of Microprocessors

BASIC ANALOG MODULES
217
100 ko.
BAND-REJECT
OUTPUT
INPUTC>--""'IIr-1~
LOW-PASS
OUTPUT
lOOkn
oCONTROL
CURRENT
0.5 mA = 0,5
O.5)JA= 500
FREQUENCY
CONTROL
CURRENT
1.0 mA=lOkHI
1.0}JA=IOHz
Fig. 6--27. Practical state-variable filter
these resistors is not critical as long as they are matched (1 % is fine) and not
so large that the compliance range of the control current source is exceeded.
The 3080s may, however, have to be matched somewhat by hand and held in
good thermal contact for optimum results. The two halves of a 3280 would
be an ideal solution in this case. Another alternative is the use of a matched
trio of transistors for exponential conversion. The third transistor is
connected in parallel with the usual exponential output transistor except for
the collector, which becomes a second output terminal. The harmonic
distortion introduced by the 3080 gain elements is much less than in the
VeA application, since the filter is a dosed-loop, negative-feedback network.
Controlling Q
Proper control of Q is an interesting problem. First, it is desirable that
Q be an exponential function of control voltage, since it has a useful range
from 0.5 to over 500. This 1,000-to-l range is best handled by having a I-V
change in Qcontrol voltage correspond to doubling or halving ofQ. Also, it
is probably desirable that high Q correspond to a high control voltage. Since
high Q corresponds to low control currents in this circuit, the polarity of the
Q control voltage will have to be inverted somewhere in the exponential
converter circuit. Finally, there may be occasions when constant bandwidth is
desired as the filter is tuned rather than constant Q. If the Q control is
exponential at "1 V per double" and two Q control inputs are provided,
constant bandwidth operation is achieved by feeding the frequency control
voltage into one of the Q control inputs as well as a frequency input. This
causes the Q to increase in direct proportion to center frequency and thereby
provide constant bandwidth.