In this chapter you will encounter some applications of positive feedback in op amp and comparator circuits. You will see how uncontrolled feedback can cause unwanted oscillation, and how controlled positive feedback (hysteresis) can be used to eliminate unwanted oscillation or produce intentional oscillation. There is also an optional active-filter application at the end.

Apparatus required

Breadboard, dual-trace oscilloscope with two attenuating probes, two 741 and one LF411 op amp, 311 comparator, 555 timer, one 100 Ω, one 820 Ω, two 1 k, two 3.3 k, three 10 k, one 100 k, one 1 M, and one 10 M 14 W resistor, three 0.033 μF, one 0.01 μF, and one 1 μF capacitor, one red LED, and two 3.3 V Zener diodes.

Experiments

Op amp as comparator

Begin by wiring up a 741 in open-loop mode as you did in a previous lab (Fig. 9.1(a)). With no negative feedback, the saturated output that results allows the op amp to be used as a voltage comparator – a circuit that tells you whether an input voltage is higher or lower than a ‘threshold’ voltage (the threshold is ground in this case). Since op amps are not specifically engineered for open-loop operation, it is not a very good voltage comparator (in ways that we shall soon see), but in some situations (when highspeed response and high sensitivity are not required) an open-loop-741 ‘comparator’ is perfectly adequate.