UNIVERSITI MALAYSIA PERLIS

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1 UNIVERSITI MALAYSIA PERLIS ANALOG ELECTRONICS II EMT /2010 EXPERIMENT # 3 OP-AMP (OSCILLATORS) 1

2 1. OBJECTIVE: 1.1 To demonstrate the Wien bridge oscillator 1.2 To demonstrate the RC phase-shift oscillator 2. INTRODUCTION: Oscillators are circuits that spontaneously generate a periodic output voltage due to positive feedback. Two important types of sinusoidal oscillators are the Wien bridge and the RC phase-shift oscillator. An operational amplifier is ideal for use in oscillator circuits because of its large input impedance, large gain, and the ease with which positive feedback can be introduced around it. The positive feedback required for oscillation is specified by the Barkhausen criterion: (i) the total gain from input to output and back through the feedback circuitry must equal at least one, and (ii) the total phase shift from input to output and back through the feedback circuitry must equal 0, or a multiple of Wien bridge Oscillator A Wien bridge oscillator is shown in Figure 4.1. It may be regarded as a bridge whose two branches are the resistive voltage divider across the inverting terminal and the reactive voltage divider across the non-inverting terminals of the operational amplifier. The circuit oscillates at the frequency at which the ac voltages at the two input terminals are equal. If R 1 and R 2 (see Figure 4.1) are made equal-valued resistors and C 1 and C 2 are made equal-valued capacitors, then the ratio of R F to R in must be 2:1 to satisfy the Barkhausen criterion. The oscillation frequency for the Wien bridge oscillator, given these stipulations, can be calculated from: f 1 2 RC (2.1) where R = R 1 = R 2, C = C 1 = C RC phase-shift Oscillator An example of an RC phase-shift oscillator is shown in Figure 4.2. The RC phaseshift oscillator uses three cascaded stages of RC high-pass filters, with the output of the last stage fed back to the inverting input of the operational amplifier. The purpose of the RC filters is to provide a phase shift of 180. Since the output of these filters is fed back to the inverting terminal, the amplifier itself provides another phase shift of 180. The total phase shift of the circuit is therefore 360 or 0. Given the stipulation that R 1, R 2, and R 3 are equal-valued resistors, and that C 1, C 2, and C 3 are all equalvalued capacitors, the oscillation frequency of the RC phase-shift oscillator can be calculated using the following equation: 1 f (2.2) 2RC 6 2

3 where R = R 1 = R 2 = R 3, C = C 1 = C 2 = C 3 This equation is exact only if the input resistor on the inverting terminal (100 k in Figure 4.2) is large enough to prevent any loading of the cascaded RC stages. 3. COMPONENT AND EQUIPMENT: 3.1 Resistors: M k k (2) kω k (3) (3) 3.2 Potentiometers: k k 3.3 Capacitors F (5) F (3) F (25V) (2) 3.4 LM 741 OP-AMP 3.5 DC Power Supply 3.6 Function Generator 3.7 Breadboard 3

4 4. PROCEDURE: 4.1 To demonstrate the Wien bridge oscillator, connect the following circuit in Figure k 1 k 1k 2 3 _ V V C 2 R 2 V out C 1 R 1 GND Figure 4.1 : Wien Bridge Oscillator Connect an oscilloscope set to ac input coupling so that V out can be viewed. With R 1 = R 2 = 10 k and C 1 = C 2 = 0.1 F, carefully adjust the 1 k potentiometer until the output waveform has the least amount of distortion. Measure and record the frequency of this waveform in Table Repeat procedure step using values in Table To demonstrate the RC phase-shift oscillator, connect the following circuit in Figure k 10 k 2 3 _ V C 1 C 2 C 3-15V V out R 1 R 2 R 3 GND Figure 4. 2 : RC phase-shift oscillator 4

5 4.2.1 Connect an oscilloscope set to ac input coupling so that V out can be viewed. With R 1 = R 2 = R 3 = 1 k and C 1 = C 2 = C 3 = 0.22 F, carefully adjust the 500 k potentiometer until the output waveform has the least amount of distortion. Measure and record the frequency of this waveform in Table Repeat procedure step using values in Table 2. 5

6 EXPERIMENT # 3 OP-AMP (OSCILLATORS) NAME : MATRIX NUMBER : PROGRAMME : DATE : TOTAL MARKS : MARKS T1 T2 Q C Total 100%

7 RESULTS: TABLE 1 R 1 = R 2 (ohms) C 1 = C 2 (microfarad) f (Hz) 1 f 2 RC (pre-calculate) 0.1 F 10 k 0.22 F 0.47 F 0.1 F 1 k 0.22 F 0.47 F 12 marks R 1 = R 2 = R 3 (ohms) 1 k 560 C 1 = C 2 = C 3 (microfarad) 0.22 F 0.47 F 0.22 F 0.47 F TABLE 2 1 f f (Hz) 2RC 6 (pre-calculate) 8 marks 7

8 QUESTIONS: 1. What are the basic components of an oscillator circuit? 2. What are the requirements of an oscillation? 3. How to vary the frequency of Wien bridge oscillator? 4. Based on the circuit in Figure 4.1, design a Wien bridge oscillator which will oscillate at a frequency of 10 khz. 5. Based on the circuit in Figure 4.2, design an RC phase-shift oscillator which will oscillate at a frequency of 10 khz. CONCLUSION (5 marks) Based on your experiment, make an overall conclusion for oscillator. 8