In this lab, we explored one of the (more useless) ways of using an operational amplifier. In this particular exercise, we used the op amp to create a negative constant which will be multiplied to the input voltage. This particular setup is known as an inverting op amp, and the circuit we will use in our experiment is pictured below:
We set up the circuit so that the input resistance R1 is approximately 32.7 kOhms, R2 is 68.1 kOhms, and V+ and V- from the analog discovery were plugged into inputs 4 and 7 of the op amp, which will theoretically limit the output voltage to +/- 5 V. The source, which goes through R1, was connected to the inverting input, and V_out was measured from the output port 6.
Before starting the lab, we were instructed to determine the theoretical output voltage of the circuit. Using the spec chart for the OP27 op amp, we were able to determine its internal resistance, and solve for V_out using simple nodal analysis. According to the calculations, the output voltage should have been -2.08 * V_in, or in other words, the circuit gain is -2.08.
To test this, we set the input voltage to be 1 V, which means the output voltage should read somewhere near ~ -2 V.
On top of this, we were instructed to tabulate the results of output voltages for input voltages ranging from -3 V to +4 V in increments of 0.5 V.
The rate of change of output voltage to input voltage represents the circuit gain due to the inverting op amp. Thus, the slope of the line while the op amp is in operating range represents this circuit gain which is approximately -2. One thing to note is that the op amp doesn't actually saturate at +/- 5 V; instead, it tops out at 4.31 V and bottoms out at -3.54 V.
In Class Examples:
1. A 741 op amp has an open-loop voltage gain of 2 × 10 5 , input resistance of 2 M, and output resistance of 50 Ohms . The op amp is used in the circuit shown below. Find the closed-loop gain vo /v s . Determine current i when v s = 2 V.
2. Using the same 741 op amp calculate the closed-loop gain vo/vs. Find io when vs = 1V.
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