SKEU 3741 BASIC ELECTRONICS LAB

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1 Faculty: Subject Subject Code : SKEU 3741 FACULTY OF ELECTICAL ENGINEEING : 2 ND YEA ELECTONIC DESIGN LABOATOY eview elease Date Last Amendment Procedure Number : 1 : 2013 : 2013 : PK-UTM-FKE-(0)-10 SKEU 3741 BASIC ELECTONICS LAB EXPEIMENT 3 OP-AMP

2 PAT A : LINEA OP-AMP APPLICATIONS : INETING SUMMING AND DIFFEENCE AMPLIFIES OBJECTIES : To demonstrate the use of Operational Amplifier for performing mathematical operations summation and difference. PATS AND EQUIPMENT: DC Power Supply Oscilloscope Function Generator LM 741 Op-amp esistors BACKGOUND 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. 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 input is v in A v i v 2 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, 1

3 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 voltage are summed. In general, O f f = 47 kω...etc. F = 100 kω = 100 kω O S = 1 pk 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 4. It can be shown that the gain of the difference amplifier can be calculated using the following: f 3 O This equation can be simplified by making 3 = F differential amplifier with unity gain: O f 1 1 = 1 = 2, yielding a simple 2

4 PEPAATION/PE-LAB ASSIGNMENTS/SIMULATIONS 1. For the Inverting Summing Amplifier circuit of Figure 2, (i) derive the output equation. (ii) generate the output voltage waveform using Multisim if 1 is a sine wave or a square-wave of 1 olt peak, and 2 is a DC voltage of 5. Print out the curve plot as a function of amplitude against time. 2. For the Difference Amplifier circuit of Figure 3, (i) derive the output equation. (ii) generate the output voltage waveform using Multisim if 1 is a sine wave or a square-wave of 1 olt peak, and 2 is a DC voltage of 5. Print out the curve plot as a function of amplitude against time. POCEDUES Part 1 : Inverting Summing Amplifier 1) To demonstrate the use of an operational amplifier as a summing amplifier, connect the circuit of Figure 2. 2) With S adjusted to produce a 1 peak sine wave at 1 khz, observe the output voltage O (and S to note the phase relationship) on an oscilloscope set to dc input coupling. 3) Sketch the output voltage waveform. Be sure to note the dc level in the output. 4) Interchange the 5 dc power supply and the 1 peak signal generator. epeat procedure step 2. 3

5 Part 2 : Difference Amplifier 5) To investigate the use of an operational amplifier in a difference amplifier configuration, connect the circuit of Figure 3. 6) With S adjusted to produce a 1 peak sine wave at 1 khz, observe the output voltage O (and S to note the phase relationship) on an oscilloscope set to dc input coupling. 7) Sketch the output voltage waveform. Be sure to note the dc level in the output. 8) Interchange the 5 dc power supply and the 1 peak signal generator. epeat procedure step 7. 4

6 PAT B : COMPAATO AND SCHMITT TIGGE OBJECTIES : After completing this experiment, you will be able to 1. Compare the input and output waveforms for comparator and Schmitt Trigger circuits 2. Use an oscilloscope to plot the transfer curve for a comparator circuit, including one with hysteresis. PATS AND EQUIPMENT: DC Power Supply Oscilloscope Function Generator LM 741 Op-amp esistors Protoboard BACKGOUND oltage comparators are used in numerous applications in practice, including waveshaping, 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 Fig 2a can be used as a comparator. When the noninverting input is slightly larger than the inverting input, the output goes to positive saturation, otherwise it goes to negative saturation. A 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 is shown in Fig 2b. 5

7 o CC 3 OH 7 I + 6 LM 741 T T 2-4 i Generally, -CC Figure 2a OL Figure 2b If If + > -, o = + sat = CC (Ideal) + < -, o = - sat = - CC (Ideal) From Fig 2a and 2b, If If ( + = i ) > ( - = T ), o = + sat = OH ( + = i ) < ( - = T ), o = - sat = OL 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 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 circuit is shown in Fig 2c with associated transfer characteristic shown in Fig 2d. 6

8 I - OH O + O FI F2 I 2 1 OL r The upper threshold point can be calculated from = = 2 ( ) + 1 ( ) UTP F2 2 1 sat 2 1 r The lower threshold point can be calculated from LTP =F1 = 2 ( sat ) + 1 ( r ) The output state is given by in > UTP, o = + sat = OH in < LTP, o = - sat = OL 7

9 PEPAATION/PE-LAB ASSIGNMENTS/SIMULATIONS Comparator 15 in 15 EF 741 Sig KΩ O -15 Figure 4 a) Simulate the circuit shown in Figure 4, in Multisim or of any equivalent software. The purpose of potentiometer is to set a variable reference voltage between + CC and CC that can be applied as EF to the op-amp. Note that changing the setting (%) of the potentiometer changes the voltage on the associated terminal. Set the signal generator ( sig ) for a 3 pp triangle waveform at 1 KHz. Set the value of EF for several positions of the potentiometer. Select both positive and negative values of EF. Print out the sig and out as a function of time waveforms for 5 different combinations of potentiometer settings. Choose 3 positive values of EF (including EF = 0) and 2 negative values of EF. b) Print out the tranfer curve (o vs sig) for the comparator by clicking B/A on the oscilloscope. Choose 3 values of EF including the one that will produce a square wave at the output. Mark the transition points ( sig that causes out to change state) and record the reference voltage ( ref ). 8

10 POCEDUES The Schmitt Trigger O sig KΩ 2 47KΩ + 5 Figure 5 i) Construct the Schmitt Trigger circuit shown in Figure 5. Drive the circuit with 1 KHz sine wave, beginning with an amplitude of a few tenths of a volt, and observe the output waveform on an oscilloscope. The input coupling of both channels should be set to DC.Try increasing the input signal amplitude, and record the observed output waveforms for different values of input. Prepare a table (Table 1) that shows sig and o for 3 different values of input ( sig ). Plot the waveforms on the same axis. Be sure to note the levels of sig at which the output voltage o changes levels (UTP, LTP). ii) In this step, you will plot the tranfer curve for the comparator on the oscilloscope. Place sig on channel 1 and out on channel 2. Set the olts/div control so that both signals are on the screen. Neither channel should be inverted. Then switch the oscilloscope to the XY mode. ary the input signal amplitude and sketch (and label) the transfer curve you see. Prepare a table (Table 2) that shows sig vs out for 3 different values of input ( sig ). Identify the transition points (UTP, LTP). 9

11 EFEENCES 1. Boylestad, and Nashelsky. (2006). Electronic Devices and Circuit Theory, 9 th Edition, Prentice Hall. 2. Floyd, Thomas L. (2005). Electronic Devices, 7 th Edition, Prentice Hall. 3. Paynter,.T. (2003). Introductory Electronic Devices and Circuits, 6 th Edition, Prentice Hall. 4. Neaman, D.A. (2001). Electronic Circuit Analysis and Design, 2 nd Edition, Mc Graw Hill. 5. Horenstein, M.N. (1996). Microelectronic Circuits and Devices, 2 nd Edition, Prentice Hall. 6. Fleeman, S.. (1990). Electronic Devices Discrete and Intergrated, Prentice Hall. Prepared By Camallil Omar Abd Hamid Ahmad 10