LM13600M datasheet(10/24 Pages) NSC

10 / 24 page

Voltage Controlled Resistors (Continued)

Voltage Controlled Filters

OTA’s are extremely useful for implementing voltage con-

trolled filters, with the LM13600 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/gm 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 ampli-

tude with a −3 dB point defined by the given equation, where

m is again 19.2 x IABC at room temperature. Figure 12

shows a VC High-Pass Filter which operates in much the

same manner, providing a single RC roll-off below the de-

fined 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 and the

varied bias of the buffer Darlingtons, these filters perform

well over several decades of frequency.

DS007980-14

FIGURE 7. AGC Amplifier

DS007980-15

FIGURE 8. Voltage Controlled Resistor, Single-Ended

LM13600M Datasheet(PDF) 10 Page - National Semiconductor (TI)