of Microprocessors

BASIC ANALOG MODULES 205
fore, must be cancelled with another op-amp inverter in the control-current
input processor. A complete circuit for an exponential response VCA is
shown in Fig. 6-16.
Improving Linearity
There are still several problems that limit the overall accuracy of a
transconductance gain-control element. Probably the most serious is the
remaining nonlinearity of the device. A related problem is the marginal
signal-to-noise ratio created by the necessity of very small signal amplitudes
at the gain element's input. Proper shielding can reduce coupled noise, but
the signals are so small that semiconductor device noise is significant. With
the lO-mV peak levels used by the preceding circuits, nonlinearity will cause
about 1.3% harmonic distortion and give a signal-to-noise ratio of about 66
dB. Used as a control voltage processor, nonlinearity is nearly 5% of full
scale and rms noise is 0.05% of full scale. Tradeoffs are possible, that is, less
distortion but more noise or vice versa, but neither parameter changes
dramatically with input signal level. Clearly, much improvement is necessary
if the VCA is to be useful in the control path to a VCO! A final problem
is that the gain drifts with temperature according to the semiconductor
junction equation. The magnitude of this drift is the same as the control
voltage sensitivity drift in a VCO, about 0.33%fc.
Concentrating on linearity first, it is seen that the linearity error is
independent of the control current. This means that a lO-mV input will
produce the same percentage of distortion at a low-gain setting as it will at a
high-gain setting. Furthermore, the effect of the nonlinearity is always a
reduction in the actual instantaneous output below what it ideally should be.
It should therefore be possible to predistort the input signal with an opposite
nonlinearity to compensate for the gain cell nonlinearity and therefore improve
things considerably.
Figure 6-17 shows a simple predistorter that can be added directly to
the 3080-based circuits given earlier. Analysis of the circuit is rather in-
RI
SIGNAL
33 kfi
INPUT
-50 TO 05
+50 mV
06
R2
R3
33 kil
+ 15 Vo----'VI/\r---«
33 kfi
~-'VVI~-----o-15 V
04
ALL DIODES IN A CA3019
MATCHED DIOOE ARRAY
Fig. 6--17. Diode bridge predistortion circuit