Day 10: Op Amp/ Op Amp Lab

Part 1: Op Amp 

Today, we study about op amp and its structure and properties. 

The terminals of primary interest are: inverting input, noninverting input, output, positive power supply, negative power supply. 

The voltage transfer characteristic describes how the output voltage varies. There are 3 distinct regions of operation. Based on the values of the difference of the input voltages, the output voltage is a linear function of the input voltages or saturates. A is the multiplying constant or gain. 

Then the professor asked us what are the open loop and the closed loop. In terms of electric circuits, the closed loop is when a signal is fed back from the output terminal to the inverting input terminal(-), also called negative feedback. While the open loop is when the circuit has no feedback. 

The professor introduce about a op amp practice. We use the realistic op amp with a large input resistor, a large gain, and a small out resistor. By applying the nodal technique at the inverting input voltage (Vn), and the output voltage (Vo), we gain two equation and solve for Vn, Vo. We calculate the gain by A=Vout/Vin

The ideal op amp. We retrieve two conditions for the ideal op amp. The first one is the constraint on input voltage of the op amp: Vn = Vp. Consequently, the gain A is infinite. The second condition is the current constraint ip = in = 0. Analysis of the op amp integrated circuit reveals that the equivalent resistance seen by the input terminals of the op amp is very large, typically 1M Ohm or more. 

Part 2: The Inverting Voltage Amplifier

Purpose: The lab aims to put the theory of the op amp into the realistic experiment. Specifically, we analyze the inverting voltage amplifier with a closed loop. We expect to retrieve the graph with three distinct regions of an op amp. 

We do the pre-lab to find the value of R2 so that we designed an amplifier which provides a gain of 2 and an input resistance R1 =2k Ohm. We end up with R2 = 4k Ohm. From the condition of an ideal op amp and the nodal analysis at the inverting input, we get vo=-(R2/R1)*vi. 

The guidance for setting up an op amp OP27 on the breadboard.

We set up the experiment, then adjust the input voltage and record the output voltage values. 

The measured output voltage table and the graph Vin vs. Vout. We observe that the graph has 3 distinct region as a op amp's property: positively saturated region, linear region, negatively saturated region. Besides, the negative and positive saturation are not equal to the value of the power supply because of the imperfection of a an op amp.

The professor's explanation why the saturation is not equal to the power supply value. This is called rail to rail performance, op amp effective measurement.

Conclusion:

We learned about a new concept/component of electric circuits: op amp. We discussed how to distinguish  between the open loop and the closed loop by the negative feedback of the inverting input. We also discussed on three conditions of the ideal op amp: infinite input resistance, infinite gain, zero input current. In the experiment, we observe the op amp property with the graph v(in) vs. v(out), and the rail to rail performance.