Summing Amplifier Lab
Overview: In this lab, we used an op amp to sum two input voltages.
Design: We designed the following circuit:
The left voltage supply (Va) was equal to voltages varying in whole integer values from -4 to +5.
The right voltage supply (Vb) was set to a constant 1V.
R1 and R2 were set to 6.8 K ohms, while R3 was set to 1 K ohms.
With the elements set, we were ready to build and test the circuit. The circuit was expected to have an output equal to Vout = (-R3/R1)(Va + Vb)
Construction and Execution:
^^^^ Measured values of the 1K ohm resistor and one of the 6.8K ohm resistors ^^^^
^^^^ Measured value of the second 6.8K ohm resistor ^^^^
^^^^ Overhead and side view of the constructed circuit ^^^^
Results: The following is a table of our results.
^^^^ Table of our results, throughout the spectrum of input voltages ^^^^
Analysis: Unfortunately, the results did not match our expectations. Strangely enough, the output voltage sign wasn't even correct, which leads me to believe that either our voltmeter or our op amp wasn't set up correctly. Based on the vast differences between our true results and our expected results, I would tend to believe it was an op amp setup error. Although there were slight differences in values between the two 6.8 K ohm resistors, I don't believe the difference was large enough to cause the drastic errors that we saw. I'm tempted to believe that the resistors were switched out by accident due to us being rushed to complete two labs in a minimal amount of time, and the resistors looking very similar. The method of using op amps as summing amplifiers still stands though.
Difference Amplifier Lab
Overview: In this lab, we used an op amp to subtract two input voltages.
Design: We designed the following circuit:
The left voltage supply (Va) was equal to voltages varying in whole integer values from -4 to +5.
The right voltage supply (Vb) was set to a constant 1V.
However, instead of using 10 K ohm resistors, we used four 9.8 K ohm resistors.
The circuit was expected to have an output equal to Vb - Va.
Construction and Execution:
The four resistors (starting at the bottom resistor, and moving in a clockwise fashion through the circuit) were measured as follows:
R1 --> 9.87 K ohms
R2 --> 9.81 K ohms
R3 --> 9.81 K ohms
R4 --> 9.84 K ohms
^^^^ The overall constructed circuit up close (left) and the overall circuit including the power supplies (right) ^^^^
Results: The following is a table of our results:
Analysis: Fortunately, this lab went much better; the results were very close to what we expected. Each step of Va experienced a correlating Vout that was approximately equal to Vb - Va. The only exception to that was when the op amp hit saturation at Va = -4V and Va = +5 V. Those values were as close to the desired Vout that the op amp could achieve with the supplied power. With these observations, it's obvious why this circuit is called a difference amplifier.
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