T H AT
Corporation
Trimmable IC
Voltage Controlled Amplifiers
THAT
2181A, 2181B, 2181C
FEATURES
·
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·
Wide Dynamic Range: >120 dB
Wide Gain Range: >130 dB
Exponential (dB) Gain Control
Low Distortion:
~0.0025% (typical 2181A)
~0.005% (typical 2181C)
Wide Gain-Bandwidth: 20 MHz
Dual Gain-Control Ports (pos/neg)
Pin-Compatible with 2150-Series
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APPLICATIONS
Faders
Panners
Compressors
Expanders
Equalizers
Filters
Oscillators
Automation Systems
·
·
·
Description
THAT 2181 Series integrated-circuit voltage con-
trolled amplifiers (VCAs) are very high-performance
current-in/current-out devices with two oppos-
ing-polarity, voltage-sensitive control ports. They offer
wide-range exponential control of gain and attenuation
with low signal distortion. The parts are selected after
packaging based primarily on after-trim THD and con-
trol-voltage feedthrough performance.
The VCA design takes advantage of a fully comple-
mentary dielectric isolation process which offers
closely matched NPN/PNP pairs. This delivers perfor-
mance unobtainable through any conventional pro-
cess, integrated or discrete. The parts are available in
three grades, allowing the user to optimize cost vs.
performance. Both 8-pin single-in-line (SIP) and sur-
face mount (SO) packages are available.
Pin Name
Input
7
SIP Pin
1
2
3
4
5
6
7
8
SO Pin
1
2
3
4
5
6
7
8
Ec+
Ec–
Sym
2k
Vcc
BIAS CURRENT
COMPENSATION
2
Ec+
25
Vbe
MULTI-
PLIER
V–
Gnd
V+
Output
Ec-
3
8
Input
1
6
Output
4
Gnd
Iadj
Sym
Table 1. 2181 Series Pin Assignments
Max Trimmed THD Plastic
SIP
@1 V, 1 kHz, 0 dB
0.005%
0.008%
0.02%
Plastic
SO
5
V-
Iset
2181LA 2181SA
2181LB 2181SB
2181LC 2181SC
Figure 1. 2181 Series Equivalent Circuit Diagram
Table 2. Ordering Information
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Document 600030 Rev 01
Page 2
THAT2181 Series IC VCAs
SPECIFICATIONS
1
Absolute -Maximum Ratings (T
A
= 25°C)
Positive Supply Voltage (V
CC
)
Negative Supply Voltage (V
EE
)
Supply Current (I
CC
)
Max
DE
E
C+
- (E
C-
)
+20 V
-20 V
10 mA
±1V
Power Dissipation (P
D
) (T
A
= 75°C)
Operating Temperature Range (T
OP
)
Storage Temperature Range (T
ST
)
330 mW
0 to +70°C
-40 to +125°C
Recommended Operating Conditions
2181A
Parameter
Positive Supply Voltage
Bias Current
Signal Current
Symbol
V
CC
I
SET
I
IN
+I
OUT
V
CC
- V
EE
= 30 V
I
SET
= 2.4mA
Conditions
Min
+4
-4
1
—
Typ
+15
-15
2.4
0.35
Max
+18
-18
3.5
2.5
Min
+4
-4
1
—
2181B
Typ
+15
-15
2.4
0.35
Max
+18
-18
3.5
2.5
Min
+4
-4
1
—
2181C
Typ
+15
-15
2.4
0.35
Max
+18
-18
3.5
2.5
Units
V
V
mA
mA
Negative Supply Voltage V
EE
Electrical Characteristics²
2181A
Parameter
Supply Current
Symbol
I
CC
Conditions
No Signal
No Signal
No Signal
R
out
= 20 kW
0 dB gain
+15 dB gain
+30 dB gain
Gain Cell Idling Current I
IDLE
Gain-Control Constant
E
C+
/Gain (dB)
E
C-
/Gain (dB)
Gain-Control TempCo
DE
C
/
DT
CHIP
Gain-Control Linearity
1 kHz Off Isolation
Output Noise
e
n(OUT)
T
A
=25°C (T
CHIP
@35°C)
-60 dB < gain < +40 dB
Pin 2 (Fig. 15)
Pin 3
Ref T
CHIP
= 27°C
-60 to +40 dB gain
E
C+
= -360 mV, E
C-
= +360 mV
20 Hz ~ 20 kHz
R
out
= 20kW
0 dB gain
+15 dB gain
Voltage at V-
V
V-
No Signal
—
—
-98
-88
-97
-86
—
—
-98
-88
-96
-85
—
—
-98
-88
-95
-84
dBV
dBV
V
6.0
6.1
6.2
6.0
6.1
6.2
6.0
6.1
6.2
mV/dB
mV/dB
%/°C
%
dB
-6.2 -6.1 -6.0
— +0.33 —
—
110
0.5
115
2
—
-6.2 -6.1 -6.0
— +0.33 —
—
110
0.5
115
2
-6.2 -6.1 -6.0
— +0.33 —
—
110
0.5
115
2
—
—
—
—
—
0.5
1
3
20
1
3
12
—
—
—
—
—
1
1.5
5
20
2
4
15
—
—
—
—
—
1.5
3
9
20
3
10
30
—
mV
mV
mV
mA
Min
—
—
—
Typ
2.4
2
±5
Max
4
10
—
Min
—
—
—
2181B
Typ
2.4
2
±5
Max
4
12
—
Min
—
—
—
2181C
Typ
2.4
2
±5
Max
4
15
—
Units
mA
nA
mV
Equiv. Input Bias Current I
B
Input Offset Voltage
V
OFF(IN)
Output Offset Voltage V
OFF(OUT)
