SKEE 2742 BASIC ELECTRONICS LAB

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1 Faculty: Subject Subject Code : SKEE 2742 FACULTY OF ELECTRICAL ENGINEERING : 2 ND YEAR ELECTRONIC DESIGN LABORATORY Review Release Date Last Amendment Procedure Number : 1 : 2013 : 2013 : PK-UTM-FKE-(0)-10 SKEE 2742 BASIC ELECTRONICS LAB EXPERIMENT 3 (BRIEF THEORY) OP-AMP

2 PART A : LINEAR OP-AMP APPLICATIONS : INVERTING SUMMING AND DIFFERENCE AMPLIFIERS BRIEF THEORY Op-Amp circuits employing negative feedback can be used in various configurations. Since in these applications there is a linear relation between input(s) and output, we usually refer to these application circuits as linear applications. Negative feedback produces bounded input-bounded output stability; i.e. a finite input voltage cannot produce an infinite output voltage. V CC V in V 1 V 2 A V out -V CC Figure 1 Figure 1 shows the schematic symbol of an op-amp. A is the voltage gain. The non-inverting input is V 1, and the inverting input is V 2. The differential output is given in equation 1 V out = A(V 1 V 2 )... (eq.1) Notice that V 1, V 2 and V out are node voltages. This means they are always measured with respect to ground. The differential input V in is the difference of two node voltages, V 1 and V 2. When the first operational amplifiers were constructed, their primary function was to perform mathematical operations in analog computers. These included summation, subtraction, multiplication, division, integration and differentiation. The summation circuit is also used to mix or combine analog signals together. a) Inverting Summing Amplifier Figure 2 shows an example of how an operational amplifier is connected to perform voltage summation. In this figure, an ac and a dc voltages are summed. In general,... (eq.2) 1

3 Note: pk = peak Figure 2 b) Difference Amplifier A difference amplifier has two inputs and the output voltage is proportional to the voltage difference of the input voltages. In fact, the (open-loop) Op-Amp itself is a difference amplifier, except that the gain is ideally infinity. Here we want a difference amplifier with finite gain. One such circuit using a single Op-Amp is shown in Figure 3. It can be shown that the gain of the difference amplifier can be calculated using the following:... (eq.3) Equation 3 can be simplified by making R 3 = R F = R 1 = R 2, yielding a simple differential amplifier with unity gain:... (eq. 4) 2

4 Figure 3 PART B : COMPARATOR AND SCHMITT TRIGGER BRIEF THEORY Voltage comparators are used in numerous applications in practice, including wave-shaping, waveform generation, interfacing between analog and digital circuits, controllers etc. There are in general three classes of voltage comparators: single threshold comparator (open loop comparator), two threshold with hysteresis (Schmitt Trigger), and two threshold without hysteresis (window comparator). In this experiment we will study the first two types. a) Comparator Single threshold voltage comparators compare two voltages and provide a voltage output that indicates which of the two voltages is higher. Most of the times a high gain op-amp operated open loop as shown in Figure 4 can be used as a comparator. When the non-inverting input (V in ) is slightly larger than the inverting input or the reference input (V ref ), the output goes to positive saturation; Otherwise it goes to negative saturation. For comparator in Figure 4, the reference voltage can be determined using voltage divider as in equation (eq. 5) 3

5 12 V V in V R 1 R 2 V ref V 6 V O Figure 4 The comparator circuit is characterised 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). The associated transfer characteristic for a comparator in Figure 4 (assuming Vref is a positive value) is shown in Figure 5. VO VOH Vref Vin -VOL Figure 5 Generally, If... (eq. 6) If... (eq. 7) 4

6 b) Schmitt Trigger Because of the sensitivity to a small input change, the output of a comparator may change due to noise on the input when the input changes very slowly. To avoid this, hysteresis is added to the comparator circuit by introducing positive feedback. The circuit is called Schmitt Trigger, has two switching threshold one for rising input voltage, the other for a falling input. By separating the two threshold, noise effects such as false triggering can be eliminated. In other word, the Schmitt trigger is a voltage comparator with positive feedback, as opposed to the open loop (no feedback) comparator. The input values that cause the output change are usually called threshold, transition or firing voltages. The distance between the threshold voltages is called the hysteresis. Schmitt triggers using Op-Amps can be configured in two ways; inverting and non inverting. A typical inverting Scmitt trigger circuit is shown in Figure 6(a) with associated transfer characteristic shown in Figure 6(b). 12 V V in 2 7 VOH V 6 V O VLT VUT Vin R 2 R 1 -VOL Vref Figure 6(a) Figure 6(b) The upper threshold point can be calculated from equation 8,... (eq. 8) The lower threshold point can be calculated from equation 9,... (eq. 9) Assuming the output is initially at V sat, then the reference level is V UT and the output will switch to Vsat only when the input rises above the V UT ; that is when V > V then V o = V sat. When the output switches to V sat, then the reference level is no longer V UT, but changes to V LT. Hence, the output will switch to V sat only when the input drops below the V LT ; that is, when V < V then V o = V sat. As a result, the output V o changes the state only when the input signal rises above the upper threshold or drops below the lower threshold, and does not respond to changes of V in within the boundaries of the two threshold levels. Because of this important features, the Schmitt triggers has some advantages over the simple single op-amp comparator and is widely used in certain applications, where the input signal is likely to be affected by spurious noise, which is to be ignored by the comparator. The transfer characteristics of the Schmitt trigger exhibit the hysterisis loop, as illustrated in Figure 6(b). 5

7 REFERENCES th 1. Boylestad, R and Nashelsky. (2006). Electronic Devices and Circuit Theory, 9 Edition, Prentice Hall. 2. Floyd, Thomas L. (2005). Electronic Devices, 7 th Edition, Prentice Hall. 3. Paynter, R.T. (2003). Introductory Electronic Devices and Circuits, 6 Hall. th Edition, Prentice 4. Neaman, D.A. (2001). Electronic Circuit Analysis and Design, 2 Edition, Mc Graw Hill. 5. Aminian, A. & Kazimierczuk, M. (2004). Electronic Devices: A Design Approach, Pearson International Edition. 6. Horenstein, M.N. (1996). Microelectronic Circuits and Devices, 2 Edition, Prentice Hall. 7. Fleeman, S.R. (1990). Electronic Devices Discrete and Intergrated, Prentice Hall. nd nd 6