ECE 291 - Electrical Engineering Laboratory

# LAB 11: OPERATIONAL AMPLIFIERS

## INTRODUCTION

Operational amplifiers (so called Op-Amps) are wonderful devices.  General principles of their operation are easy to understand, if you know what feedback is.  They are also easy to use and very handy in many applications.  If you need to process analog signals you will most likely use Op-Amps rather than discrete transistors.  The devices are made as integrated circuits and come in a variety of packages, often in multiple units.  One of the oldest successful designs, still in demand, is the famous μA741 which you will meet in this laboratory.  The circuit consists of  over twenty bipolar transistors but, thanks to the miracles of modern IC manufacturing technology, it sells for a fraction of a dollar a piece.

Other courses in EE curriculum will provide you with the knowledge of applications and design with op-amps.  This laboratory exercise is just an introduction to this important field.

## PRELAB

Draw schematic diagrams of an inverting amplifier and a non-inverting amplifier utilizing operational amplifiers.  Express their gains as functions of the values of resistors used in the circuit.

## LABORATORY

### 1.      INVERTING AMPLIFIER

Using an op-amp in your parts kit wire an inverting amplifier.  Choose two sets of resistors in the circuit to obtain two gain values between five and a hundred.  Measure the gains and compare them with the values calculated from the known resistances in the circuit.  Measure the resistors with a digital ohmmeter.

### 2.      NONINVERTING AMPLIFIER AND THE GAIN BANDWIDTH PRODUCT (GBP)

Assemble a non-inverting amplifier with low gain, up to 10.  Check its frequency response.  Op-amp performance is limited at high frequency; it behaves as a low-pass filter.  Measure its bandwidth, which is defined by f-3db frequency.  Recall your work on filters, Experiments VII.  It is not necessary to measure the whole frequency distribution.

Repeat the bandwidth measurements for two more resistor sets, giving the gain in tens and hundreds range.  Measure the gain and the bandwidth in each case.

There is a rule that applies to these circuits which says that the gain bandwidth product (GBP) is a constant.  It means that, just like in life, you can not have something for nothing  In this case, to get more gain you have to pay for it with frequency you can amplify, and the other way around.  Test this rule for your three circuits.  Consider also the op-amp specifications which give you open loop gain (no feedback) as 200,000 and the bandwidth limiting frequency of 5 Hz.

### 3.      FOLLOWER

Build a voltage follower circuit, which is an op-amp version of an emitter follower circuit.  It is also called a buffer, because with its high input impedance and low output impedance it is used for isolating circuits, in a sense that they do not influence one another.

HINT: The gain of the follower is one and it does not invert the phase.  You do not even need resistors!

## REPORT

• Draw the schematics and list the values of the resistors used.
• Compare measured and calculated gains.
• Comment on GBP rule applied to the circuits you built.