THAT2151 Datasheet, PDF (9/10 Pages) List of Unclassifed Manufacturers – IC Voltage-Controlled Amplifiers

Rev. 10/25/96
symmetry voltage (<±2.5 mV) to correct for VBE mis-
matches within the VCA IC. For this purpose, the 2150
series devices were designed for optimum performance
with an impedance of approximately 50Ω at pin 4. A
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 RSYM to provide the
required adjustment range.
It is also possible to use pin 2 and pin 4 together as
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.
It is also possible (and advantageous) to combine
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 EC– drive) or Q3
(for EC+ drive). When either of these transistors satu-
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-
Page 9
erwise required to command high attenuation
(+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
It has already been noted that the control port
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 Ω resistor in series with a 1.5 nf capacitor from the
control port to ground will usually suffice to prevent
the instability.
Noise Considerations
It is second nature among good audio designers to
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.
Less well known, however, is the effect of noisy cir-
cuitry and high impedance levels in the control path of
INPUT
10u
+15V
2₂1₁5₅0₀
S_Se_eri_re_ies_s
V^VC^CA^A
7
V+
3
20k
1
Ec-
OUT
-IN
Ec+
Ec+
GND
4
8
V-
2
6
5
51
5.1k
-15V
Ec+
47p
20k
-
LF351
+
OUTPUT
+15V
Rsym
300²k⁴⁰(^k2155)
390k (2150A)
470k (2151)
50k
SYM
ADJ
-15V
Figure 14. Positive Control Port Using Pins 2 and 4
THAT Corporation; 734 Forest Street; Marlborough, Massachusetts 01752; USA
Tel: [phone]; Fax: [phone]; Web: http://www.thatcorp.com