Question

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 Figure 4. Comparator circuit. Now consider the circuit of Figure 5. a) Which LED will be turned on when vi = 1 V? Which LED will be turned on when v = -1 V? 2001} LED 10 {2001 LED 2 Figure 5. Comparator with LEDs. Now consider the circuit of Figure 6. b) Write an equation for v, as a function of the pot setting F. Assume F = 0 corresponds to the slider being all the way up. 5V [ - 1kn sna _vi(t) n(t) - 5v0 Figure 6. Input signal. To make things more interesting, we add some "noise" to v . The resulting "noisy" signal is vi(t) = V1 + n(t). Assume that the noise n(t) is a square wave of amplitude v = 0.1 V (Vpp = 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 O V, rather than above and below 1 V as shown in Figure 7.

a)Which LED will be turned on when vi = 1V? Which LED will be turned on when vi=-1V?

b) Write an equation for v1 as a function of the pot setting ?. Assume ? = 0 corresponds to the slider being all the way up.

c) What do you expect the LEDs to do for a pot setting of ? =  0.4? How about ? =  0.5? How about ? = 0.6?

d) What is the threshold ? for the Schmitt trigger as shown?

Answer 1

Answer a)

When Vi is 1V, LED1 will glow.

Explanation: Note that Vi is connected to non-inverting input of the op-amp. And inverting input is grounded (i.e. 0V). Therefore the net input to op-amp (V₊ - V_-) is positive. Therefore Op-amp out will saturate to nearly +5V. Also notice that anode of LED 1 is towards the op-amp output. Therefore LED 1 will get forward-biased in this case and hence this LED will glow.

Similarly when Vi is -1V, the op-amp output will be nearly -5V. Hence LED2 will get forward-biased and hence it will glow

Answer b)

The required equation is

V1 = -10*F + 5

Where F varies from 0 to 1. Zero mean slider is all the way up and 1 means that the slide is all the way down.

Answer c)

Please see the image below for the analysis of the three values of F.

Answer d) The threshold voltage for the configuration of Schmitt trigger is given by

Vt = ± (R1/R2)*Vs

Here R1 = 20K and R2 = 300K and Vs = 5V

Hence

Vt = ± (20/300)*5 = ±0.33V

Hope this helps