Lab 5: Operational Amplifiers

Abstract:

The use of operational amplifiers as circuit building blocks is explored in this experiment. Basic properties of op amps are observed using simple resistive networks. A few of the practical limitations of op amp integrated circuits are also introduced and compared with several of the ideal assumptions.

Introduction:

Operational amplifiers are an extremely important component for a wide range of power electronic circuits. The term operational refers to the use of op amps in electronic circuits which perform arithmetic operations on the input voltages (or currents) applied to the circuit. Although the concept of an operational amplifier dates back to the era of World War II, the development of integrated circuits (ICs) from the 1960’s to the present has resulted in a large number of op amp types and features. Many general purpose op amps in IC form are very inexpensive. In fact, the socket that op amp IC plugs into may cost as much or more than the op amp itself!

The op amp is depicted schematically as shown in Figure 5-1. The figure shows the two op amp inputs: “-” for the inverting input and “+” for the non-inverting input, the op amp output, and the power supply connections. NOTE that the power supply connections are not always shown in diagrams, but they must be included in the actual circuit.

Figure 5-1: Basic op amp schematic.

The integrated circuit op amp used in this experiment is shown in Figure 5-2. The circuit is contained in a dual in-line package, or DIP for short. The DIP has a notch or stripe at one end to indicate the correct orientation of the circuit. The standard part number is usually printed on the top of the DIP. Note that the pin numbers are assigned in counter clockwise order beginning at the notch end. In addition to the two inputs, the two power supply pins, and the output, notice that this particular op amp has three other pins: one labeled NC, meaning no connection, and two labeled offset null. The offset null pins allow us to make small adjustments to the internal currents in the IC in order to force the output voltage to be zero (null) when the inputs are both zero in order to compensate for the anticipated manufacturing variations from chip to chip. We will not need to use the offset null feature in this experiment, so no connections will be made to offset null pins. It is also important to realize that there is no “ground” pin on the op amp: the op amp receives its ground reference via the external components and connections of the complete circuit. While the particular IC used in this experiment contains a single op amp, many other IC types are produced which contain two or more op amps in a single DIP package.

Integrated circuit op amps behave very much like the conceptually “ideal” op amp used in circuit analysis. There are some important limitations to keep in mind, however. First, the supply voltages cannot exceed some maximum rating, typically +/- 18V DC. The op amp will usually operate using lower voltage supplies, but exceeding the maximum rating will destroy the IC. Second, the output voltage from an IC op amp is usually limited to be a volt or two smaller than the power supply voltages, e.g., the output voltage swing of an op map with +/- 15V supplies is, perhaps, +/- 13V. Third, the output current from most op amps is limited to 30mA or so, meaning the load resistance attached to the output must be large enough that no more than the maximum current flows when the output voltage is maximum.

Figure 5-2: 741 op amp pin-out diagram.

IC op amps have many other characteristics that will be considered in subsequent electrical engineering courses. When in doubt about the limitations of an op amp, it is best to refer to the manufacturer’s data sheet.

It is common to include power supply bypass capacitors in op amp circuit designs. Capacitors are charge storage elements which will be discussed in more detail at a later time. Bypass refers to the good design practice of placing capacitors across the power supply connections to help stabilize the DC power supply voltages and “bypass” any noise or interference on the supply lines to ground. This means connecting small (typically 0.01μF) capacitors between the positive supply voltage and the circuit ground, as shown in Figure 5-3. The bypass capacitors should be placed as close to the IC as possible.

Figure 5-3: Op amp with power supply bypass capacitors.

Many different types of op amps are available from commercial manufacturers. The type 741 op amp used in this lab was originally introduced by Fairchild Semiconductor in 1968, so it is an “old”, reliable, well-understood, and inexpensive IC. The 741 is by no means the best op amp for every purpose: more recent designs reflect the advances of integrated circuit technology that have taken place over the last 20+ years. The 741 op amp is used here because it is a good example of the so-called general purpose operational amplifiers that are used in everything from radios and wireless telephones to car engine control systems and the space shuttle.

Figure 5-4: Non-inverting amplifier circuit.

Figure 5-5: Inverting amplifier circuit.

Figure 5-6: Inverting summer circuit.

Experiment:

When working with electronic devices ALWAYS assemble and verify the circuit with the power OFF. Once the circuit has been checked, then apply the power. ICs can be damaged by incorrect voltage connections. Work carefully and methodically.

(1) Assemble the inverting amplifier circuit of Figure 5-5. Measure and record the actual value of the resistors. Remember the power supply connections: use V+ = +12 volts to IC pin 7 and use V- = -12 volts to IC pin 4. Include 0.01μF bypass capacitors between the +12V and ground and between the -12V and ground, placing the capacitors as close the IC as possible. Use the bench power supply for the supply connections and for V_in. Make sure all of the grounds are connected. Record the input and output voltages as you vary V_in from -6 volts to +6 volts in 1 volt steps. You will want to use smaller steps near the output saturation levels in order to determine for what input the output begins to saturate.

(2) Now use the signal generator in place of the bench power supply for V_in (still for the inverting amplifier). Observe the signal generator signal on channel 1 of the oscilloscope and the op amp output on channel 2. Make sure the ‘scope inputs are set for DC coupling. Adjust the signal generator for a sine wave with a ~500Hz frequency, and adjust the amplitude so that the signal out of the op amp is 4 volts peak-to-peak. Determine the input peak-to-peak voltage.

(3) Modify the circuit of Figure 5-5 by adding another input as shown in Figure 5-7. Use the signal generator for V₁ and the bench power supply for V₂ set to 0 volts DC and readjust the signal generator if necessary so that the op amp output is a 4 volt peak-to-peak sine5wave at ~500Hz. Display the bench supply voltage on ‘scope channel 1 and the op amp output on channel 2. Observe how the output of the op amp changes as the bench supply voltage is varied between 0 and 6 volts. Record the maximum and minimum values of the output voltage when the bench supply is +/- 1, +/- 2, and +/- 5 volts DC.

Figure 5-7: Inverting summer circuit with two inputs.

Results:

(a) Present your measurements from part (1) of the experiment in the form of a graph with V_in as the abscissa (x-axis) and V_o as the ordinate (y-axis). Label your plot. What is the slope of the curve? Does it pass exactly through the origin? How does the result compare to your prediction using the ideal op amp model?

(b) What was the input peak-to-peak voltage required in part (2) of the experiment to get a 4 volt peak-to-peak output waveform? How does this relate to your results from part (1)?

(c) Describe the function of the circuit used in part (3) of the experiment. What is the mathematical relationship between the output voltage and the two input voltages? Present your measurements for the various DC input voltages. Discuss your results.

(d) What can be done to improve this experiment?