Laboratory 6. Comparators and Schmitt triggers

Equipment: Large leads, small leads, protoboard, potentiometer, 741 op amp, 2-10x scope probes, handheld dvm, 1 bnc-banana lead.Resistors: 1k, 2k,9.1k.

Purpose: Learn proper protoboard etiquette and troubleshooting techniques. Learn the basics of comparator and Schmitt trigger operation.

Protoboard layout:

Protoboards allow the ready assembly of low power, low voltage electronic circuits. Good assembly practices will minimize the problems associated with poor connections, and will aid troubleshooting.

Most protoboard wiring is done with low gauge, single strand wiring. A wire is inserted into a slot in the board, where it is held by a spring loaded clip. Groups of slots are bussed together to provide electrical connection. These connections would be between groups of wires, or between wires and components such as capacitors, inductors, resistors, or electronics. The protoboard slots are laid out so that common integrated circuits packages can be readily inserted. In particular, standard dual in line (DIP) in single in line (SIP) packages are convenient to connect using protoboards.

When you receive the protoboard from the instrument room, you will also receive two boxes of leads, one set with banana jacks and alligator clips for the off board wiring, and the other a set of small wires for on board wiring. These latter wires should be a varying length, and (mostly) with the insulation stripped back at both ends. By keeping your circuit assembly neat and efficient, you will produce a circuit that is easily inspected. This can be done through the careful layout of main circuit components, and the use of appropriately sized wires. A pair of needlenose pliers with cutters and strippers will aid in the assembly of your circuit.

Preliminary steps:

A. Use an ohmmeter to verify which points on your protoboard are bussed (connected together internally). In order to make this measurement, insert a small lead into the alligator clip from each meter lead, and insert the small wire into the protoboard.

Initial that you have determined the sets of bussed connections:_____________________

It is common for circuits to have many connections to ground, positive and negative power supply, and similar nodes. Protoboards accommodate this need by providing longer sets of bussed slots for these nodes. They also include several banana plug terminals so that power supply and ground connections can be made securely to the board.

B. Power supply and ground leads should ALWAYS be connected to the board using these banana plugs. Connection of these leads with alligator clips is to be avoided, due both to the likelihood of poor connections and shorting problems. An alligator clip should only be used to connect to the protoboard when the clip will be dead if the connection is broken. The clip also should be shielded to prevent shorting when the circuitry is jostled. _______________

C. Protoboard notes: poor connections are not uncommon on protoboards. They can result from weak springs in the slots, breaks in the solid wires that are hidden by the wire insulation, etc. Another common error is tightening a terminal onto insulation rather than onto bare wire. When a circuit is not working properly, a visual inspection is not sufficient to prove connectivity. You should expect to make voltage measurements at both ends of a wire connection to verify the connection. ____________

D. Wire color code conventions are often used to aid in troubleshooting circuits. The common convention is to use red wires for the positive or hot leads, and black for the ground or negative leads. On dc circuits

Figure 1. Comparator schematic

with both positive and negative power supply inputs, use red for the positive, black for common/ground, and yellow for the negative supply. This leaves plenty of other colors to use for the other circuit wiring. __________

Procedure:

E. We will investigate voltage comparators in order to gain practice in protoboard wiring, measurements, and troubleshooting. A simple op amp circuit is the voltage comparator. Go to the lab web site, and review the 741 op amp spec sheet and pin diagram. According to the spec sheet, what is the open loop gain of the op amp, expressed in units of volts per volt? __________

F. In the simplest comparator, the inverting (-) input terminal is grounded, and the input voltage is connected to the non-inverting (+) input, as shown in Figure 1. When the input voltage is positive, the output voltage will approach the positive power supply voltage (positive rail), while when the input voltage is negative, the output voltage will approach the negative power supply voltage (negative rail). Examine op amp equations, and explain this operation:

1. Record the pin diagram for the 741 op amp, in the package that you have been given

______________________________2. You will use a potentiometer in this experiment. A potentiometer is a three terminal device, with a

schematic as shown below. Use an ohmmeter to determine which terminals on the potentiometer correspond to terminals 1, 2 and 3 on the potentiometer. Draw a physical diagram of the potentiometer to the right of the schematic diagram, showing where the terminals are on the device.

