Lab 6: Oscillators using Op-amps - Phase-shift and Multivibrator, Lab Reports of Electrical and Electronics Engineering

Download Lab 6: Oscillators using Op-amps - Phase-shift and Multivibrator and more Lab Reports Electrical and Electronics Engineering in PDF only on Docsity! EE 429L Lab 6 Oscillators using Op-amps (1week) 1st Dec. 2009 Remember to get your circuits verified before you break them down. Compare results with your SPICE simulations in your lab report. Also record ALL key voltages and currents and print out all relevant plots from the oscilloscope. Answer all the questions in the lab. Required Parts Various Resistors & capacitors Transistors – NPN 2N2222 and PNP 2N2907 Diodes, 1N914 or equivalent Op-amps, 741 or equivalent Required Equipment Oscilloscope Function generator Power supplies Potentiometer Multi-meter Breadboard Objective To study the operation and characteristics of phase-shift and multivibrator oscillators based on op-amps. Phase-shift oscillators In general, oscillator occurs in a circuit when there is positive feedback from the output to the input, i.e., a phase shift of 0o (or 360o), and simultaneously the overall gain of the circuit is equal to or greater than one. For a circuit to oscillate at a single frequency, i.e., for a sine-wave oscillator, this condition should occur at only one frequency. A network consisting of a capacitor and a resistor in series has a phase that varies with frequency. The largest phase shift that can be obtained with one capacitor and one resistor is less than 90o. However, with three identical RC networks, one after the other, it is easy to obtain an 180o-phase shift at some particular frequency. We can then combine this with an inverting amplifier for a total phase shift of 360o. The analysis is a little complicated, because the three different RC networks load each other. It turns out that the total attenuation of all three networks is a factor of 29, so that in order to oscillate they must be combined with an amplifier that has at least that gain. The frequency at which this occurs is given by: (1) Careful: Do not confuse this formula with that for a 45o-phase –shift RC network, which does not contain a factor of 6 . Procedure 1. Design an inverting op-amp circuit with a gain of about 60 (Figure 1). Then using equation (1), design a three-stage RC filter network which has a gain of 1/29 and a phase shift of 180o at a frequency near 1KHz. Note that R2 should be greater than R. Why? Construct RCo f 62 1   these and connect them as shown in Figure 1. What values did you chose for the resistors, capacitors and supply voltages? Apply a signal to the input and observe the signal at point D, as a function of frequency. Point A is 180o out of phase with the input. At what frequency is point D in phase with the input? What is the overall gain, from the input to point D, at his frequency? Careful: The gain at the output of the circuit, at the center tap of the potentiometer R, in general is smaller than the gain at point D. 2. Adjust the potentiometer R so that the overall gain to the “output” terminal shown in Fig. 1 at the frequency found in step 1 is about 1. Connect the output to the input. Does the circuit oscillate? Readjust the potentiometer R so that the circuit just barely oscillates. 3. Compare the theoretical frequency and the nominal attenuation of 29 in the RC network with the values experimentally obtained. Why should there be differences? 4. Set a function generator to approximately the same frequency as that at which the circuit oscillates. Apply a very small signal, about 10-mV p-p, to the “sync” input of the oscillator. This will cause the oscillator to synchronize, or lock into the same frequency as the signal generator. Over how large a range in frequency can you “pull” the oscillator so that it remains in synchronism with the signal generator? You can also lock on to multiples, or harmonics of the frequency, especially with a larger input signal. Multivibrator Op-amps can also be used in an entirely different type of oscillator called an unstable multivibrator or Schmitt-trigger oscillator. In this case the frequency of the oscillation is set by time constants and the operation is more like that of a digital circuit. Procedure 1. Consider the circuit in Figure 2. Note that there is both positive and negative feedback. The positive feedback acts quickly, since it is generated by resistors. The negative feedback is delayed by the time constant of a resistor and capacitor. However, it will eventually overcome the positive feedback. Since it has a large voltage swing. The nominal frequency of this circuit is given by: