VOLTAGE COMPARATORS WITH OPERATIONAL AMPLIFIERS - HYSTERESIS COMPARATORS

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1 OLTAGE COMPAATOS WTH OPEATONAL AMPLFES - HYSTEESS COMPAATOS. OBJECTES a) Determining the oltage transfer characteristics (TC) for hysteresis comparators. b) Determining the output oltage in accordance with the configuration of the circuits and the input oltage. c) Determining the effects of modifying the supply and reference oltages on the TC of hysteresis comparators. d) Deduction of the noise effect, oerlapped on the input oltage on the comparator s switchings.. COMPONENTS AND NSTUMENTATON Use the experimental board equipped with two operational amplifiers 74, a 0KΩ potentiometer, resistances of different alues and a 0nF capacitor. n order to supply the assembly, you will use a double dc regulated source, and as a sinusoidal signal source you will use a signal generator. n order to isualise the oltages, you need a dual channel oscilloscope and for some dc oltages you need a dc oltmeter.. THEOETCAL ASPECTS Comparators with Hysteresis Simple comparators, without feedback hae two shortcomings: for input signal with slow ariations, the output switching from one alue to another can be slow as well; if the input signal has, as it usually does hae, noise superimposed on it (with a frequency much higher than that of the signal), the signal might cross the threshold alue a number of times around each threshold-crossing. At the output, there will be multiple unwanted switching, instead of only one at each signal threshold-crossing. The unwanted switching can be aoided if two threshold alues depending on the output oltage alues are introduced. f the two threshold oltages are denoted by ThH (the high threshold) and ThL (the low threshold), the difference between these two threshold Δ Th ThH ThL must be greater than the peak-to-peak alue of the noise. At a certain moment there is (actie) only one threshold oltage, depending on the output oltage alue. We name such a circuit a comparator with hysteresis. Since eery threshold alue depends on one of the two possible alues of the output oltage, we must find a way to feed back the output oltage to the input of the comparator. The used technique is called positie feedback and this means to bring a fraction of the output oltage back to the noninerting input of the op amp through a resistie diider.. nerting Comparator with PF The circuit of an inerting comparator with hysteresis looks like in Fig..a) and the TC looks like in Fig..b). The potentials at the op-amp inputs are (op-amp inputs do not draw any current):

2 _ O O OH D - PS _ O 4 ThL 0 ThH - PS a) from the output oltage O is brought to the noninerting input of the op amp. To know the output state we must determine D: D O O Th 0, Th O Suppose that has a negatie alue, low enough to hae D>0. n this case O OH, and we are in the point on the TC in Fig..b). Therefore, we hae: Due to the positie feedback, through and, a fraction ( ) D Assume now that increases, so D decreases getting close to zero. The moment when D crosses through zero towards negatie alues, the op-amp output switches. The alue of for which D0 between the points and on the TC, is the ThH threshold alue: ThH OH f continues to increase, > ThH, D<0 and O OL (we are for example in point 3 on the characteristic). We consider now that starts to decrease, which means an increase towards zero of D. The moment when D crosses through zero towards positie alues, the output of the op amp switches oer. The alue of for which D0 (between the points 4 and 4 on the TC) is the ThL threshold oltage: OH ThL OL f continues to decrease, D<0 and O OH (we are for example in point on the characteristic). We proed that the op-amp circuit with positie feedback in Fig..a) behaes as a comparator with hysteresis with the TC presented in Fig..b). The hysteresis is centered on zero, the width of the hysteresis being: Th ThH 4 OL b) Fig.. nerting comparator with hysteresis: a) circuit; b) TC. ThL ( ) The direction of going oer the hysteresis cure is clockwise. Note that at a certain moment only one threshold is actie. To center the hysteresis on a different from zero alue, a reference oltage is applied to the noninerting input. OH OL 3

