Datasheet

LM13700
SNOSBW2E –NOVEMBER 1999–REVISED MARCH 2013 www.ti.com
Voltage Controlled Filters
OTA's are extremely useful for implementing voltage controlled filters, with the LM13700 having the advantage
that the required buffers are included on the I.C. The VC Lo-Pass Filter of Figure 32 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 × I
ABC
at
room temperature. Figure 33 shows a VC High-Pass Filter which operates in much the same manner, 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 34 and the state variable filter of Figure 35. Due to the excellent g
m
tracking of the two
amplifiers, these filters perform well over several decades of frequency.
Figure 31. Floating Voltage Controlled Resistor
Figure 32. Voltage Controlled Low-Pass Filter
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