THAT2181 Datasheet, PDF (4/10 Pages) List of Unclassifed Manufacturers – Trimmable IC Voltage Controlled Amplifiers

Page 4
THAT2181 Series IC VCAs
Theory of Operation³
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.)
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 6. Gain vs. Control Voltage (E_C+, Pin 2) at 25°C
Figure 7. Gain vs. Control Voltage (Ec-, Pin 3) at 25°C
+
D1
D2
Figure 8. Gain vs. Control Voltage (Ec-) with Temp (°C)
2 Q1
Ec+
1
IN
I_IN
Q3
25
D3
Voltage
Bias
Generator
Q2 3
Ec-
8
OUT
Q4 4
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-.
V₃
Icell
Iadj
5
V-
Figure 5. Simplified Internal Circuit Diagram
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 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: [phone]; Fax: [phone]; Web: www.thatcorp.com