of Microprocessors

BASIC ANALOG MODULES
187
EXPONENTIAL CONTROL
VOLTAGE INPUT
~
LINEAR
CONTROL
VOLTAGE
FOR
CONSTANT
MODULATION
INDEX
lout EXPONENTIAL
Y------------~______o~L~ER~~T
OUTPUT
(8)
Fig. &-6. Practical exponential converter. (A) Exponential voltage to current
converter. (B) Linear voltage to current converter.
op-amp bias currents, is typically set to around 10 /LA. When connected to
the oscillator to be described later, a current range from about 0.25 /LA to
0.25 rnA will cover the audible range. The resistor in series with the
amplifier output serves to limit the maximum output current to a safe value.
Like the two-diode circuit, the matched transistors cancel the major temperature
dependence of the exponential converter. The output current flows into
this exponential converter and the voltage-compliance range is from roughIy
ground to the positive collector breakdown voltage. Note that negative-going
input voltages result in increasing magnitudes of output current. The polarity
may be reversed simply by swapping connections to the bases of the two
transistors. This circuit performs well but still has two remaining imperfections,
which will be remedied later.
Linear Control Input
A linear control input for the dynamic depth frequency modulation
described in Chapter 3 can easily be added to this exponential converter. One
possibility is to simply modulate the reference current by summing the linear
control voltage with the reference voltage in the reference current regulator.
The effect can be understood by noting the overall I/O relation for the
exponential converter: lout = I",je39vin. Thus, the linear control voltage
input multiplies the oscillator frequency by factors greater than unity for
positive inputs or less than unity for negative inputs. Linear frequency modulation
implemented through this input will be such that the modulation
index (ratio offrequency deviation to center frequency) will be constant as the