-3.1 -2.85 -2.6
-3.1 -2.85 -2.6
-3.2 -2.85 -2.6
1. All specifications subject to change without notice.
2. Unless otherwise noted, T
A
=25°C, V
C
= +15V, V
EE
= –15V. Test circuit is as shown in Figure 2. SYM ADJ is ad-
justed for minimum THD at 1 V, 1 kHz, Ec– = –Ec+ = 0 V
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
600030 Rev 01
Page 3
Electrical Characteristics (Cont'd.)
2181A
Parameter
Total Harmonic Distortion
Symbol
THD
Conditions
1 kHz
V
IN
= 0 dBV, 0 dB gain
V
IN
= +10 dBV, -15 dB gain
V
IN
= -5 dBV, +15 dB gain
V
IN
= +10 dBV, 0 dB gain
Slew Rate
Symmetry Control Voltage V
SYM
Gain at 0 V Control Voltage
R
in
= R
out
= 20 kW
A
V
= 0 dB, Minimum THD
E
C-
= 0 mV
— 0.0025 0.005
—
—
—
—
-0.5
-0.1
0.018 0.025
0.018 0.025
0.004 0.008
12
—
0.0
—
+0.5
+0.1
—
—
—
—
—
-1.5
0.004 0.008
0.025 0.035
0.025 0.035
0.006 0.010
12
—
—
+1.5
+0.15
—
—
—
—
—
-2.5
-0.2
0.005
0.035
0.035
.0015
12
—
0.0
0.02
0.07
0.07
—
—
+2.5
+0.2
%
%
%
%
V/ms
mV
dB
Min
Typ
Max
Min
2181B
Typ
Max
Min
2181C
Typ
Max
Units
-0.15 0.0
Vcc
2181
Series
VCA
IN
10u
20k
1
Ec-
7
V+
-IN
GND
V-
22p
20k
OUT
8
3
Ec-
SYM
Ec+
-
OP275
5
6
2
4
OUT
Vcc
+
Power Supplies
Vcc = +15 V
Vee = -15 V
5.1k
Rsym
680k (2181A)
220k (2181B)
130k (2181C)
50k SYM
ADJ
Vee
Vee
Figure 2. Typical Application Circuit
Figure 3. 2181 Series Frequency Response Vs. Gain
Figure 4. 2181 Series Noise (20kHz NBW) Vs. Gain
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Page 4
THAT2181 Series IC VCAs
Theory of Operation
3
The THAT 2181 Series VCAs are designed for high
performance in audio-frequency applications requiring
exponential gain control, low distortion, wide dynamic
range and low control-voltage feedthrough. These parts
control gain by converting an input current signal to a
bipolar logged voltage, adding a dc control voltage, and
re-converting the summed voltage back to a current
through a bipolar antilog circuit.
Figure 5 presents a considerably simplified internal
circuit diagram of the IC. The ac input signal current
flows in pin1, the input pin. An internal operational
transconductance amplifier (OTA) works to maintain
pin 1 at a virtual ground potential by driving the emitters
of Q1 and (through the Voltage Bias Generator) Q3.
Q3/D3 and Q1/D1 act to log the input current, producing
a voltage, V3, which represents the bipolar logarithm of
the input current. (The voltage at the junction of D1 and
D2 is the same as V3, but shifted by four forward V
be
drops.)
Figure 6. Gain vs. Control Voltage (E
C+
, Pin 2) at 25°C
Gain Control
Since pin 8, the output, is usually connected to a vir-
tual ground, Q2/D2 and Q4/D4 take the bipolar antilog
of V3, creating an output current which is a precise rep-
lica of the input current. If pin 2 (Ec+) and pin 3 (Ec-)
are held at ground (with pin 4 - SYM - connected to a
high impedance current source), the output current will
equal the input current. For pin 2 positive or pin 3 nega-
tive, the output current will be scaled larger than the in-
put current. For pin 2 negative or pin 3 positive, the
output current is scaled smaller than the input.
