Oct 10 & 17 EGR 220: Engineering Circuit Theory Due Oct 17 & 24 Lab 4: Op Amp Circuits

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1 Oct 10 & 17 EGR 220: Engineering Circuit Theory Due Oct 17 & 24 Lab 4: Op Amp Circuits Objective The objective of this lab is to build simple op amp circuits and compare observed behavior with theoretical and predicted behavior. A second objective is to learn how to create a wiring diagram for a circuit, and how to wire a circuit using a chip, on the breadboard. Op Amp Overview Op amps require an external power supply to be able to function and amplify their input signal this is analogous to turning a stereo receiver on with the power button first and then having a second input in the form of the radio station signal. The amplifier circuit (the op amp chip) includes transistors, which are active circuit elements (resistors, capacitors and inductors are all passive elements). The 741 op amp is powered with +/ 15 Volts (and common ground) from the power supply. ** Never apply an input to the op amp (pins 2 & 3) when the power supply (the ±15V) is turned off ** * Always disconnect the op amp input signal before disabling the dc power supply * This means that pins 2 & 3 (the inputs ) must be energized only after pins 4 & 7 (the ±15 Volts) are already powered Op Amps have two inputs (inverting and non-inverting + ) and one output (vo). o You should review the Two Golden Rules before you design and wire the circuits Specific wiring information is given below for the 741 op amp (image from

2 Pre-lab 1. ** Complete Problem 4 in the homework this week ** - you do not need to hand it in with the pre-lab, but be sure you have completed it so that you know what to expect from your circuits in lab. (Hand it in with the homework, as usual) 2. Perform calculations / write expressions as indicated below. 3. Complete the two circuit designs specified below creating a circuit diagram for each circuit that includes wiring information (using the pin-out figure above; see sample below) and specific values for all quantities (such as resistors) in the circuit. 4. Sketch and label a graph (by hand is fine) showing the input(s) and the expected output for each of the op amp circuits you will build. Label each input and the output along with the voltage values for each input and output signals. a. For the summing amplifier, you can select any DC voltage level for the DC input signal, and any function that you can obtain from the function generator for the AC input. Sketch the two input signals and the output (which is the result of summing or adding the inputs together). Bring a USB drive to lab this week. You need to capture the oscilloscope images of your circuit s input and output signals as digital figures by saving the oscilloscope screen to your USB drive. Hand drawn input and output graphs will not be accepted for the lab memo. Pictures taken with your phone or other camera of the oscilloscope screen will not be accepted for the lab memo. page 2 2

3 Notes for performing this lab 1. You will need to take time before wiring the circuit to plan how you will proceed a. You need to plan where to place the op amp chip, given the other elements in the circuits and the need for power to the chip as well as input signals and oscilloscope probes. i. Note that the chip must be placed across the center channel of the breadboard. ii. Place the chip over the holes you will use, make sure each leg is over the intended hole, and gently push the chip straight down into the breadboard. iii. If you need to remove the chip, grasp both ends and pull straight up, with no rocking, to avoid breaking the legs. b. You should use consistent wire colors for the same functions in your circuit. This allows you to keep your work neat and also aids in trouble shooting the circuit. i. Use red wires (or orange) for power (the +15V supply to the chip) ii. Use black for ground / common iii. Select a different color for the power (the 15V supply), such as green iv. As needed, and if possible, select unused colors for wiring inputs and outputs 1. Note that it is best to simply clip the probes directly to circuit elements, and avoid the need for additional wire. c. Use wires as short as possible, to avoid clutter on your board, BUT do not cut wires from the jumper wire kit if you need custom length and/or colors, use the spools of wire by the windows. d. Keep the leads on circuit elements straight and neat (i.e., the resistor leads), with one bend each. If you need to straighten them, use needle-nosed pliers. 2. For the function generator, recall that some of the function generators provide an output twice the peak-to-peak value from that stated on the function generator screen. Test your function generator before proceeding and record whether this occurs for you or not, so that you will know what output signal to expect from your op amp circuits. R1 = Ω Rf = Ω ground 2 +Vcc Vcc 6 +Vcc = V Vcc = V vi = V (Note that these values can be written directly on the circuit diagram) Figure 1: Sample wiring diagram for a non-inverting amplifier (figure 5.16 in text) page 3 3

4 Lab 4: Building and Testing Op Amp Circuits Build and test an Inverting Amplifier The inverting amplifier is among the most common feedback configurations for op amps. PL: Write the expression for the gain, vo/vi for the circuit below In lab test this relationship by constructing the circuit and observing the behavior of the 741 op amp. Specifically Design an op-amp circuit with a voltage gain of 10 (which means you need to select values for resistors R1 and Rf). Build your circuit and measure the gain experimentally. o Measure the gain first using a DC source and second with a signal generated by the function generator. o Use input voltage of at least 1V or 1Vpp (not 100mVpp) to get able to record good data for the circuit behavior. You should view both the input and output signals simultaneously on the oscilloscope. o You can use the multimeter to confirm the operation of the circuit with the DC source. You must use the oscilloscope when testing the circuit with the AC function input signal. Do NOT take your circuit apart. You can simply put an additional resistor on this circuit for the next experiment. Do the experimental results agree with the theory? Figure 2: Text figure Determine appropriate values for the resistors and test the circuit. page 4 4

5 Build and test a Summing Amplifier The summing amplifier can add input signals. PL: Write the expression for vo in terms of each input signal vi (v1 and v2) separately, such that you have vo = f(vi, resistance). Then using the concept of superposition, write (and simplify) an expression for the total vo from the addition of the two input signals In lab test this prediction by constructing and observing the behavior of the circuit below. Specifically adding to your circuit from part 1 above, and NOT starting over... Design an op-amp circuit that adds two signals. o Note, the figure allows for three signals, and you must change this to have only two input signals. o The signal input in the box would be the third input if you were to sum three signals. o You can do this if you want to, but start with summing only two Build your circuit and measure the output experimentally. o Use one DC input signal and one AC input signal and confirm that they are summed at the output. o Use input voltage of at least 1Vpp (not 100mVpp) to get able to record good data for the circuit behavior. You should view both of the input signals as well as the output signal simultaneously on the oscilloscope. o This may take some thought, to think about how you will view them all simultaneously, in a way that is clear and useful. o Note that you can save the image of what is on the scope screen to a USB drive (to then include in your memo if desired). Bring a USB drive to lab. Do the experimental results confirm theory? Figure 3: Text figure 5.21 sums three signals. page 5 5