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22 Comparators and the Schmitt Trigger Reading: Floyd and Buchla, Analog Fundamentals: A Systems Approach, Section 8-1 Objectives: After performing this experiment, you will be able to: 1. 2. Compare the input and output waveforms for comparator and Schmitt trigger circuits. Use an oscilloscope to plot the transfer curve for a comparator circuit, including one with hysteresis truct and test a relaxation oscillator using a Schmitt trigger. Summary of Theory: A comparator is a switching device that produces a high or low output, depending on which of the two inputs is larger. The comparator is run with the very high open-loop gain. When the noninverting input is very slightly larger than the inverting input, the output goes to positive saturation; otherwise it goes to negative saturation. Although general-purpose op-amps can be used as comparators, specially designed op-amps can switch faster and have additional features not found on general-purpose op-amps. For noncritical applications, general-purpose op-amps are satisfactory and will be used in this experiment. A comparator circuit is characterized by its transfer characteristic. The transfer characteristic (curve) is a plot of the output voltage (plotted along the y-axis) as a function of the input voltage (plotted along the x-axis). Consider the comparator shown in Figure 22-1(a). The reference voltage is +6 V. When the input is greater than +6.0 V, the output will go to positive saturation (approximately +13 V); when the input is less than +6.0 V, the output will be in negative saturation. A glance at the transfer curve will show the output for any given input voltage. +15 V positive 741C +6.0 V 1.0 uF (a) Circuit (b) Transfer curve Figure 22-1 145