Laboratory Design Project II

I forgot to bring home the book to use in completing the writeup for the lab. I have used a question mark in the following for numbers that I do not remember. Use the book to find the right number. I hope this does not cause any confusion.

The object of this experiment is to assemble, evaluate, and simulate a free-running multivibrator and a triangular/square-wave generator. You can read the textbook's treatment of the circuits in sections 10.2 and 10.4 of the text.

For all of the circuits, you should have 100 μF decoupling capacitors from each power supply rail to circuit ground. You should be aware that these capacitors are polar electrolytics which can explode if they are put in with the wrong polarity.

Op amps can oscillate when equipment such as an oscilloscope is connected to a circuit. This is caused by the capacitance of the connecting leads. To minimize these problems, clip a 100 Ω resistor in series with the signal lead to the oscilloscope. Use the other end of the resistor to connect to the proto board.

The 741 op amp has a fairly low slew rate and is not an appropriate op amp for this experiment. You should use one of the BiFET op amps such as the TL071, TL081, LF351, etc. These have a slew rate that is a little more than 10 times that of the 741.

Part One

1. Assemble the circuit in Fig. 10.7 omitting the resistor R and capacitor C.
• Connect the output of the function generator to the negative (-) input to the op amp and the oscilloscope to its output. The op amp is connected as a Schmidt Trigger comparator.
• Connect the x input of the oscilloscope to the circuit input and the y input of the oscilloscope to the circuit output.
• Switch the operation of the oscilloscope to the x-y mode, and set the input coupling of both inputs to dc coupling.
• With power applied to the circuit, apply a triangular wave to the negative (-) input of the op amp.
• Increase the function generator output to 10 V peak at a frequency of 30 Hz. The waveform displayed on the oscilloscope is a plot of output voltage versus input voltage. In SPICE, you would display this using a dc sweep.
• Read from the oscilloscope the values of the input voltage that causes the output of the op amp to switch between its two stable states. Verify that these values are the same that are predicted theoretically. Record the oscilloscope waveform.
2. Power down the circuit and replace the 10 kΩ resistor with a 5.1 kΩ resistor in series with a 10 kΩ potentiometer connected as a variable resistor. The resistance of the combination can be varied from 5.1 kΩ to 15.1 kΩ.
• Apply power to the circuit.
• Vary the resistance of the potentiometer and note how the input levels at which the op amp switches states can be varied. Remove the potentiometer and replace the 5.1 kΩ resistor with the 10 kΩ value.
3. With C = 0.1 μF, calculate the value of the resistor R in Figure 10.7 that gives an oscillation frequency of 1 kHz.
• Power up the circuit with these values and observe the waveforms at the op-amp output and across C. It should be a square wave with a frequency of 1 kHz.
• Record the waveform.
4. With the original value of C, observe the effect of changing the value of R to 10 times its original value. The frequency should be 100 Hz. With the original value of R, observe the effect of changing the value of C to 0.1 times its original value. The frequency should be 10 kHz.

Part Two

1. The circuit of Figure 10.19 shows a 2 op-amp triangular/square-wave generator. We have analyzed this circuit in class with the exception that the resistor R₃ was short circuit and the 5 diodes were omitted. The addition of these elements allow the square-wave output to be limited to a value 2V_D + V_Z independently of the value of the power supply voltages, where V_D is the forward bias diode drop and V_Z is the breakdown voltage of the zener. Two back-to-back zener diodes could be used for this, but the circuit given in the text is a faster switching circuit because the zener diode is never operated in its forward bias region. Thus the switching time is not limited by its diffusion capacitance, which is the capacitance when it is forward biased. There is a design example in the text for this circuit. The author of the text has the values for R₁ and R₂ reversed.
• Using the values from the example in the text, assemble the circuit. Use 1N4148 diodes for the limiter and a zener diode with the zener voltage assumed in the example. If a zener diode is not available with the desired voltage, use the closest value available, preferably on the larger voltage side. Note that the required value of R₃ is a function of the zener voltage.
• Use the oscilloscope to observe the two outputs from the circuit. Record the waveforms.
2. Like the square-wave generator circuit, the frequency can be changed by changing one R, one C, or both. Observe the change in frequency by changing each one by a factor of l0 as is specified for the square-wave generator of Part I. Record all waveforms.