Feedback and operational amplifiers

232 Chapter 4 is easy for Schmitt triggers that have a small amount of hysteresis. Use the circuit of Figure 4.62B. First choose a resis-tive divider (RI, R 2) to put the threshold at approximately the right voltage; if you want the threshold near ground, just use a single resistor from noninverting input to ground. Next, choose the (positive) feedback resistor R 3 to produce the required hysteresis, noting that the hysteresis equals the output swing, atlenuated by a resistive divider formed by R 3 and R I IIR 2 • Finally, choose an output pullup resistor 14 small enough to ensure nearly full supply swing, taking account of the loading by R 3. For the case where you want thresholds symmetrical about ground, connect an offsetting resistor of appropriate value from the non inverting input to the negative supply. You may wish to scale all resistor values in order to keep the output current and impedance levels within a reasonable range.