of Microprocessors

208 MUSICAL ApPLICATIONS OF MICROPROCESSORS
veA Using the 2020
Rather than describe the IC itself in detail, let's look instead at a
complete VCA circuit using it in Fig. 6-20. The lO-V signal input is first
attenuated to 5 V by R1 and R2. In a 5-V system, R1 may be omitted and
R2 increased to lOOK. The input impedance of the 2020 itself is tens of
megohms with a bias current requirement of about 500 nA. The current
output, which at 5 V signal input will have a peak value roughly one-third of
the control current, is converted into a lO-V peak output by AI, which is
connected as a current to voltage converter. R3 should be adjusted for unity
gain through the circuit with 1 rnA ofcontrol current. The offset trim circuit
at the 2020 noninverring signal input is necessary to cancel the device's offset
voltage and minimize control feedthrough.
The control circuitry shown provides simultaneous linear and exponential
gain control. Study of the configuration of A2 and the transistor pair in
the 2020 should reveal that it is exactly the same exponential conversion
structure as used in the VCO circuit. In normal operation, one control input
would be at zero, while the other is exercised over the full control range.
With both inputs at zero, a reference current of0.1 fJ.-A flows into the 2020.
This gives a gain of 0.0001 (-80 dB) relative to the I-rnA value which,
V+
'---------.,.---o6~~~5~T
L-_-+- ---,
SIGNAL
IN+
EXP CONV
COLLECTOR
EXP CONV
EMITTERS
V-
HALF OF DUAL UNIT SHOWN
Fig. 6-19. Simplified schematic of SSM 2020 VCA IC