394CHAPTER 8. OPERATIONAL AMPLIFIERS−+VoutCRRRVrampVrefVrefVrampVout is a square wave just like Vref, only tallerOscillator circuit using positive feedbackWhen the output is saturated positive, the Vref will be positive, and the capacitor willcharge up in a positive direction. When Vramp exceeds Vref by the tiniest margin, the outputwill saturate negative, and the capacitor will charge in the opposite direction (polarity). Os-cillation occurs because the positive feedback is instantaneous and the negative feedback isdelayed (by means of an RC time constant). The frequency of this oscillator may be adjustedby varying the size of any component.• REVIEW:• Negative feedback creates a condition of equilibrium(balance). Positive feedback createsa condition of hysteresis(the tendency to ”latch” in one of two extreme states).• An oscillatoris a device producing an alternating or pulsing output voltage.8.13Practical considerationsReal operational have some imperfections compared to an “ideal” model. A real device deviatesfrom a perfect difference ampliﬁer. One minus one may not be zero. It may have have an offsetlike an analog meter which is not zeroed. The inputs may draw current. The characteristicsmay drift with age and temperature. Gain may be reduced at high frequencies, and phasemay shift from input to output. These imperfection may cause no noticable errors in someapplications, unacceptable errors in others. In some cases these errors may be compensatedfor. Sometimes a higher quality, higher cost device is required.