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Schmitt Trigger and Relaxation Oscillator

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Schmitt Trigger and Relaxation Oscillator Lab 1: Schmitt Trigger and Relaxation Oscillator UC Davis Physics 116B Rev 1/06/2020 Introduction V CC + Vin V V - out th V EE Figure 1: An open loop op-amp as a comparator An open loop op-amp can be thought of as a comparator, as shown in Figure 1, with the following behavior: Vin > Vth ! Vout = VCC Vin < Vth ! Vout = VEE As we discussed in class, this circuit can be unstable for inputs near the threshold. A Schmitt Trigger uses feedback to change the threshold depending on the output state, in order to improve its stability. Schmitt Trigger The Schmitt trigger circuit is shown in Figure 2. Build this circuit using an LM741, with ±15V power supplies. Use R1 = R2 =10kΩ, and R3=100kΩ. Use the fixed 5V supply for Vbb. Drive the circuit with a 1kHz triangle wave, with a 20V peak-to-peak amplitude. Verify the transition threshold for both the rising and filling slopes and verify that they match the prediction of: R1jjR2jjR3 R1jjR2jjR3 Vth = Vbb + Vout R2 R3 For your lab report, include images of the scope traces, including a close up on both the rising and falling transition. 1 Lab 1: Schmitt Trigger U.C. Davis Physics 116B (Rev. 05) INTRODUCTION V = 15V Like the Wien bridge oscillator in 116A, the bb Schmitt trigger is an application of positive feedback. This circuit is a voltage comparator R with hysteresis. A voltage comparator will give 2 +15V as its output one of two voltages: V 2 _ 7 in +15V for V > V 6 V � in th 3 + out Vout = � 4 �-15V for Vin < Vth -15V where Vth is a threshold voltage. A voltage comparator with hysteresis will have a different R V for rising input voltages than for falling 3 th R input voltages. 1 In this lab, we will build a Schmitt trigger using the 741 op amp, our analog circuit Fig 1: Schmitt trigger circuit. workhorse. We will measure Vth and verify the circuit's operation as a voltage comparator. 1. SCHMITT TRIGGER For each set of resistor values, apply a sine wave or a triangle wave to the input and sketch The Schmitt trigger circuit is shown in figure the output. For your lab report, sketch these 1. Build this circuit at least twice, each time waveforms, showing the input voltages where 15V with a different set of resistors. Some the + output switches and compare these to the recommended values are 10kΩ, 10kΩ, 100kΩ; or 2 V 7's you measured with the voltmeter. V th 10kΩ, 10kΩ, 1MΩ; or 10k Ωin, 22kΩ, 100kΩ. - 6 V Use the variable voltage supply for and out Vin 2. SIMULATED NOISY SOURCE V + measure it with the voltmeter. bb Measure Vout 4 R 3 To see how a Schmitt trigger might be useful, with the oscilloscope and note how2 fast the we − 1will5V construct a source with some fake noise output switches from to . For each +15V −15V introduced into it. Figure 2 shows how to do set of resistors, record V for both positive and th Rthis using two function generators and a second negative transitions. See how close they are to 3 R op amp, used as a summing amplifier. Build the predicted values: 1 this circuit and use it as the input to your RR|| ||R RRR| | || Schmitt trigger. Regard the 1 kHz sign wave as V =+123V 123V th R bb R out (1) the "signal" and the 10 kHz sine wave as the 2 3 "noise". For your lab report, sketch the input For your lab report, report these measuredFigure a 2:nd A Schmittand Trigger output waveforms, being careful to show predicted voltages and draw them on a Vout vs Vin each time the input crosses a Vth and what the curve showing the hysteresis loop. (Make a output does in response. Try to determine separateSimulated drawing for Noisy each set Sourceof R's.) approximately how much hysteresis is required to filter out a given amount of noise. large amplitude R 1 kHz sine wave R (the signal) _ to Schmitt trigger Vout + small amplitude R 10 kHz sine wave (the noise) Fig 2: Simulated noisy source. Figure 3: A simulated noise source. To see how a Schmitt trigger might be useful,22 we will construct a source with some fake noise introduced into it. Figure 3 shows how to do this using two function generators and a second 741 op-amp, used as a summing amplifier. Build this circuit and use it as the input to your Schmitt trigger. Make all resistors equal, with values of a few kΩ. Regard the 1kHz sign wave as the “signal” and the 10 kHz sine wave as the “noise”. For your lab report, include a picture of the scope trace showing both the input and the outpu, being careful to show each time the input crosses a Vth and what the output does in response. Determine the amplitude of the “noise” signal that will cause multiple transitions on the rising edge of the signal. Include an image of this in your report. 2 R F +15V VF - CF V out V T + −15V R 2 R 1 Figure 4: Relaxation oscillator. Relaxation Oscillator Construct the relaxation oscillator shown in Figure 4. Use R1 = R2 = RF = 10kΩ and CF =10nF. Verify that the period of the oscillation is 2R1 + R2 T = 2RF CF ln R2 as we derived in class. For your lab report, include a picture of the output waveform. 3 Appendix: LM741 Pinout 4.
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