University of Nueva Caceres

College of Engineering and Architecture

City of Naga

25 kHz WEIN-BRIDGE OSCILLATOR

A Report

Submitted in Partial Fulfillment of the Requirements

For Communications I Subject

Submitted By:

Submitted To:

OCTOBER 2011

CONTENTS

I. INTRODUCTION

II. COMPUTATIONS

III. SCHEMATIC DIAGRAM

IV. SIMULATION

V. COMPONENT AND PCB LAYOUT

VI. ACTUAL PHOTOS

VII. BILL OF MATERIALS

VIII. DATASHEETS

IX. GRADING SHEET

I. INTRODUCTION

Wave generators play a prominent role in the field of electronics. They generate signals

from a few hertz to several gigahertz. Modern wave generators use many different circuits and

generate such outputs as sinusoidal, square, rectangular, sawtooth, and trapezoidal

waveshapes. These waveshapes serve many useful purposes in different electronic circuits.

One type of wave generator is known as an oscillator. An oscillator can be thought of as

an amplifier that provides itself, through feedback, with an input signal. Oscillators are

classified according to the waveshapes they produce and the requirements needed for them to

produce oscillations.

The primary purpose of an oscillator is to generate a given waveform at a constant peak

amplitude and specific frequency and to maintain this waveform within certain limits of

amplitude and frequency.

Oscillators are important in many different types of electronic equipment. For example, a

quartz watch uses a quartz oscillator to keep track of what time it is. An AM radio transmitter

uses an oscillator to create the carrier wave for the station, and an AM radio receiver uses a

special form of oscillator called a resonator to tune in a station. There are oscillators in

computers, metal detectors and even stun guns.

A Wien bridge oscillator is a type of electronic oscillator that generates sine waves. It can

generate a large range of frequencies. The oscillator is based on a bridge circuit originally

developed by Max Wien in 1891. The bridge comprises four resistors and two capacitors. The

oscillator can also be viewed as a positive gain amplifier combined with a bandpass filter that

provides positive feedback.

It is so called because the circuit is based on a frequency-selective form of the Whetstone

bridge circuit. It is a two-stage RC coupled amplifier circuit that has good stability at its resonant

frequency, low distortion and is very easy to tune making it a popular circuit as an audio

frequency oscillator. It uses a feedback circuit consisting of a series RC circuit connected with a

parallel RC of the same component values producing a phase delay or phase advance circuit

depending upon the frequency. At the resonant frequency ƒr the phase shift is 0o.

Basic Wein Bridge Oscillator Circuit

The output of the operational amplifier is fed back to both the inputs of the amplifier. One

part of the feedback signal is connected to the inverting input terminal (negative feedback) via

the resistor divider network of R1 and R2 which allows the amplifiers voltage gain to be

adjusted within narrow limits. The other part is fed back to the non-inverting input terminal

(positive feedback) via the RC Wien Bridge network. The RC network is connected in the

positive feedback path of the amplifier and has zero phase shift a just one frequency. Then at

the selected resonant frequency, ƒr, the voltages applied to the inverting and non-inverting

inputs will be equal and "in-phase" so the positive feedback will cancel out the negative

feedback signal causing the circuit to oscillate.

Also the voltage gain of the amplifier circuit must be equal to three (Gain=3) for

oscillations to start. This value is set by the feedback resistor network, R1 and R2 for an

inverting amplifier and is given as the ratio -R1/R2. Also, due to the open-loop gain limitations

of operational amplifiers, frequencies above 1MHz are unachievable without the use of special

high frequency op-amps.

II. COMPUTATIONS

The voltage gain of a Wein bridge oscillator circuit must be equal to three (GAIN=3) for

oscillations to start to start. Using a 10k resistor for R1, it follows:

kR

kR

ARR

R

RA

kR

A

v

v

v

20

)13)(10(

)1)((

1

10

3

2

2

12

1

2

1

Using a 1nF capacitor to obtain a 25 kHz of oscillations, we will get the value of R needed

for the Wein bridge oscillator circuit:

kR

nFkHzR

fCR

kHzf

nFC

4.6

)1)(25(2

1

2

1

25

1

But since there is no available value of resistor rated 6.4 k , we will use a 8k trimmer

(also known as bias) to adjust the frequency of oscillation to 25 kHz.

VI. ACTUAL PHOTOS

VII. BILL OF MATERIALS

QUANTITY MATERIAL UNIT PRICE TOTAL PRICE

1 LM741 P 15.00 P 15.00 1 8-Pin IC Holder 5.00 5.00 1 20 kΩ 1W Resistor 2.50 2.50 1 10 kΩ 1W Resistor 2.50 2.50 1 5 kΩ 1W Resistor 2.50 2.50 1 1 kΩ 1W Resistor 2.50 2.50 2 5 kΩ Bias 8.00 8.50 2 1 nF Ceramic Capacitor 1.50 3.00 1 Copper Clad 22.00 22.00 1 Ferric Chloride 35.00 35.00 2 Soldering Lead 12.00 12.00 1 m. Connecting Wire 3.50 3.50

TOTAL: P 114.00

IX. GRADING SHEETS

SUBJECT: Communications I SCHOOL YEAR: 2011-2012

NAME: DATE SUBMITTED: __________________

CRITERIA PERCENTAGE GRADE REMARKS

LAYOUT 10%

DOCUMENTATION 10%

DESIGN 30%

FUNCTIONALITY 50%

TOTAL 100%