THAT2181 Datasheet, PDF (8/10 Pages) List of Unclassifed Manufacturers – Trimmable IC Voltage Controlled Amplifiers

Page 8
THAT2181 Series IC VCAs
There is no reason why both control ports cannot be
used simultaneously.
Symmetry
The SYM pin (pin 4) is actually a sort of additional
positive-sense control port. It is provided to allow V_be
mismatches in the core transistors to be adjusted after
packaging and installation in the circuit board. It should
only be used for this purpose. Connect pin 4 only to a
high-impedance source as shown in Figures 2 and 15.
Control Port Drive Impedance
The control ports (pins 2 through 4) are connected
directly to the bases of the logging and/or antilogging
transistors. The accuracy of the logging and antilogging
is dependent on the E_C+ and E_C- voltages being exactly
as desired to control gain. The base current in the core
transistors will follow the collector currents, of course.
Since the collector currents are signal-related, the base
currents are therefore also signal-related. Should the
source impedance of the control voltage(s) be large, the
signal-related base currents will cause signal-related
voltages to appear at the control ports, which will inter-
fere with precise logging and antilogging, in turn causing
distortion.
The 2181 Series VCAs are designed to be operated
with zero source impedance at pins 2 and 3, and a high
(³50 kW) source impedance at pin 4. To realize all the
performance designed into a 2181, keep the source im-
pedance of the control voltage driver well under 50 W.
This often suggests driving the control port directly
with an opamp. However, the closed-loop output imped-
ance of an opamp typically rises at high frequencies be-
cause open loop gain falls off as frequency increases. A
typical opamp's output impedance is therefore inductive
at high frequencies. Excessive inductance in the control
port source impedance can cause the VCA to oscillate in-
ternally. In such cases, a 100 W 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 con-
sider the effects of noisy devices on the signal path. As is
well known, this includes not only active devices such as
opamps and transistors, but extends to the choice of im-
pedance levels as well. High value resistors have higher in-
herent thermal noise, and the noise performance of an
otherwise quiet circuit can be easily spoiled by the wrong
choice of impedance levels.
Less well known, however, is the effect of noisy circuitry
and high impedance levels in the control path of volt-
age-control circuitry. The 2181 Series VCAs act like multi-
pliers: when no signal is present at the signal input, noise at
the control input is rejected. So, when measuring noise (in
the absence of signal – as most everyone does), even very
noisy control circuitry often goes unnoticed. However, noise
at the control port of these parts will cause noise modula-
tion of the signal. This can become significant if care is not
taken to drive the control ports with quiet signals.
The 2181 Series VCAs have a small amount of inherent
noise modulation because of its class AB biasing scheme,
where the shot noise in the core transistors reaches a mini-
mum with no signal, and increases with the square root of
the instantaneous signal current. However, in an optimum
circuit, the noise floor rises only to -94 dBV with a
50 mA rms signal at unity gain — 4 dB of noise modulation.
By contrast, if a unity-gain connected, non-inverting 5534
opamp is used to directly drive the control port, the noise
floor will rise to 92 dBV — 6 dB of noise modulation.
Vcc
2181
Series
VCA 7
V+ ³
IN
10u
20k
1
Ec-
OUT
-IN
SYM
Ec+
GND
V- ₆
2
4
8
5
22p
20k
-
OP275
+
OUT
Vee
Power Supplies 5.1k
Rsym
Vcc = +15 V
Vee = -15 V
680k (2181A)
220k (2181B)
Vee Ec+ ^{130k (2181C)}
Figure 15. Positive Control Port Using Pin 4
50k
SYM
ADJ
Vcc
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