THAT2150A ETC, THAT2150A Datasheet - Page 9

• Manufacturer Part Number: THAT2150A
• Description: IC Voltage-Controlled Amplifiers
• Manufacturer: ETC

symmetry voltage (< 2.5 mV) to correct for V
matches within the VCA IC. For this purpose, the 2150
series devices were designed for optimum performance
with an impedance of approximately 50
trim pot is used to adjust the voltage between pin 4
and pin 2 as shown in Figure 3, Page 3. For supply
voltages other than shown, scale R
required adjustment range.
an opposite-sense voltage control port. A typical circuit
using this approach is shown in Figure 14. Pin 3 may
be grounded, and pin 2 driven against the symmetry-
adjustment voltage. The change in voltage at pin 4
does have a small effect on the symmetry voltage, but
this is of little practical consequence in most applica-
tions. Using the opposite sense of control can some-
times save an inverter in the control path.
both control ports with differential drive (see Fig-
ure 15). While the driving circuitry is more complex,
this configuration offers better performance at high
attentuation levels (<-90 dB) where the single-control-
port circuits begin to saturate Q1 (for E
(for E
rates, the internal opamp will accomodate the change
in current demand by responding with a small change
in its input offset voltage. This leads to an accumula-
tion of charge on the input capacitor, which in turn
can cause thump when the high attenuation is sud-
denly removed (e.g., when a muted channel is opened).
Differential control drive avoids the large dc levels oth-
Rev. 10/25/96
It is also possible to use pin 2 and pin 4 together as
It is also possible (and advantageous) to combine
C+
drive). When either of these transistors satu-
INPUT
THAT Corporation; 734 Forest Street; Marlborough, Massachusetts 01752; USA
Tel: [phone]; Fax: [phone]; Web: http://www.thatcorp.com
10u
Figure 14. Positive Control Port Using Pins 2 and 4
SYM
C–
20k
to provide the
Series
drive) or Q3
Series
at pin 4. A
2150
2150
VCA
VCA
5.1k
BE
-15V
mis-
1
+15V
7
-IN
V+
V-
5
GND
6
Ec+
3
Ec+
Ec-
2
Ec+
4
erwise
(+610 mV for -100 dB gain at pin 3 alone, vs. 305 mV
when using both pin 3 and pins 2 and 4).
Control Port Drive Impedance
should be driven by a low source impedance for mini-
mum distortion. This often suggests driving the control
port directly with an opamp (see below under Noise
Considerations). However, the closed-loop output im-
pedance of an opamp typically rises at high frequencies
due to falling loop gain. The output impedance is
therefore inductive at high frequencies. Excessive in-
ductance in the control port source impedance can
cause the VCA to oscillate internally. In such cases, a
51
control port to ground will usually suffice to prevent
the instability.
Noise Considerations
consider the effects of noisy devices on the signal path.
As is well known, this includes not only active devices
such as op amps and transistors, but extends to the
choice of impedance levels as well. High value resistors
have inherent thermal noise associated with them, and
the noise performance of an otherwise quiet circuit can
be easily spoiled by the wrong choice of impedance lev-
els.
cuitry and high impedance levels in the control path of
51
OUT
8
It has already been noted that the control port
It is second nature among good audio designers to
Less well known, however, is the effect of noisy cir-
resistor in series with a 1.5 nf capacitor from the
required
390k (2150A)
300k (2155)
470k (2151)
240k
Rsym
-
+
LF351
47p
20k
to
command
-15V
50k
SYM
ADJ
+15V
high
OUTPUT
attenuation
Page 9