3 . Noninerting Comparator with PF The circuit of a noninerting comparator with hysteresis is depicted in Fig..a) and the TC is depicted in Fig..a). O OH - PS O ThL 0 ThH PS a) b) Fig.. Noninerting comparator with hysteresis: a) circuit; b) TC. The threshold oltages are determined for D D 0, when becomes ThL O 0 ThL OH O Th Th O 0 ; ThH OL, respectiely ThH. emark that for the noninerting comparator the low threshold alue ThL is obtained for the high alue of the output oltage OH and ice-ersa, ThH is obtained for OL. The hysteresis is centered on 0 haing the width Th : Th ThH ThL To center the hysteresis on a different from zero alue, a reference oltage is applied to the inerting input. OL ( OH OL ). PEPAATON.P. NONNETNG COMPAATO For the schematic of Fig. 3 the following data is gien: PS, - PS-. Which is TC for the non-inerting comparator with positie feedback? What does O(t) look like for (t) sinusoidal oltage with 3 amplitude and 00Hz frequency? What happens if the amplitude of is 8? 3

4 .P. NETNG COMPAATO Use the schematic of Fig. 4. Which are the expression of the threshold oltages Th,L and Th,H? What are the alues of Th,L and Th,H for EF 0, PS, - PS-? Which is the TC O( )? What is the sense of moement on the hysteresis cure? What is the width of the hysteresys cure Δ Th Th,H- Th,L? What are the effects of modifying the supply oltage oer TC? What are the effects of modifying EF oer TC? What does O(t) look like when 8 sin π 00t [][HZ], for the aboe data? What happens if the amplitude of is? 3.P. NETNG COMPAATO FO NOSY SGNAL Use the schematic in Fig. 5. We want to study the effect of the noise oerlapped on the input oltage, on the switches of the comparator without feedback. n Fig. 5 the input signal of the comparator ( ) is obtained by summing the sinusoidal input oltage ( s), with 0 amplitude and 00Hz frequency with a triangular signal n (considered to be the noise) with. amplitude and.7khz frequency. The summing is made using the and resistors. The noise oltage is generated by a multiibrator circuit (framed by the marked area). What do s(t), n(t), (t) look like? What does O(t) look like?. NONNETNG COMPAATO Use the circuit shown in Fig. 3.. EXPLOATONS AND ESULTS Supply the assembly with PS, - PS-. is a sinusoidal oltage with 8 amplitude and 00Hz frequency from the signal generator. Using the calibrated oscilloscope, you will see (t), O(t) and TC O( ). esults (t), O(t). TC Al 5K 0K O - Al Fig. 3. Noninerting comparator with positie feedback 4

5 . NETNG COMPAATO Use the experimental schematic from Fig. 4. PS, - PS- from the dual dc regulated power supply. Adjust EF 0, using P. 8sin π 00t[][Hz] from the signal generator. Using the oscilloscope, Y-t mode, isualize (t) and O(t). Using the oscilloscope, Y-X mode isualize TC O( ), applying the oltages and O on the X and Y inputs of the oscilloscope. Modify the amplitude of to. Using the oscilloscope, Y-t mode, isualize (t) and O(t). esults (t) and O(t) for a amplitude of 8 and. TC O( ) when the amplitude of is 8. What are the alues of the threshold oltages Th,H and Th,L, and of the output oltage OH and OL? A. The effects of modifying the oltage supply Modify the supply oltages to PS9, - PS-9. Using the oscilloscope isualize (t), O(t), then TC O( ) for EF 0 and amplitude of 8. Modify the supply oltages to PS5, - PS-9. isualize (t), O(t) and TC. esults (t), O(t), O( ) for PS9, - PS-9., and for PS5, - PS-9. B. The effects of modifying EF Supply the assembly with PS, - PS-. Adjust P until EF 3. Using the oscilloscope, isualize (t), O(t) and O( ). Set EF -3. Using the oscilloscope isualize (t), O(t) and O( ). 5

6 esults (t), O(t), O( ) for EF 3 and for EF -3 Al P 0K EF 5K 0K O - Al Fig. 4. nerting comparator with positie feedback 3. NETNG COMPAATO FO NOSY SGNAL Use the circuit shown in Fig. 5. Supply the assembly with PS, - PS-. Using the oscilloscope, isualize and O at the same time. esults (t), O(t). Al 0K K 4 Al 5 0nF C - Al 3 K8 n 0K 5K 5K 0K O s - Al Fig. 5 nerting comparator with positie feedback and noisy signal 6

7 EFEENCES. Oltean, G., Electronic Deices, Editura U.T. Pres, Cluj-Napoca, SBN , 006. Sedra, A. S., Smith, K. C., Microelectronic Circuits, Fifth Edition, Oxford Uniersity Press, SBN: ,