Blackmer
®
Trimmable IC
Voltage Controlled Amplifiers
THAT
2181A, 2181B, 2181C
FEATURES
• 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
APPLICATIONS
• Faders
• Panners
• Compressors
• Expanders
• Equalizers
• Filters
• Oscillators
• Automation Systems
Description
THAT 2181 Series integrated-circuit voltage
controlled amplifiers (VCAs) are very high-
performance current-in/current-out devices with
two opposing-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 control-voltage
feedthrough performance.
The VCA design takes advantage of a fully
complementary dielectric isolation process which
offers closely matched NPN/PNP pairs. This deliv-
ers performance unobtainable through any
conventional process, 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 surface 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
2k
Vcc
Ec+
Ec-
BIAS CURRENT
COMPENSATION
2
Sym
V-
Gnd
V+
Output
Ec+
25
Vbe
MULTI-
PLIER
Ec-
3
8
Input
1
6
Output
4
Gnd
Iadj
Sym
Table 1. Pin Assignments
Max Trimmed
THD
@1V,1kHz,0dB
Plastic
SIP
Plastic
SO
5
V-
Iset
0.01%
0.02%
0.05%
2181AL08-U 2181AS08-U
2181BL08-U 2181BS08-U
2181CL08-U 2181CS08-U
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
Copyright © 2008, THAT Corporation
Document 600030 Rev 02
Document 600030 Rev 02
Page 2 of 12
THAT 2181 Series
Blackmer® Trimmable IC VCAs
SPECIFICATIONS
1
Absolute Maximum Ratings
2,3
Positive Supply Voltage (V
CC
)
Negative Supply Voltage (V
EE
)
Supply Current (I
CC
)
Maximum
∆E
C
E
C+
- (E
C-
)
+20 V
-20 V
10 mA
±1V
Power Dissipation (P
D
) (T
A
= 75 ºC)
Operating Temperature Range (T
OP
)
Storage Temperature Range (T
ST
)
330 mW
0 to +70 ºC
-40 to +125 ºC
Recommended Operating Conditions
Parameter
Positive Supply Voltage
Symbol
V
CC
Conditions
2181A
Min Typ Max
+4
-4
V
CC
- V
EE
= 30 V
I
SET
= 2.4 mA
1
—
+15
-15
2.4
0.35
+18
-18
3.5
2.5
2181B
Min Typ Max
+4
-4
1
—
+15
-15
2.4
0.35
+18
-18
3.5
2.5
2181C
Min Typ Max
+4
-4
1
—
+15
-15
2.4
0.35
+18
-18
3.5
2.5
Units
V
V
mA
mA
Negative Supply Voltage V
EE
Bias Current
Signal Current
I
SET
I
IN
+ I
OUT
Electrical Characteristics
2
2181A
Parameter
Supply Current
Equiv. Input Bias Current
Input Offset Voltage
Output Offset Voltage
Symbol
I
CC
I
B
V
OFF(IN)
V
OFF(OUT)
Conditions
No signal
No Signal
No Signal
R
out
= 20 kΩ
0 dB gain
+15 dB gain
+30 dB gain
Min
—
—
—
—
—
—
—
Typ
2.4
2
±5
0.5
1
3
20
Max
4
10
—
1
3
12
—
Min
—
—
—
—
—
—
—
2181B
Typ
2.4
2
±5
1
1.5
5
20
Max
4
12
—
2
4
15
—
Min
—
—
—
—
—
—
—
2181C
Typ
2.4
2
±5
1.5
3
9
20
Max
4
15
—
3
10
30
—
Units
mA
nA
mV
mV
mV
mV
µA
Gain Cell Idling Current
Gain-Control Constant
I
IDLE
T
A
=25°C (T
CHIP
≅35°C)
-60 dB < gain < +40 dB
Pin 2 (Fig. 15)
E
C+
/Gain (dB)
Pin 3
E
C-
/Gain (dB)
∆E
C
/∆T
CHIP
Ref T
CHIP
= 27°C
-60 to +40 dB gain
6.0 6.1
-6.2 -6.1
6.2
-6.0
6.0 6.1
-6.2 -6.1
6.2
-6.0
6.0 6.1
-6.2 -6.1
6.2 mV/dB
-6.0 mV/dB
%/°C
%
dB
Gain-Control TempCo
Gain-Control Linearity
1 kHz Off Isolation
Output Noise
— +0.33 —
—
0.5
115
2
—
— +0.33 —
—
110
0.5
115
2
—
— +0.33 —
—
110
0.5
115
2
—
E
C+
= -360mV,E
C-
=+360mV 110
e
n(OUT)
20 Hz ~ 20 kHz
R
out
= 20kΩ
0 dB gain
+15 dB gain
No Signal
—
—
-98
-88
-97
-86
-2.6
—
—
-98
-88
-96
-85
-2.6
—
—
-98
-88
-95
-84
-2.6
dBV
dBV
V
Voltage at V-
V
V-
-3.1 -2.85
-3.1 -2.85
-3.2 -2.85
1. All specifications are subject to change without notice.
2. Unless otherwise noted, T
A
=25ºC, V
CC
=+15V, V
EE
= -15V. Test circuit as shown in Figure 2. SYM ADJ is adjusted for minimum THD at 1 V, 1 kHz, Ec- = -Ec+ = 0 V.
3. Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only; the functional operation of
the device at these or any other conditions above those indicated in the operational sections of this specification is not impli ed. Exposure to absolute maximum rating condi-
tions for extended periods may affect device reliability.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Copyright © 2008, THAT Corporation
THAT 2181 Series
Blackmer® Trimmable IC VCAs
Page 3 of 12
Document 600030 Rev 02
Electrical Characteristics (con’t)
2
Parameter
Symbol
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
R
IN
= R
OUT
= 20 kΩ
2181A
Min Typ Max
2181B
Min Typ Max
2181C
Min Typ Max
Units
Total Harmonic Distortion THD
—
—
—
—
—
0.0025
0.018
0.018
0.004
12
—
0.0
0.005
0.025
0.025
0.008
—
+0.5
+0.1
—
—
—
—
—
-1.5
0.004
0.025
0.025
0.006
12
—
0.008
0.035
0.035
0.010
—
+1.5
+0.15
—
—
—
—
—
-2.5
-0.2
0.005 0.02
0.035 0.07
0.035 0.07
0.015 —
12
—
0.0
—
+2.5
+0.2
%
%
%
%
V/µs
mV
dB
Slew Rate
Symmetry Control Voltage V
SYM
A
V
= 0dB, Minimum THD -0.5
Gain at 0 V Control Voltage
E
C-
= 0 mV
-0.1
-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
Copyright © 2008, THAT Corporation
Document 600030 Rev 02
Page 4 of 12
THAT 2181 Series
Blackmer® Trimmable IC VCAs
Theory of Operation
4
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 inter-
nal circuit diagram of the IC. The ac input signal
current flows in pin 1, 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 repre-
sents 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
virtual ground, Q2/D2 and Q4/D4 take the bipolar
antilog of V3, creating an output current which is a
precise replica of the input current. If pin 2 (E
C+
) and
pin 3 (E
C-
) 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 negative, the output current will be
scaled larger than the input current. For pin 2
negative or pin 3 positive, the output current is
scaled smaller than the input.
Figure 7. Gain vs. Control Voltage (E
C-
, Pin 3) at 25°C
Figure 8. Gain vs. Control Voltage (E
C-
) with Temp (°C)
In normal operation, the output current is
converted to a voltage via an opamp-based I-V
converter, as shown in Figure 2, where the conver-
sion ratio is determined by the feedback resistor
connected between the output and inverting input.
The signal path through the VCA and the output
opamp is non-inverting.
The scale factor between the output and input
currents is the gain of the VCA. Either pin 2 (Ec+) or
pin 3 (Ec-), or both, may be used to control gain.
Gain is exponentially proportional to the voltage at
pin 2, and exponentially 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
Figure 5. Simplified Internal Circuit Diagram
4. For more details about the internal workings of the 2181 Series of VCAs, see An
Improved Monolithic Voltage-Controlled
Amplifier,
by Gary K. Hebert (Chief Technology Officer, 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
Copyright © 2008, THAT Corporation
THAT 2181 Series
Blackmer® Trimmable IC VCAs
Page 5 of 12
Document 600030 Rev 02
negative
control port. Because of the exponential
characteristic, the control voltage sets gain
linearly
in decibels.
Figure 6 shows the decibel current gain
of a 2181 versus the voltage at Ec+, while Figure 7
shows gain versus the Ec-.
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
Ω
resistor 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
Ω
resistor along with a 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 middle-level, middle-frequency signal (e.g. 1 kHz at
1 V) to the audio input, setting the VCA to 0 dB gain,
and adjusting the SYM trim while observing THD at
the output. In the 2181, this adjustment coincides
closely with the setting which produces minimum
control-voltage feedthrough, though the two settings
are not always identical.
Temperature Effects
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 relationship is temperature dependent. There-
fore, the gain of any log-antilog VCA depends on its
temperature.
Figure 8 shows the effect of temperature on the
negative control port. (The positive control port
behaves in the same manner.) Note that the gain at
Ec = 0 V is 0 dB, regardless of temperature. Chang-
ing 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
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
Gain
=
E
C+
−
E
C−
(0.0061)(1+0.0033
T)
,
Eq. 1
where
∆T
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
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
Figure 9. Representative DC Offset vs. Gain
Gain
=
E
C−
0.0061
,
Eq. 4
Audio Performance
The 2181-Series VCA design, fabrication and
testing ensure extremely good audio performance
when used as recommended. The 2181 maintains
low distortion over a wide range of gain, cut and
signal levels. Figures 10 through 12 show typical
distortion performance for representative samples of
each grade of the part. At or near unity gain, the
2181 behaves much like a good opamp, with low
distortion over the entire audio band. Figure 13
shows typical THD for a 2181A over frequency at 0
dB gain, with a 1 V input signal, while Figure 14
details the harmonic content of the distortion in a
typical A–grade part.
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 kΩ, which provides approximately
2.4 mA. This current is split into two paths: 570
μA
is used for biasing the IC, and the remainder
becomes I
CELL
as shown in Figure 5. I
CELL
is furth
er split in two parts: about 20
μA
biases the core
transistors (Q1 through Q4), the rest is available for
input and output signal current.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com
Copyright © 2008, THAT Corporation
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