ELEC 242 Lab

Experiment 4.1

The 741 Op-Amp

Equipment

Components

The 741 operational amplifier, or op-amp, comes in an 8-pin dual inline package (DIP) which looks like this

If you look closely at the package, you will find a notch at one end or a dot in one corner. This tells us how to find Pin 1: the dot is located next to Pin 1 and the notch is located between Pins 1 and 8. The rest of the pins are numbered like this:
Pin 8 is not connected (NC). Pins 1 and 5 are used to eliminate the offset voltage. We won't be using this feature, so don't connect anything to these pins. The remaining pins give us the following circuit symbol for our op-amp:
For more information, see the 741 data sheet.

In order to function, the op-amp must be connected to an external power supply. Since we want to produce both positive and negative output voltages, we need both positive and negative voltages for the power supply. These are labeled and on the diagram. For a 741, the nominal values are =15 V and =-15 V.

To avoid clutter, we won't show the power supply terminals (pins 4 and 7) on any of the subsequent circuit diagrams. However, they must be connected or your amplifier will not operate.

Note that there is no ground terminal on the op-amp. The zero reference point is established by the external circuit and is not important to the op-amp itself.

Part 1: Powering up the Op-Amp



Step 1:

 If you have not already done so, wire the bus strips on your breadboard to provide positive power, negative power and ground buses. Whatever color scheme you have chosen for your wires, you should use the green binding post for ground, the black for -15 V, and the red for +15 V.

Step 2:

 Connect a capacitor between the +15 V power bus and ground. Connect another capacitor between the -15 V power bus and ground. The power buses for your board should look like this:

These capacitors are the first of several bits of "magic" we will employ to try to avoid amomolous behavior. As we will see when we study control systems, feedback also has a dark side. In particular, feedback which becomes positive at some frequency can cause instabilities. Although we have not deliberately introduced any positive feedback, feedback can occur where we don't intend it. The purpose of these capacitors is to prevent it from occurring via the power supply, which at high frequencies is not a very ideal voltage source.

Step 3:

 Plug an op-amp into the breadboard so that it straddles the gap between the top and bottom sections of the socket strip. If you have wired the power buses as suggested above, Pin 1 should be to the left.

Warning
Do not try to unplug the op-amp with your thumb and forefinger. It's a good way to end up with the op-amp plugged into your fingertip. Use the IC puller from your toolkit.


Step 4:

 Connect Pin 4 ( ) to the negative power supply bus (-15 V). Connect Pin 7 ( ) to the positive power supply bus (+15 V).


Step 5:

 Set the METER SELECTOR on the power supply to +20V. With the power supply disconnected from the breadboard, turn on the supply and adjust the left-hand voltage control until the meter reads 15 volts.

Step 6:

 Turn off the supply and connect the supply to the breadboard with banana plug patch cables. Connect the 0 to -20V terminal (black) to the black binding post on your breadboard, the 0 to +20V terminal (red) to the red breadboard binding post, and the COMMON terminal (light blue) to the green breadboard binding post. Note that none of the power supply output terminals are connected to ground. If we want the power supply zero volt reference connected to ground, we must make the connection ourselves.

Part 2: The Op Amp as a Comparator

The gain of the op-amp without feedback (so called "open loop" mode) is too high to be useful as an amplifier. But this high gain provides a very sharp threshold if we use the op amp as a switch or comparator (so called because it compares one voltage to another and gives an output signifying which is greater).

Caution

The components we've used so far have been simple (only two terminals) and fairly rugged (connecting a resistor or capacitor "backwards" won't harm it). The op-amp has four times as many pins, so it's easier to make a mistake in wiring it. Unfortunately, it's also considerably more delicate, so connecting it incorrectly can destroy it (often without so much as a puff of smoke to let you know that it has become an inoperational amplifier.

The moral: always wire your circuit with the power turned off and check your wiring carefully before turning the power on.



Step 1:

 With the power turned off, wire the following circuit. This will compare the function generator output with an adjustable threshold proficed by the 0-6V supply.


Step 2:

 Set the function generator to produce a 4 V p-p, 100 Hz triangle wave.

Step 3:

 Pull out the DC OFFSET control and adjust it so that the waveform has an average value of 2.5 V (i.e. the negative peaks have a value of +0.5 V).

Step 4:

 Connect the function generator output to of the circuit above. Connect CH1 of the scope to and CH2 to . Set the CH2 VOLTS/DIV to 5. Make sure both channels of the scope are on DC.

Step 5:

 Turn on the power supply and set the 0-6V output to zero.

Step 6:

 Slowly increase the 0-6V control until begins to change. Sketch the waveform.

Step 7:

 Increase in steps of 0.4 V until stops changing. At each step sketch , noting the positive and negative peak values and the duration of the high and low states.

Step 8:

 Set to zero and push the DC OFFSET control back in. should be approximately a square wave. Is this what you expect?

Question 1:

 Explain the waveforms observed in this Part. Develop an expression for the duty cycle (the percentage of the time that the waveform is in the "high" state) as a function of .