2181-DEMO THAT Corporation, 2181-DEMO Datasheet - Page 4

• Manufacturer Part Number: 2181-DEMO
• Description: Audio Modules & Development Tools Trimmable VCA Demo Board
• Manufacturer: THAT Corporation

Specifications of 2181-DEMO

• Description/function: Audio Amplifiers
• Operating Supply Voltage: 15 V
• Product: Audio Modules
• For Use With/related Products: THAT2181
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Document 600030 Rev 02
high performance in audio-frequency applications
requiring exponential gain control, low distortion,
wide
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.
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
Gain Control
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
pin 3 (E
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.
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.
The THAT 2181 Series VCAs are designed for
Figure 5 presents a considerably simplified inter-
Since pin 8, the output, is usually connected to a
dynamic
Figure 5. Simplified Internal Circuit Diagram
C-
) are held at ground (with pin 4 - SYM -
range
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
and
low
be
drops.)
control-voltage
Copyright © 2008, THAT Corporation
Theory of Operation
C+
) and
Page 4 of 12
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.
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 8. Gain vs. Control Voltage (E
Figure 6. Gain vs. Control Voltage (E
Figure 7. Gain vs. Control Voltage (E
In normal operation, the output current is
The scale factor between the output and input
4
Blackmer® Trimmable IC VCAs
C-
C+
C-
THAT 2181 Series
) with Temp (°C)
, Pin 3) at 25°C
, Pin 2) at 25°C