that2151 THAT Corporation, that2151 Datasheet - Page 4

• Manufacturer Part Number: that2151
• Description: Ic Voltage-controlled Amplifiers
• Manufacturer: THAT Corporation

performance in audio-frequency applications requiring
exponential gain control, low distortion, wide dynamic
range and low dc bias modulation. These parts control
gain by converting an input current signal to a bipolar
logged voltage, adding a dc control voltage, and re-con-
verting the summed voltage back to a current through
a bipolar antilog circuit.
circuit diagram of the IC. The ac input signal current
flows in pin 1, the input pin. The internal op amp
works to maintain pin 1 at a virtual ground potential
by driving the emitters of Q1 and (through the Voltage
Bias Generator) Q3. For positive input currents (I
fined as flowing into pin 1), the op amp drives the emit-
ter of Q1 negative, turning off its collector current,
while simultaneously driving the emitter of Q3 nega-
tive, turning it on. The input signal current, therefore,
is forced to flow through Q3 and D3.
Logging & Antilogging
is logarithmic with collector current, the voltage from
the base of Q3 to the cathode of D3 is proportional to
the log of the positive input current. The voltage at the
cathodes of D3 and D4 is therefore proportional to the
log of the positive input currents plus the voltage at
pin 3, the negative control port. Mathematically,
where V
Page 4
The THAT 2150 Series VCAs are designed for high
Figure 6 presents a considerably simplified internal
Because the voltage across a base-emitter junction
V
3
IN
Figure 6. Simplified Internal Circuit Diagram
1
E
3
Ii n
C
is the voltage at the junction of D3 and D4;
Ec+
2V
-
+
T
2
ln
V-
V 3
Q1
Q3
THAT Corporation; 734 Forest Street; Marlborough, Massachusetts 01752; USA
D1
D3
I
5
I
C3
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Tel: [phone]; Fax: [phone]; Web: http://www.thatcorp.com
,
Generator
Voltage
Bias
D2
Q2
Q4
D4
3
8
4
Theory of Operation
(SYM)
Ec-
OUT
Ec+
in
de-
V
rent of Q3; and I
Q3. It is assumed that D3 matches Q3 (and will be as-
sumed that they match Q4 and D4, as well).
is connected to a voltage source at ground or nearly
ground potential. Pin 8 is connected to a virtual
ground (usually the inverting input of an op amp with
negative feedback around it). With pin 4 near ground,
and pin 8 at virtual ground, the voltage at the cathodes
of D3 and D4 will cause an exponentially-related cur-
rent to flow in D4 and Q4, and out via pin 8. A similar
equation governs this behavior:
Exponential Gain Control
begs further exploration. Accordingly:
rent in Q4/D4 will precisely mirror that in Q3/D3.
When pin 3 is positive with respect to pin 4, the voltage
across the base-emitter junction of Q3 is higher than
that across the base-emitter junction of Q4, so the
Q4/D4 current remains proportional to, but less than,
the current in Q3/D3. In the same manner, a negative
voltage at pin 3 with respect to pin 4 causes the
Q4/D4 current to be proportional to, but greater than
that in Q3/D3.
ence in the voltages E
“deci-linear” control. Mathematically, this is:
ture (25˚C), allowing for a 10˚C internal temperature
rise, and converting to a base of 10 for the exponential,
this reduces to:
T
In typical applications (see Figure 3, Page 3), pin 4
V
The similarity between the two preceeding equations
V
E
Rearranging terms,
I
If pin 3 and pin 4 are at ground potential, the cur-
The ratio of currents is exponential with the differ-
A
For pin 4 at or very near ground, at room tempera-
is the thermal voltage,
C4
3
3
V
C
E
E
I
I
I
C4
C3
C3
E
C
C
C
e
e
E
2V
2V
C
2V
2V
2V
E
C
T
T
2V
E
T
T
T
S
ln
ln
C
E
T
ln
ln
is the reverse-saturation current of
C
.
I
I
, where A
I
I
C4
C4
I
I
I
S
S
C+
I
C4
C4
C3
S
.
and E
.
E
kT
q
2V
C
; I
V
T
C–
C3
is the current gain.
ln
2V
, providing convenient
2150 Series IC VCAs
is the collector cur-
T
I
I
C3
ln
S
I
I
C3
S