Note that misconnection of the potentiometer in this lab can result in a short circuit and the destruction of the potentiometer.

3. Wire

the circuit shown in Figure 1. Measure Vin and Vout with dvm’s. Test the circuit to make sure that it is working correctly—in particular, measure and record the range of values of Vin :_________________.

Record the value of Vin at the point where Vout switches from negative to positive:____________.

4. Also record the value of Vin where Vout switches from positive to negative:_____________.

5. Replace the potentiometer with the function generator set to produce a 5 volt (peak), 100 hz triangle wave. This signal will continually sweep Vin between -5v and +5v. Connect Vin to Channel 1 of the scope, and Vout to Channel 2 of the scope. Note that we are using probe leads for the oscilloscope. Each probe has a spring loaded clamp at the sensor end. In order to take protoboard measurments, take a short piece of protoboard wire, and clamp one end of the wire in the probe clamp. Insert the other end of the wire into the protoboard slot. DO NOT insert the probe directly into the protoboard. The probes we are using are 10:1 probes, which means that the probe reduces the voltage by a factor of 10. Go to both the Ch1 and Ch2 controls, and select the 10x probe option for both. Verify that the traces on the scope match the data taken in steps 3 and 4.

6. Put the scope into the x-y mode of operation, and plot Vout vs. Vin. Capture the scope trace with this setting, and verify that it shows the comparator switching at zero volts. Port it into a word document, label it Figure 1, and give it an appropriate caption.

7. Remove the wire connecting the inverting input to ground. Attach the variable bench power supply to the inverting input of the comparator. Vary the value of this voltage from –2 volts to +2 volts in steps, and record the effect of this voltage on the comparator operation.—what is the changes in these traces as compared to the result of Step 6?

8. Change the scope to the Y-T (time domain) mode. Connect the Ch1 input to the variable dc supply. Describe the effect of changing this value on the waveshape of the output (Ch2) signal:

G. Schmitt triggers. The comparator circuit of the previous section is subject to noise problems, as illustrated in Figure 2. In order to avoid this problem, a circuit modification known as the Schmitt trigger is often used. In this circuit, positive feedback is used to obtain a deadband in the performance of the comparator.

Figure 2. Illustration of unwanted chatter in the output signal when input signal noise causes multiple zero crossings.

Consider the circuit of Figure 3. The voltage at the op amp + input is:

[1]

The second term in this equations provides hysteresis offset in the operation of the comparator. If Rin/(Rin+Rf)=0.1, this term provides a voltage shift of +1.2v when Vout=+12v and a shift of -1.2v when Vout=-12v. Because of this, the voltage will shift up 1.2v higher than it would for a comparator, and it will shift 1.2v lower than it would for a comparator. This provides a deadband of 2.4 volts in the operation of the Schmitt trigger, and will create a deadband in the operation of the circuit. This deadband is desirable in many applications. Figure 4 shows the Vout vs. Vin plot for this circuit with the hysteresis and deadband shown. Note that the arrows show that that the two vertical portions of the plot can only be traversed in one direction. The horizontal portions are bi-directional. Figure 4 is centered on the Vin=0v axis, but note that Schmitt triggers can be set to switch around any voltage in the same manner as for comparators.

1. Build the circuit shown in Figure 3 with Rin=1kohms and Rf=9.1kohms. Use a 10 volt triangle wave for the input voltage. Observe the YT plot, and note the values of the Vin when the output voltage switches.

Positive going switching Vin=___________________

Negative going switching Vin=___________________

2. What is the width of the deadband?

How does this compare with the theoretical deadband width?

Figure 3. Simple Schmitt trigger circuit

3. Put the oscilloscope into X-Y mode, and observe the resulting trace. Label it Figure 2 in the same word document that Figure 1 is in, and include a suitable caption. Print this document and attach it to your report.

OPTIONAL (2.5%)

Change Rin to 2k and 3k in turn. Include plots of both in the word document that has Figures 1 and 2 already, and hand in with your report. Measure the width of the dead band in both of these cases. Compare with theoretical values.

Last changed: 1/4/07 tho

Figure 4. Vout vs. Vin plot of the Schmitt trigger showing the hysteresis offset and the deadband.