of Microprocessors

BASIC ANALOG MODULES 189
(or 0.9766 for a binary-calibrated system) and C could be varied from zero up
to perhaps 3 octavesN. The "tune" control determines the basis frequency
(frequency with all inputs at zero) by feeding a variable dc voltage directly
into the summer. A fine-tuning control and more inputs can be added to this
sttucture essentially without limit. Note that algebraic summation of the
input voltages is inherent; thus, a negative voltage at one of the inputs will
counteract a positive voltage at another input.
The output voltage from the input processor is scaled by adjusting the
value of R relative to the input resistors. Since this circuit will drive the base
of an exponential converter transistor directly, the transistor equation must
be solved to determine the range of output voltages needed. It turns out that
a 0.018-V increase in base voltage will double the collecror current at room
temperature. It is common practice to set R to 2,000 ohms when WOK
input resistors are used which would scale a 1-V input down to 0.020 V. An
internal trimming potemiometer between the op-amp output and the exponential
converter base is then used to adjust to the exact value needed
around 18 mY. Note that the polarity inversion (positive-going input voltages
produce negative-going outputs) precisely matches the requirements of
the exponential converter.
Assuming that the tuning control is set to midrange (no net effect on
the control voltage sum) and all control inputs are zero, the output of this
circuit would also be zero. The exponential converter would then produce a
current equal to the reference current, which is typically set to 10 !LA.
Positive control voltage sums (more negative input to exponential converter)
give higher currents from the exponential converter, while negative sums
give lower currents. For normal operation, the tuning control would be set
negative so that 0 V from the other inputs would produce the lowest normal
audio frequency. Then positive control voltages from 0 V to 10 V would
cover the audio range. Negative control inputs in addition to the negative
contribution of the tuning control could produce even lower frequencies,
useful as control voltages themselves.
Sawtooth Oscillator
The current from the exponential converter could be used to charge
(actually discharge since it is a negative current) a capacitor directly. Greater
accuracy is obtained, however, if the exponential converter collector remains
at a constant voltage near ground, since then the collector-base voltage is
near zero and leakages are minimized. This desire is satisfied by feeding the
current directly into the summing node of an integrator as shown in Fig.
6-8. The negative current is integrated and inverted by the op-amp and
appears as a positive-going ramp at its output. The op-amp used for the
integrator must have low bias current yet high speed for optimum low- and
high-frequency performance, respectively, which usually means a FET opamp.