Voltage Controlled Resistors (Continued) DS007980-14 FIGURE 7. AGC Amplifier DS007980-15 FIGURE 8. Voltage Controlled Resistor, Single-EndedVoltage Controlled Filters g is again 19.2 x I at room temperature. Figure 12 m ABC shows a VC High-Pass Filter which operates in much the OTA’s are extremely useful for implementing voltage con- same manner, providing a single RC roll-off below the de- trolled filters, with the LM13600 having the advantage that fined cut-off frequency. the required buffers are included on the I.C. The VC Lo-Pass Additional amplifiers may be used to implement higher order Filter of Figure 11 performs as a unity-gain buffer amplifier at filters as demonstrated by the two-pole Butterworth Lo-Pass frequencies below cut-off, with the cut-off frequency being Filter of Figure 13 and the state variable filter of Figure 14. the point at which X /g equals the closed-loop gain of C m Due to the excellent g tracking of the two amplifiers and the (R/R ). At frequencies above cut-off the circuit provides a m A varied bias of the buffer Darlingtons, these filters perform single RC roll-off (6 dB per octave) of the input signal ampli- well over several decades of frequency. tude with a −3 dB point defined by the given equation, where www.national.com 10
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