ELEC 243 Lab

Experiment 5.1

The 741 Op-Amp

Components

741 Op Amp
Resistors: 1-10 $\Omega$ , 1-10 k $\Omega$

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 this week, so don't connect anything to these pins. The remaining pins give us the following circuit symbol for our op-amp: $\includegraphics[scale=0.640000]{opamps/cktsym.ps}$ 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 $V_{CC+}$ and $V_{CC-}$ on the diagram. For a 741, the nominal values are $V_{CC+}$ =15 V and $V_{CC-}$ =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 (as described in the Before Lab section) to provide positive power, negative power and ground buses.

Step 2:

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 pliers or IC puller from your toolkit.

Step 3:

Connect Pin 4 ( $V_{CC-}$ ) to the negative power supply bus (-15 V). Connect Pin 7 ( $V_{CC+}$ ) to the positive power supply bus (+15 V).

Step 4:

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 5:

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 banana jack on your breadboard, the 0 to +20V terminal (red) to the red breadboard banana jack, and the COMMON terminal (light blue) to the green breadboard banana jack. 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: Open-Loop Response

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. Note that this is a 1000:1 voltage divider, so that a 1 V signal at $v_{in}$ results in a 1 mV signal at the input of the op-amp. $\includegraphics[scale=0.650000]{opamps/open_loop.ps}$

Step 2:

Set the function generator to produce a 2 V p-p, 20 Hz sine wave.

Step 3:

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

Step 4:

You should see a badly distorted (clipped) waveform at $v_{out}$ . If you don't, try increasing the function generator output.

Step 5:

Pull out the DC OFFSET control on the function generator and adjust it until the waveform is roughly a symmetrical square wave.

Step 6:

Note the positive and negative peak values of $v_{out}$ .

Step 7:

Connect a 100 $\Omega$ resistor from $v_{out}$ to ground. What happens to the output signal?

Step 8:

Remove the 100 $\Omega$ resistor from the op-amp output. Set the function generator output to square wave. Note the shape of the $v_{out}$ waveform.

Step 9:

Set the function generator to produce a 1 kHz sine wave. Pull out the AMPL knob to engage the 20 dB attenuator. Adjust the AMPL and DC OFFSET controls until the op-amp output is a 10 V p-p unclipped sine wave. Measure the amplitude of the function generator output.

Question 1:

Based on your measurements in the previous step, what is the open loop gain of the op-amp? Is it consistent with the specifications in the data sheet?

Step 10:

Push the DC OFFSET control on the function generator back in.

Remark:

We have just seen a number of the shortcomings of a real (as opposed to an ideal) operational amplifier: clipping, which limits the maximum amplitude of the output; slew-rate limiting, which limits the maximum slope of the output; and offset, which gives a non-zero output for zero input. When we reduce the overall gain with feedback, some of these (e.g. offset) are reduced significantly, and we get an output which is a faithful reproduction of the input. However, other limits (such as maximum output level) must be respected for this fidelity to remain.