The scale factor between the output and input cur-
rents is the gain of the VCA. Either pin 2 (Ec+) or pin 3
(Ec-), or both, may be used to control gain. Gain is expo-
Figure 7. Gain vs. Control Voltage (Ec-, Pin 3) at 25°C
+
D1
D2
Figure 8. Gain vs. Control Voltage (Ec-) with Temp (°C)
3
2
Q1
Voltage
Bias
Generator
Q2
Ec+
1
8
Ec-
OUT
Q4
4
IN
I
IN
25
D3
Q3
SYM
D4
nentially proportional to the voltage at pin 2, and expo-
nentially proportional to the negative of the voltage at
pin 3. Therefore, pin 2 (Ec+) is the
positive
control port,
while pin 3 (Ec-) is the
negative
control port. Because of
the exponential characteristic, the control voltage sets
gain
linearly
in
decibels.
Figure 6 shows the decibel cur-
rent gain of a 2181 versus the voltage at Ec+, while Fig-
ure 7 shows gain versus the Ec-.
Temperature Effects
V
3
5
Icell
Iadj
V-
Figure 5. Simplified Internal Circuit Diagram
The logging and antilogging in the VCA depends on
the logarithmic relationship between voltage and current
in a semiconductor junction (in particular, between a
transistor's V
be
and I
c
). As is well known, this relation-
3. For more details about the internal workings of the 2181 Series of VCAs, see
An Improved Monolithic Volt-
age-Controlled Amplifier,
by Gary K. Hebert (Vice-President, Engineering, for THAT Corporation), presented at the 99th
convention of the Audio Engineering Society, New York, Preprint number 4055.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
600030 Rev 01
Page 5
ship is temperature dependent. Therefore, the gain of
any log-antilog VCA depends on its temperature.
Figure 8 shows the effect of temperature on the nega-
tive control port. (The positive control port behaves in the
same manner.) Note that the gain at Ec = 0 V is 0 dB, re-
gardless of temperature. Changing temperature changes
the
scale factor
of the gain by 0.33%/°C, which pivots the
curve about the 0 dB point.
Mathematically, the 2181's gain characteristic is
Gain
=
E
C
+
-
E
C
-
,
(0.0061)(1
+
0.0033
D
T)
Eq. 1
larger value resistor to form a voltage divider connected
to the wiper of a trim pot across the supply rails.
This trim should be adjusted for minimum harmonic
distortion. This is usually done by applying a mid-
dle-level, middle-frequency signal (e.g. 1 kHz at 1 V) to
the audio input, setting the VCA to 0 dB gain, and adjust-
ing the SYM trim while observing THD at the output. In
the 2181, this adjustment coincides closely with the set-
ting
which
produces
minimum
control-voltage
feedthrough, though the two settings are not always iden-
tical.
where
DT
is the difference between room temperature
(25°C) and the actual temperature, and Gain is the
gain in decibels. At room temperature, this reduces to
Gain
=
E
C
+
-
E
C
-
,
0.0061
Eq. 2
DC Feedthrough
Normally, a small dc error term flows in pin 8 (the
output). When the gain is changed, the dc term changes.
This control-voltage feedthrough is more pronounced
with gain; the –A version of the part produces the least
feedthrough, the –C version the most. See Figure 9 for
typical curves for dc offset vs. gain
If only the positive control port is used, this becomes
Gain
=
E
C
+
,
0.0061
Eq. 3
If only the negative control port is used, this becomes
Gain
=
-
E
C
-
,
0.0061
Eq. 4
DC Bias Currents
The 2181 current consumption is determined by the
resistor between pin 5 (V-) and the negative supply voltage
(V
EE
). Typically, with 15V supplies, the resistor is 5.1 kW,
which provides approximately 2.4 mA. This current is
split into two paths: 570
mA
is used for biasing the IC,
and the remainder becomes Icell as shown in Figure 5.
Icell is further split in two parts: about 20
mA
biases the
core transistors (Q1 through Q4), the rest is available for
input and output signal current.
Figure 10. 1 kHz THD+Noise Vs. Input Level, 0 dB Gain
Trimming
The 2181-Series VCAs are intended to be adjusted for
minimum distortion by applying a small variable offset
voltage to pin 4, the SYM pin. Note that there is a 25
W
re-
sistor internal to the 2181 between pin 4 and pin 2. As
shown in Figure 2, Page 3, the usual method of applying
this offset is to use the internal 25
W
resistor along with a
Figure 11. 1 kHz THD+Noise Vs. Input Level, +15 dB
Gain
Figure 9. Representative DC Offset Vs. Gain
Figure 12. 1 kHz THD+Noise Vs. Input Level, -15 dB
Gain
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
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