that2162 THAT Corporation, that2162 Datasheet - Page 4

• Manufacturer Part Number: that2162
• Description: Dual Pre-trimmed Blackmer Voltage Controlled Amplifier
• Manufacturer: THAT Corporation

Page 4 of 12
performance in audio-frequency applications requir-
ing exponential gain control, wide dynamic range,
low control-voltage feedthrough, and low cost. This
part controls 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.
internal circuit diagram of the IC. The ac input signal
current flows in pin 7 [10], the input pin. An internal
operational transconductance amplifier (OTA) works
to maintain pin 7 [10]
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 Vbe drops.
Gain Control
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
6 [11] (E
the output current will equal the input current. For
pin 6 [11] positive or pin 4 [13] negative, the output
current will be scaled larger than the input current.
For pin 6 [11] negative or pin 4 [13] positive, the
output current is scaled smaller than the input.
currents is the gain of the VCA. Either pin 6 [11]
(E
control gain. Gain is exponentially proportional to
the voltage at pin 6 [11], and exponentially propor-
tional to the negative of the voltage at pin 4 [13].
1
IN
Pin number references are for VCA1, with VCA2 shown in brackets.
C+
7[10]
) or pin 4 [13] (E
The THAT 2162 VCA is designed for high
Figure 4 presents a considerably simplified
Since pin 2 [15], the output, is usually connected
The scale factor between the output and input
I in
V+
C+
) and pin 4 [13] (E
Ec+
Figure 4. Simplified internal circuit
6[11]
30
V 3
V-
Q1
Q3
D1
D3
5
1
C-
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
at a virtual ground potential
), or both, may be used to
Generator
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
Voltage
I cell
Bias
30
C-
) are held at ground,
D2
D4
Q2
Q4
Copyright © 2007, THAT Corporation
V+
Theory of Operation
12
4[13]
2[15]
3[14]
I adj
Ec-
OUT
SYM
Therefore, pin 6 [11] (E
port, while pin 4 [13] (E
port. Because of the exponential characteristic, the
control voltage sets gain linearly in decibels. Figure 5
shows the decibel current gain of a 2162 versus the
voltage at E
Temperature Effects
on the logarithmic relationship between voltage and
current in a semiconductor junction (in particular,
between a transistor's Vbe and Ic). As is well known,
this relationship is temperature dependent. There-
fore, the gain of any log-antilog VCA depends on its
temperature.
Figure 7. Gain Vs. Control Voltage (Ec-) with Temp (ºC)
The logging and antilogging in the VCA depends
Figure 5. Gain Vs. Control Voltage (Ec+) @ 1 kHz
Figure 6. Gain Vs. Control Voltage (Ec-) @ 1 kHz
C+
, while Figure 6 shows gain versus E
2162 Dual Pre-trimmed Blackmer® VCA
C+
C-
) is the positive control
) is the negative control
C-
.