EECS 100 Operational Amplifier Laboratory L Chua University of California Berkeley Department of Electrical Engineering and Computer Sciences EECS 100 Professor Leon Chua LABORATORY 5 v1 OPERATIONAL AMPLIFIER Integrated operational amplifiers opamps for short became widely available with the introduction of the A709 designed by the legendary Bob Widlar in 1965 This part was rapidly superseded by the 741 which has better performance and is still in wide use Today well over a hundred different versions of opamps are available Opamps are arguably the most widely used analog circuit components The ideal opamp Figure 1 produces an output that is the difference Vi Vi of its inputs gained up by infinity Practical opamps deviate from this ideal somewhat For example the gain of the opamp we are using in this lab is only about two million In many applications these deviations from ideality do not introduce significant errors Real operational amplifiers of course must be connected to a power supply which to reduce clutter is often now shown in the schematic diagram Figure 1 Ideal operational amplifier Although an amplifier with infinite gain does not appear to be particularly useful using only few extra components opamps can be configured to perform a very wide range of tasks and find almost universal application in interfacing sensors to other electronic circuits In this laboratory we will focus on amplification and buffering two tasks operational amplifiers excel at We will also design the electronics for a pH acidity meter Before reading on please download the datasheet for the LMC6483 It contains a lot of information such as the supply voltage and temperature range over which the amplifier can be used Like most datasheets this one also has a section on applications with many circuit suggestions Datasheets are usually a very valuable source of information and I recommend that you make it a habit to check them out at the minimum to get the connection diagram of the device Page 1 EECS 100 L Chua Operational Amplifier Laboratory LAB REPORT Lab Session Name 1 SID Name 2 SID In this laboratory we will be using the LMC6482 from National Instruments An 8 pin package contains two identical operational amplifiers check the datasheet for the pinout You can use either opamp in these experiments It s always a good idea to tie unused inputs to a known potential e g ground to avoid excessive power dissipation or other problems but for this laboratory you probably will get by with just ignoring the unused part We will power the operational amplifiers from 5V i e V 5V and V 5V in all experiments described in this laboratory and will not show the supply connections in schematics Note that the operational amplifier has no dedicated terminal for ground 1 Openloop Operation Let s first check that the operational amplifier is working and indeed has a very large gain Set up the circuit below and adjust the potentiometer such that Vout 0V It s unlikely that you in fact will be able to do this because explain of 3 P 5V 1kOhm Vout 5V Page 2 EECS 100 Operational Amplifier Laboratory L Chua Draw the openloop Vout versus Vin characteristic of the operational amplifier Draw the expected Vout versus Vin characteristics on the plot and copy the plot to your prelab Turn on the oscilloscope Change the scope to XY mode by pressing the Main Delayed button followed by the XY soft key Set the function generator to sine wave output at 10 Hz with 100mV peak to peak amplitude Label the axes variable units ticks in the graph below and show the measured result in a different color than the expected result Expected characteristic of 3 P Measured characteristic of 3 M Explain discrepancies of 3 M Page 3 EECS 100 Operational Amplifier Laboratory L Chua 2 Positive and egative Feedback Most practical opamp circuits use feedback to set the gain to an accurate and reasonable e g 10 value This works very well provided that the feedback is connected correctly Here we compare opamps with positive and negative feedback Which of the two circuits A or B is configured for negative feedback Circuit with negative feedback of 2 P Simulate and measure Vout versus Vin for the circuit with negative feedback Setup a DC sweep of the input Vin from 5V to 5V and plot the output as a function of input Attach your simulation plot to your prelab and copy to the plot below Build the negative feedback circuit and generate the XY plot just as you did in part 1 Copy your measured results to the plot below Repeat all above with a positive feedback circuit Explain what s happening and summarize your result in the graph Simulation of 3 P Measurement of 3 M Page 4 EECS 100 Operational Amplifier Laboratory Explanation L Chua of 3 M 3 Voltage Gain Now let s use the operational amplifier with feedback as shown below What is the value of R1 that results in a gain Vout Vin 10 Value for R1 of 2 P Calculate the expected Vin to Vout relationship and plot below Measure the gain Vin to Vout relationship using the XY plot on the oscilloscope with a 1kHz 1V peak to peak sine wave Include your measured result below Explain discrepancies Page 5 EECS 100 Operational Amplifier Laboratory L Chua Simulation of 3 P Measurement of 3 M Discrepancies of 3 M 4 Buffers In the previous experiment you had to take the 50 output resistance of the function generator into account to get the correct gain This is possible when the output impedance is known However often the output impedance of a source often a sensor is not known and may even vary from part to part or with temperature The problem with the inverting amplifier configuration is its finite input resistance R1 Noninverting gain stages have infinite or nearly infinite input resistance Since no current is flowing the value of the source resistance does not matter for the gain Build and measure the IO characteristics of a non inverting amplifier for Rs 0 1k and 100k Set Vout Vin 11 choose the smaller of the gain setting resistors to be equal to 1k Calculate the correct value of the other resistor Circuit diagram for a non inverting amplifier Label the resistors including Rs the series output resistance of the sensor Diagram of 5 P Measurement 3 traces of 5 M Page 6 EECS 100 L Chua Operational Amplifier Laboratory 5 Electronic Interface for a pH Sensor In this part you will design the electronic circuits for a pH sensor The pH is an important for characterizing acidity You can read up on it e g on the Wiki http en wikipedia org wiki
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