Voltage Controlled Resistors
(Continued)
DS007981-15
FIGURE 8. Voltage Controlled Resistor, Single-Ended
DS007981-16
FIGURE 9. Voltage Controlled Resistor with Linearizing Diodes
Voltage Controlled Filters
OTA’s are extremely useful for implementing voltage con-
trolled filters, with the LM13700 having the advantage that
the required buffers are included on the I.C. The VC Lo-Pass
Filter of
Figure 11
performs as a unity-gain buffer amplifier at
frequencies below cut-off, with the cut-off frequency being
the point at which X
C
/g
m
equals the closed-loop gain of (R/
R
A
). At frequencies above cut-off the circuit provides a single
RC roll-off (6 dB per octave) of the input signal amplitude
with a −3 dB point defined by the given equation, where g
m
is again 19.2 x I
ABC
at room temperature.
Figure 12
shows a
VC High-Pass Filter which operates in much the same man-
ner, providing a single RC roll-off below the defined cut-off
frequency.
Additional amplifiers may be used to implement higher order
filters as demonstrated by the two-pole Butterworth Lo-Pass
Filter of
Figure 13
and the state variable filter of
Figure 14.
Due to the excellent g
m
tracking of the two amplifiers, these
filters perform well over several decades of frequency.
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