Operational Amplifiers Lab - Basic Characteristics and Design

OBJECTIVES:
To understand the basic characteristics of operational amplifiers.
To calculate and measure the gain of an operational amplifier circuit.
Design an amplifier circuit and verify its validity using an oscilloscope. 

 

MATERIAL REQUIRED:
Breadboard
DC Power Supplies
GW Instek 1830 D power supply
GW Instek GPS3303 Power Supply or similar
Digital Multimeter (DMM)
uA741 Op-amp
1K-ohm Resistors
10 K-ohm resistor

 

Deliverable:
The deliverable for this lab will not be a formal lab report but instead will be a memo to your boss at prime electronics.  On the last page you will see a memo to your lab team from your boss (Sue Murphy). Your task is to write a reply memo to her. The format of this memo can be found at the bottom of the last page. 

 

Background:
Operational Amplifiers (Op Amps) are electronic devices that can be configured to amplify signals and perform other useful applications. Op Amps are housed in an integrated circuit (IC) which is an entire circuit, consisting usually of many components, housed in a single, small chip.  The components in an individual IC may include thousands of transistors, diodes, resistors and capacitors. These components are formed on a chip of silicon using a sophisticated photo-lithographic process. 

 

There are many different types of OpAmps available. Typically they have high input resistance, very low output resistance, and extremely high gains.  Most often op-amps are wired in a circuit that employs negative feedback in order to control the gain of the amplifier.  A standard non-inverting opamp circuit is shown in Figure 1.  A power supply called a bipolar supply (bipolar because it has both positive voltage and negative voltage) is used to power the opamp. 

 

The offset null terminals may be used in an auxiliary circuit to compensate for degradation in amplifier performance due to aging and due to imperfections that arise in the circuit during its fabrication.  However, since the degradation of performance is in most cases negligible, the offset terminals are often unused and play a secondary role in the circuit analysis problem.  Terminal number eight (8) is of no interest simply because it is an unused terminal: NC stands for no connection, which means that the terminal is not connected to the amplifier circuit. 

 

PROCEDURE:
Part I.

Because a bipolar voltage supply is required, we must set up our power supply according to the circuit shown in figure 3.  After constructing the circuit, adjust the power supplies A and B for 12.5 volts by turning knobs A and B.  Setting the tracking to ‘SERIES’ during this experiment makes it easier to control the voltages.

Objectives

 

Now that our bipolar power supply has been constructed we will turn our attention to the op-amp circuit.  Figure 4 shows how this portion of the circuit should be constructed. The 1830 D supply will produce the voltage Vin.  NOTE:  The input voltage (Vin) should always remain within the supply voltage of +12.5 V to -12.5 V.   DO NOT EXCEED 12.5 V.   The 1 K-ohm resistor between terminal 3 and ground is not necessary but tends to yield better results. If you wish, you can replace this resistor with a short circuit and investigate if there are any differences.

3.We are now ready to connect the bipolar power supply constructed in step 1 to the op-amp circuit.  Connect terminals +12.5V and -12.5V of the power supply to pins 7 and 4 respectively.  Connect the ground terminal of the power supply circuit (0.0 volt lead in Figure 3) to the ground of the op-amp circuit (point A in Figure 4).

 

4.Adjust Vin from 0 to 5 volts. in 1 volt increments and record Vout in table I.  DO NOT EXCEED 12.5V FOR Vin!  Use digital multimeter (DMM) to measure Vout.

5.Switch the Vin power supply leads so that negative voltages can be tested.  Adjust Vin from 0 to -5 VDC in 1 volt increments and record Vout in table I.

6.Calculate the theoretical gain of the op-amp circuit using the ideal op amp model.  Draw a graph using the data from Table I.  Comment on your graph and explain its significant features in the lab report.  What was the actual gain of the op- amp? At what point did saturation occur?

 

Part II.

1.Design a non-inverting op-amp circuit which will provide a gain of +2.  A large feedback resistor tends to produce better results (10 K ohm suggested) Construct and test your circuit. When you are satisfied with your design, have the instructor check your circuit to verify that you have designed and built your circuit correctly. Include a schematic showing your non-inverting op-amp design in your lab report.

 

What is the acceptable range of input voltages that can be amplified correctly by your non-inverting op-amp design? Why are the voltages in this range acceptable? To answer these questions, construct the circuit below. Use the function generator to make a triangle wave as the input to your amplifier and measure the input and output voltages of your amplifier. Adjust the function generator amplitude so that the scope output shows which voltages are and are not amplified correctly. Include a clearly labelled screenshot in your memo - a picture is worth a thousand words.

 

Normally the scope is used to display a voltage vs time. You may find it interesting to put the scope in X-Y mode to view the output voltage (ch2) vs the input voltage from the function generator (ch1). The X-Y mode is available from the “Display” menu. If you reduce the frequency to 0.2 Hz you can get a good understanding of how the trace is generated. I recommend that you include this graph in your memo – the boss did not ask for it, but it’s always a good idea to provide more value if you can.