Transconductance amp gives oscillator reciprocal response

摘要

The circuit in Figure 1 is a variation on the familiar integrator/comparator triangle-wave oscillator, which you typically implement with two op amps. An integrator and a comparator connect in a positive-feedback loop; the comparator drives the integrator and vice versa. A fixed amount of hysteresis exists in the comparator, producing a triangle wave at the integrator's output. The wave oscillates between the fixed threshold voltages. You can vary the oscillation rate using a potentiometer to set the integrator's gain and thus obtain a constant-amplitude, variable-frequency, triangle-wave output. In the circuit of Figure 2, the integrator's gain is fixed, whereas the comparator's hysteresis is variable. An OTA (operational transcon-ductance amplifier), IC_2, sets the hysteresis, producing an oscillator in which the period is a linear function of an externally applied voltage. The oscillation rate is inversely proportional to the control volt-age; in other words, the oscillator has a 1/x response. Such an oscillator is useful in A/D-converter applications and for clocking time-delay systems, such as audio-delay lines.