of Microprocessors

216 MUSICAL ApPLICATIONS OF MICROPROCESSORS
each variable resistor in the circuit has been replaced with a fixed resistor
driven by a VCA. The feedback path from the bandpass output that controls
Q has also been simplified and the relation between VCA gain and l/Q has
been made linear.
Note that a "voltage-controlled integrator" is used in two places. If the
variable-gain element is a 3080, 3280, or other current output transconductance
device, the voltage-controlled integrator can be simplified as in Fig.
6-26. The extremely high output impedance of the 3080 makes it possible
to perform integration directly with a capacitor to ground. The unity gain
buffer can often be a simple FET source follower, since its dc offset is
cancelled when the voltage-controlled integrator is used in a closed-loop
configuration. Note that positive as well as negative voltage-controlled
integrators are easily constructed by interchanging inputs to the 3080.
Practical State Variable Filter
Figure 6-27 shows a simplified but practical voltage-controlled filter
based on the state-variable principle. The circuitry to generate the frequency
control currents to the 3080s is not shown but may be the same as that used
earlier in VCA circuits. Note that the two integrators should ideally receive
the same magnitude of control current at all times. Use of the simple
resistive current sharer is generally adequate and even preferable to two
independent pairs of exponential converter transistors. The actual value of
R
INPUTo-'VVIrl--f
R
LOW-PASS
OUTPUT
F- I
- 2Tf(~)C
Q=..L 38
Q
CONTROL
FREQUENCY
CONTROL
Fig. 6-25. Voltage-controlled version of Fig. 6-23.
INPUTo-"VV'v~-I
OUTPUT
Fig. 6-26. Voltage-controlled integrator using 3080