Prelab Part 2 In class, we discussed an op amp comparator circuit as shown below in Figure 4. For this circuit, whenever vi > Vref, Vo goes to the positive power rail. Else, v goes to the negative power rail. Throughout this part, assume that the power rails are +5 V. Vi Vref Figure 4. Comparator circuit. Now consider the circuit of Figure 5. a) Which LED will be turned on when v₁ = 1 V? Which LED will be turned on when v₁ = -1 V?/nVi 200 Ω LED 1 W 200 Ω LED 2 Figure 5. Comparator with LEDS. Now consider the circuit of Figure 6. b) Write an equation for v₁ as a function of the pot setting F. Assume F = 0 corresponds to the slider being all the way up. 5 V V1 vi(t) 1 ΚΩ. n(t) 5 V Figure 6. Input signal. To make things more interesting, we add some “noise” to v₁. The resulting "noisy" signal is v₁(t) = v₁ + n(t). Assume that the noise n(t) is a square wave of amplitude vp = 0.1 V (Upp 0.2 V) and frequency f = 2 Hz. Figure 7 shows a graph of vi(t) for a pot setting F = 0.4. Make sure that you understand this figure. Changing the pot setting F will raise or lower the level of the signal in Figure 7. As another example, when F = 0.5, the square wave will be oscillating above and below 0 V, rather than above and below 1 V as shown in Figure 7./n1.0 V ↑vi(t) +0.2 V 0.5 1.0 2.0 t (seconds) Figure 7. "Noisy" signal. Consider now connecting vi(t) in Figure 6 as the input to the circuit of Figure 5. Like the improved volt meter, the input resistance of the comparator is very high. Thus, we do not need to worry about the loading effect. c) What do you expect the LEDs to do for a pot setting of F = 0.4? How about F = 0.5? How about F = 0.6? In class, we also talked about the Schmitt trigger (comparator with hysteresis) which reduces "chattering" due to noise. A Schmitt trigger circuit is shown below in Figure 8. Again, assume power rails of +5 V. d) What is the threshold T for the Schmitt trigger as shown?/nNote that this threshold is larger than upp of the noise in the example above. Thus, any chattering in the example should be eliminated by replacing the comparator with the Schmitt trigger. We will verify this in the lab. 300 ΚΩ ww Vi 20 ΚΩ w Figure 8. Schmitt trigger. νο

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