Op-amp Circuit Experiment: Voltage Follower, Inverting Amplifier, Summing Circuit, Lab Reports of Theatre

Download Op-amp Circuit Experiment: Voltage Follower, Inverting Amplifier, Summing Circuit and more Lab Reports Theatre in PDF only on Docsity! ECE2A Spring 2007 Lab #4 4 Simple Op-Amp Circuits OBJECTIVES: To gain experience with design and construction of op-amp IC circuits, explore simple amplifier concepts and algebraic functional circuits, and further develop skills with the oscilloscope. BACKGROUND: Refer to the class text, or any text on electronics, for basic information on the operational amplifier and descriptions of the standard inverting and non-inverting amplifier configurations. Although most op-amp circuits can be understood based on the characteristics of an ideal op-amp, in the lab we will use real op amps with non- ideal characteristics. In this experiment we will encounter two of these: output saturation and finite slew rate. Output saturation simply means the op amp cannot force the output voltage beyond the voltage range it is supplied with. We will be supplying the op amp with +/- 15V. As you will find, the op-amp will not be able swing above +15V or below -15V. An additional non-ideality that can often show up with amplifier circuits is unwanted oscillation. If you see occasional bursts of something on the oscilloscope trace for the op-amp output, call it to the attention of the TA; you may need to add bypass capacitance to the power supply pins (not intended to be part of this experiment, and therefore not explained further here). Note: In the circuits below the pin numbers for the various op amp connections are shown. Pins not shown are to be left unconnected. PROCEDURE: Part 1. Op-Amp Circuit Construction; Unity-gain buffer amplifier The op-amp circuits are the only type of active circuit that we will encounter in ECE 2A. Active circuits require an external source of energy to operate properly. Figure 1 below shows a picture of the LM741 op-amp that we will use, with the pin numbers conventionally labeled (for more detail, a copy of the LM741 data sheet is posted at the class web site). Most op-amps have the same pin configuration. Use the +/-20V outputs of your bench power supply, adjusted appropriately, for the +/- 15V sources. 1 2 3 4 8 7 6 5 LM 741 - + +15 V 7 42 3 6 -15 V input output V g o-scope Fig 1: Op-amp IC pin diagram and test-circuit schematic (1) Using your breadboards and the lab bench power supply, construct the circuit shown in figure 1 above. This is called the voltage follower configuration, and is a type of buffer amplifier. It is often used because the high input impedance prevents excessive loading of the source generator; i.e., no current is drawn from the source, therefore no voltage drops on any internal resistance of the source. Use your alligator clip leads to connect the breadboard to the power supply terminals. Figure 2 indicates how this circuit could be laid out on your breadboard. Also remember that we want the common terminal (COM) of the power supply to be at ground potential. It will be useful if you pick a row of sockets to be used for all ground connections. Fig. 2 Breadboard layout of fig 1 (2) Use the function generator at the bench as source Vg to provide a 1V amplitude (2V pk-pk), 1kHz sinewave excitation to the circuit, and observe the output on the oscilloscope. Set up the scope so that the input signal is displayed on channel 1, and the output signal is displayed on channel 2. Make a plot of the two waveforms in your lab notebook. What is the voltage gain of this circuit? Is it an inverting or non- inverting amplifier? - + +15 V -15 V 4.7 kΩ 1 kΩ 10 kΩ V g Vo 4.7 kΩ V dc + 7 4 2 3 6 Fig 4: Op-amp summing circuit Part 4. Non-inverting amplifier (1) Assemble the non-inverting amplifier circuit shown in figure 5. Remember to shut off the +/-15 V supply before assembling a new circuit. Use the decade resistor box for the feedback resistor 9the 1k resistor between pins 6 and 2). (2) Apply a 1 Volt amplitude, 1kHz sine wave at the input, and display both input and output on the oscilloscope. Measure the voltage gain of this circuit, and compare to the theory discussed in class. Make a plot of the input/output waveforms in your notebook. (3) Increase the feedback resistance from 1kΩ to 9kΩ . What is the gain now? (4) Increase the feedback resistance further until the onset of clipping, that is, until the peaks of the output signal begin to be flattened due to output saturation. Record the value of resistance where this happens? Increase the feedback resistance to 999kΩ. Describe and draw waveforms in your notebook. What is the theoretical gain at this point? How small an input signal would be required to produce a 10V amplitude sine wave at the output, given this gain? Try to adjust the function generator to this value. You may need to use the 0.2V button on the function generator, which reduces the output amplitude. Describe the output achieved. (5) Increase the amplitude of the function generator back to 1V and then disconnect the feedback resistor completely. The op-amp is now running "open-loop". Op amps used in this configuration are referred to as "voltage comparators". The output can assume one of only two states (voltage levels), which reflect a comparison of the input voltages. To see this, complete the following statement for this circuit: When Vg>_____, Vo=_______; when Vg<_____, Vo=______. - + +15 V -15 V 1 kΩ 1 kΩ 10 kΩ V g Vo7 4 2 3 6 Fig 5: Non-inverting amplifier circuit Discussion Questions 1. In part 1, step 3, you found the maximum rate the op-amp output could change. This is specified on the data sheet as the "slew rate", and has units of Volts/microsecond. Calculate the slew rate you observed from timerise VSlewrate _ 9010−∆= Is this greater than or equal to the guaranteed minimum from the data sheet ? 2. The DC voltage you recorded in part 3, step 4, is the output saturation voltage. Is your measured value within the limits specified on the data sheet? The relevant spec in the Fairchild data sheet is "Output Voltage Swing" Vo(p-p), and our condition corresponds to Vcc=+/-15V, 10kΩ RL. 3. The +/-voltage supplies, commonly labeled Vcc and Vee, are also referred to as the power rails. Can you guess what the output voltage swing would be for an op-amp advertised as being a "rail to rail" op amp?