ELEC 101, Spring 2005
Prof. Rich Kozick

## Laboratory 4 Operational Amplifier Introduction

The objective in lab this week is to get an introduction to operational amplifiers (op amps). We will discuss op amps in lecture next week, and then we will do some more interesting applications of op amps in Lab 5.

## 1. Electronic Lessons on Op Amps

http://www.facstaff.bucknell.edu/mastascu/eLessonsHTML/EEIndex.html
The op amp lessons are located under the Elements item, and you should read through the first two lessons, called Op-Amps-General and Op-Amps Inverter.

Use the lessons to guide you through setting up the inverting amplifier circuit in Section 2. You should read through the analysis in the E-lesson, but we will discuss op amps in detail during lecture next week.

## 2. Inverting Amplifier

Consider the inverting amplifier circuit shown in Figure 2.15 on page 75 of the Bobrow text.
1. Repeat the analysis that leads to the result that the output voltage vo is related to the input voltage vs according to vo = -(R2 / R1) vs.

For your convenience, the inverting amplifier circuit shown in Figure 2.15 of the Bobrow text is shown in Figure 1 below. The notation in Figure 1 is slightly different from the text, so the equation describing Figure 1 is .

Figure 1: "Inverting amplifier" with 741 op amp.

2. Build an inverting amplifier with gain = -(R2 / R1) = -10, using R1 = 1 k ohm, R2 = 10 k ohm.

You can obtain the pin diagram for the 741 op amp from the data sheet on the course web page under Laboratories, or you can refer to Figure 2 below.

Figure 2: Pin diagram for 741 op amp.

The pin numbers are also indicated in the circuit in Figure 1 above. We will use "supply" voltages +V and -V in Figure 2 equal to +12 volts and -12 volts, respectively.

3. If you have time, try modifying your circuit to achieve "gains" of vo / vs = -1 and -2. For each case, measure the output voltage vo for several positive and negative input voltages vs. Are there any limitations to these circuits? In other words, are there circumstances under which the desired gain is not achieved? What is the largest output voltage that your circuit can achieve? Can you explain why?

4. Suppose you want to design an inverting amplifier with a variable gain using a potentiometer (pot) whose resistance varies from a few ohms to 10 k ohms as a dial is turned. (This might be used for a volume control in a radio.) Should the pot be placed at R1 or R2? Try putting the pot in your amplifier circuit, if you have time.
Record information in your lab notebook that will help you in future weeks when construcing op amp circuits. Also, keep your inverting amplifier wired on your breadboard and we will use it next week in Lab 5.

Thank you and have fun!