SSM2164
a low cutoff frequency. The main exception to this is in
dynamic processing applications, where faster attack or decay
times may be needed.
but higher noise, and the opposite is true for less current. The
increased noise is due to higher current noise in the gain core
transistors as their operating current is increased. THD has the
opposite relationship to collector current. The lower distortion
is due to the decrease in the gain core transistors’ emitter
impedance as their operating current increases.
+5V
100pF
3
2
V
I
C
100k
This classical tradeoff between THD and noise in VCAs is
usually expressed as the choice of using a VCA in either Class A
or Class AB mode. Class AB operation refers to running a VCA
with less current in the gain core, resulting in lower noise but
higher distortion. More current in the core corresponds to
Class A performance with its lower THD but higher noise.
Figures 11 and 12 show the THD and noise performance of the
SSM2164 as the bias current is adjusted. Notice the two
characteristics have an inverse characteristic.
1µF
30k
30k
I
I
I
I
IOUT
IOUT
IOUT
IN
4
VCA1
VCA2
VCA3
1/4
OP482
V
V
V
V
V
OUT1
IN1
500
560pF
+5V
+5V
+5V
100pF
V
6
7
C
100k
1µF
30k
30k
5
I
IN
1/4
OP482
V
V
V
OUT2
OUT3
OUT4
IN2
The quiescent current in the core is set by adding a single
resistor from the positive supply to the MODE pin. As the
simplified schematic shows, the potential at the MODE pin is
one diode drop above the ground pin. Thus, the formula for the
MODE current is:
500
560pF
100pF
V
C
11
10
100k
1µF
30k
30k
I
12
IN
1/4
OP482
(V +)− 0.6V
IN3
IMODE
=
500
RB
560pF
100pF
V
I
14
15
C
With ±15 V supplies, an RB of 7.5k gives Class A biasing with a
current of 1.9 mA. Leaving the MODE pin open sets the
SSM2164 in Class AB with 30 µA of current in the gain core.
100k
1µF
30k
30k
13 IOUT
IN
VCA4
1/4
OP482
Basic VCA Configuration
IN4
500
Figure 24 shows the basic application circuit for the SSM2164.
Each of the four channels is configured identically. A 30 kΩ
resistor converts the input voltage to an input current for the
VCA. Additionally, a 500 Ω resistor in series with a 560 pF
capacitor must be added from each input to ground to ensure
stable operation. The output current pin should be maintained
at a virtual ground using an external amplifier. In this case the
OP482 quad JFET input amplifier is used. Its high slew rate,
wide bandwidth, and low power make it an excellent choice for
the current-to-voltage converter stage. A 30 kΩ feedback
resistor is chosen to match the input resistor, giving unity gain
for a 0.0 V control voltage. The 100 pF capacitors ensure
stability and reduce high frequency noise. They can be
increased to reduce the low pass cutoff frequency for further
noise reduction.
560pF
POWER SUPPLY
AND BIASING CIRCUITRY
9
8
16
1
V–
GND V+
MODE
0.1µF 0.1µF
R
(7.5kΩ CLASS A)
(OPEN CLASSAB)
B
–15V
+15V
Figure 24. Basic Quad VCA Configuration
Low Cost, Four-Channel Mixer
The four VCAs in a single package can be configured to create a
simple four-channel mixer as shown in Figure 25. The inputs
and control ports are configured the same as for the basic VCA,
but the outputs are summed into a single output amplifier. The
OP176 is an excellent amplifier for audio applications because
of its low noise and distortion and high output current drive.
The amount of signal from each input to the common output
can be independently controlled using up to 20 dB of gain or as
much as 100 dB of attenuation. Additional SSM2164s could be
added to increase the number of mixer channels by simply
summing their outputs into the same output amplifier. Another
possible configuration is to use a dual amplifier such as the
OP275 to create a stereo, two channel mixer with a single
SSM2164.
For this example, the control voltage is developed using a
100 kΩ potentiometer connected between +5 V and ground.
This configuration results in attenuation only. To produce both
gain and attenuation, the potentiometer should be connected
between a positive and negative voltage. The control input has
an impedance of 5 kΩ. Because of this, any resistance in series
with VC will attenuate the control signal. If precise control of
the gain and attenuation is required, a buffered control voltage
should be used.
Notice that a capacitor is connected from the control input to
ground. Because the control port is connected directly to the
gain core transistors, any noise on the VC pin will increase the
output noise of the VCA. Filtering the control voltage ensures
that a minimal amount of noise is introduced into the VCA,
allowing its full performance to be realized. In general, the
largest possible capacitor value should be used to set the filter at
–8–
REV. 0
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