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# U of M ECE 2006 - ASTABLE MULTIVIBRATOR

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Find the square wave’s period for a frequency of 250Hz:ECE 2006 University of Minnesota Duluth Lab 6ASTABLE MULTIVIBRATOR INTRODUCTIONThe student will analyze and construct an astable multivibrator. The circuit will be simulated using nominal component values selected by the student to providefor specific operations. The circuit component parameters will be measured. And theoretical calculations of the multivibrator’s performance will be made. The multivibrator will be constructed to observe actual perfomance.BACKGROUNDA multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip flops. An astable multivibrator has two states, neither one stable. The circuit therefore behaves as an oscillator with the time spent in each state controlled by the charging or discharging of a capacitor through a resistor.The astable multivibrator may becreated directly with transistors or withuse of integrated circuits such asoperational amplifiers (op amps) or the555 timer. Most operational amplifiersare powered by a positive and negativerail voltage, the output never able toexceed these rail voltages. Dependingupon inital conditions, the op amp’soutput will drive to either positive ornegative rail. Upon this occurance, thecapacitor will either charge or dischargethrough the resistor R2, its voltageslowly rising or falling. As soon as thevoltage at the op amp’s invertingterminal reaches that at the non-inverting terminal (the op amp’s outputvoltage divided by R1 and R2), the output will drive to the opposing rail and this process will repeat with the capacitor discharging if it had previously charged andvice versa. Once the inverting terminal reaches the voltage of the non-inverting terminal the output again drives to the opposing rail voltage and the cycle begins again. Thus, the astable multivibrator creates a square wave with no inputs. Period of astable multivibrator displayed in Figure 1: )21ln(21231RRRCT A.Dommer Page 1 January 2005Figure 1: Astable MultivibratorFigure 2:uA741 Pin LayoutECE 2006 University of Minnesota Duluth Lab 6THEORETICAL PROCEDUREFind the square wave’s period for a frequency of 250Hz:T = _______________________Find values for R1, R2, R3 and if C1 is 0.1uF (Let R1= 10R2= 100kOhms) that should produce a square wave of frequency 250Hz.R1 = ___________________ R2 = ____________________R3 = ___________________ C1 = ____________________Select components with nominal values as close as possible to those calculated in the prelab. Measure the resistor impedances with the digital multimeter and the capacitor’s capacitance with the LCR meter.R1 = ___________________ R2 = ____________________R3 = ___________________ C1 = ____________________Determine expected amplitude and frequency using the measured component values. Assume a power supply voltage of +/- 12 volts.Amplitude = _____________ Frequency = _____________EXPERIMENTAL PROCEDUREConstruct the astable multivibrator displayed in Figure 1using a uA741 operational amplifier and the componentsselected in the theoretical procedure.The operational amplifier should be powered with railvoltages of +/- 12 Volts using the DC power supply. Setthe power supply before connecting to the operationalamplifier. Do not connect the DC power supply to thecircuit with the current on. Turn the power on andobserve the operation of the multivibrator. Record theamplitude and frequency of the square wave.Amplitude = _____________ Frequency = _______________Save the oscilloscope screenshot for use in lab reportA.Dommer Page 2 January 2005ECE 2006 University of Minnesota Duluth Lab 6SIMULATED PROCEDUREConstruct a PSPICE schematic of the astable multivibrator displayed in Figure 1 using measured component values, part uA741 as the operational amplifier, DC power sources. Set the initial condition (IC) of the capacitor to 0.001 volts.Double click on the output wire to and label ‘Output’. In a similar fashion, label the inverting terminal of the operational amplifier ‘Capacitor’.Simulate the astable multivibrator in transient (time-domain) mode and add traces of both the output and the capacitor voltages.Record amplitude and determine the frequency of the square wave:Amplitude = ______________ Frequency = _____________Save a schematic screenshot and a screenshot of the transient analysis for display in lab report.Modify the initial condition of the capacitor to -0.001 volts.Simulate the astable multivibrator in transient mode and add traces of both the output and capacitor voltagesSave a screenshot of the transient analysis for use in lab reportQUESTIONSDisplay the frequency and amplitude for the theoretical, experimental and simulated portions of the lab in a table. Describe any distinctions between thetheoretical, experimental and simulated results.What differences can be noticed between the experimental and simulation waveforms? Think in terms of the wave shape, noise, rise and fall times, etc.What difference is noticed in the transient analysis when the initial condition of the capacitor is modified slightly to a negative voltage? Why does this change occur?What could have affected the initial condition of your experimental circuit, assuming that the capacitor had no initial charge across it.A.Dommer Page 3 January

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