratio is unity.

When pin 3 is at +122 mV, the output current (Q4) is

10 times (20 dB) less than the input current. At

122 mV, the output current is 10 times (20 dB)

greater than the input current. Another way of ex-

pressing this relationship is:

Gain

0.0061

, where Gain is the gain in decibels.

Negative Input Currents

For negative input currents, Q1/D1 operate with

Q2/D2 to mirror the lower-half-core behavior. Pin 2 is

normally at or very near ground (see the section below

on Symmetry Adjustment for more detail), so the same

gain scaling applied to the base of Q3 is applied to the

base of Q2. The polarity (positive/negative, in dB) of

the gain is the same for the top pair versus the bottom

pair of the four "core" transistors because their sexes

(NPN/PNP) are inverted in the top versus the bottom,

while the bases are cross-connected between the input

(left) half and the output (right) half of each pair.

The resulting control over gain is extremely consis-

tent from unit to unit, since it derives from the physics

of semiconductors. Figure 7 shows actual data from a

typical 2150 Series VCA, taken at 25°C.

Core Bias Currents

A quiescent bias current in the core transistors is

established by the Voltage Bias Generator shown in

Figure 6. This current acts like crossover bias in the

output stage of a complementary class AB power am-

plifier, smoothing the transition between turning on

the top (PNP) pair and the bottom (NPN) pair of transis-

tors in the core. This lowers distortion greatly at some

cost to noise performance, as the current noise of the

core transistors (which run at approximately 20

A) is

the dominant noise source in the 2150 Series VCAs.

Transistor Matching

The bias current flows downwards in the core (from

Q1 to Q3, and from Q2 to Q4) so long as there is good

matching between all four compound transistors (tran-

sistors plus diodes). Mismatches will cause a dc output

current to flow in pin 8, which will ultimately manifest

itself as a dc offset voltage. Static offsets are of little

consequence in most audio applications, but any mis-

match-caused dc output current will be modulated by

gain commands, and may become audible as "thumps"

if large, fast gain changes are commanded in the pres-

ence of significant mismatches.

Transistor matching also affects distortion. If the

top half of the gain cell is perfectly matched, while the

bottom half is slightly off, then the gain commanded by

the voltage at pin 3 will affect the two halves of the core

differently. Since positive and negative halves of ac

input signals are handled by separate parts of the core,

this gives rise to even-order distortion products.

Symmetry Adjustment

The monolithic construction of the devices assures

relatively good matching between the paired transis-

tors, but even small V

mismatches can cause unac-

ceptable asymmetries in the output. For this reason,

the bases of Q1 and Q4 are brought out separately to

pin 2 and pin 4, respectively. This allows a small static

voltage differential to be applied to the two bases. The

applied voltage must be set to equal the sum of the V

mismatches around the core (which varies from sample

to sample). Figure 3 (Page 3) includes a typical circuit

to apply this symmetry voltage. R

SYM

controls primarily

even-order harmonic distortion, and is usually ad-

justed for minimum THD at the output. Figure 8 plots

THD vs. the voltage between pins 2 and 4 (the two E

C+

ports) for various gain settings of a typical part.

Opposite Polarity Control

As may be seen from the mathematics, the bases of

Q1 and Q4 can also be used as an additional control

Figure 7. Gain Versus Control Voltage (Pin 3) at 25°C

Figure 8. Typical THD Versus Symmetry Voltage

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

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

Rev. 10/25/96

Page 5

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:

0.122

, where A

is the gain in volts/volt, or

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

logging and antilogging is dependent on the E

C+

and

C-

voltages being exactly as desired to control gain.

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

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

of ap-

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

resulting decibel gain modulation to a percentage.

Even 100

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

SET

, which is normally determined by a resistor from

pin 5 to V. The voltage at pin 5 is typically 2.7 V. I

SET

divides into two portions: approximately 400

A is used

for internal biasing, and the rest is available for the

current source at the bottom of the core. I

SET

should

therefore be 400

A 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

, and under peak demands,

the sum of the input and output currents plus I

SET

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

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Page 6

2150 Series IC VCAs

Applications

Input

As mentioned above, input and output signals are

currents, not voltages. While this often causes some

conceptual difficulty for designers first exposed to this

convention, the current input/output mode provides

great flexibility in application.

The input pin (pin 1) is a virtual ground with nega-

tive feedback provided internally (see Figure 6, Page 4).

The input resistor (shown as 20 k

in Figure 3, Page 3)

should be scaled to convert the available ac input volt-

age to a current within the linear range of the device.

(Peak input currents should be kept under 1 mA for

best distortion performance.) An additional consider-

ation is stability: the internal op amp is intended for

operation with source impedances of less than 30 k

at high frequencies. For most audio applications, this

will present no problem.

The choice of input resistor has an additional, sub-

tle effect on distortion. Since the feedback impedances

around the internal opamp (essentially Q1/D1 and

Q3/D3) are fixed, low values for the input resistor will

require more closed-loop gain from the opamp. Since

the open-loop gain naturally falls off at high frequen-

cies, asking for too much gain will lead to increased

high-frequency distortion. For best results, this resis-

tor should be kept to 10 k

or above. Distortion vs. fre-

quency for a 1 V signal at 0 dB gain with a 20 k

input

resistor is plotted in Figure 13.

The quiescent dc voltage level at the input is ap-

proximately +10 mV. As mentioned above, any dc input

currents will cause dc signals in the output which will

be modulated by gain, causing audible thump. There-

Figure 10. 1kHz THD+Noise Vs. Input, -15 dB Gain

Figure 11. 1kHz THD+Noise Vs. Level, 0 dB Gain

Figure 12. 1kHz THD+Noise Vs. Input, +15 dB Gain

Figure 13. THD Vs. Frequency, 0 dB Gain

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Rev. 10/25/96

Page 7

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-

rent conversion at the input, the familiar ratio of R

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

f) 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

output currents plus I

SET

, which must be supplied

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

. 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

A 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.

As implied by the equation for A

(at the foot of

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

(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

)

(

0.0033

)

where E

is in volts, and

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

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Page 8

2150 Series IC VCAs

symmetry voltage (<

2.5 mV) to correct for V

mis-

matches within the VCA IC. For this purpose, the 2150

series devices were designed for optimum performance

with an impedance of approximately 50

at pin 4. A

trim pot is used to adjust the voltage between pin 4

and pin 2 as shown in Figure 3, Page 3. For supply

voltages other than shown, scale R

SYM

to provide the

required adjustment range.

It is also possible to use pin 2 and pin 4 together as

an opposite-sense voltage control port. A typical circuit

using this approach is shown in Figure 14. Pin 3 may

be grounded, and pin 2 driven against the symmetry-

adjustment voltage. The change in voltage at pin 4

does have a small effect on the symmetry voltage, but

this is of little practical consequence in most applica-

tions. Using the opposite sense of control can some-

times save an inverter in the control path.

It is also possible (and advantageous) to combine

both control ports with differential drive (see Fig-

ure 15). While the driving circuitry is more complex,

this configuration offers better performance at high

attentuation levels (<-90 dB) where the single-control-

port circuits begin to saturate Q1 (for E

drive) or Q3

(for E

C+

drive). When either of these transistors satu-

rates, the internal opamp will accomodate the change

in current demand by responding with a small change

in its input offset voltage. This leads to an accumula-

tion of charge on the input capacitor, which in turn

can cause thump when the high attenuation is sud-

denly removed (e.g., when a muted channel is opened).

Differential control drive avoids the large dc levels oth-

erwise required to command high attenuation

(+610 mV for -100 dB gain at pin 3 alone, vs.

305 mV

when using both pin 3 and pins 2 and 4).

Control Port Drive Impedance

It has already been noted that the control port

should be driven by a low source impedance for mini-

mum distortion. This often suggests driving the control

port directly with an opamp (see below under Noise

Considerations). However, the closed-loop output im-

pedance of an opamp typically rises at high frequencies

due to falling loop gain. The output impedance is

therefore inductive at high frequencies. Excessive in-

ductance in the control port source impedance can

cause the VCA to oscillate internally. In such cases, a

resistor in series with a 1.5 nf capacitor from the

control port to ground will usually suffice to prevent

the instability.

Noise Considerations

It is second nature among good audio designers to

consider the effects of noisy devices on the signal path.

As is well known, this includes not only active devices

such as op amps and transistors, but extends to the

choice of impedance levels as well. High value resistors

have inherent thermal noise associated with them, and

the noise performance of an otherwise quiet circuit can

be easily spoiled by the wrong choice of impedance lev-

els.

Less well known, however, is the effect of noisy cir-

cuitry and high impedance levels in the control path of

Ec+

Ec-

OUT

-IN

V-

GND

V+

240k

VCA

Series

2150

Rsym

Ec+

10u

LF351

47p

20k

50k

ADJ

SYM

5.1k

20k

+15V

-15V

+15V

OUTPUT

INPUT

2150

Series

VCA

300k (2155)

390k (2150A)

470k (2151)

Figure 14. Positive Control Port Using Pins 2 and 4

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

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

Rev. 10/25/96

Page 9

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

A 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-

OUT

-IN

V-

GND

V+

VCA

Series

2150

Rsym

Ec+

10u

LF351

47p

20k

50k

ADJ

SYM

150k

5.1k

20k

+15V

-15V

+15V

OUTPUT

INPUT

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

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ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: THAT2150A