Parul Poltehynic Institute Subject Code : 3320701 Name Of Subject : Basic Electronics Name of Unit : Ch-3 Oscillator’s Topic : Oscillator’s Name of Faculty

• Parul Poltehynic Institute

Subject Code : 3320701

Name Of Subject : Basic Electronics

Name of Unit : Ch-3 Oscillator’s

Topic : Oscillator’s

Name of Faculty : H.M.Avaiya

• Introduction

• Oscillator is an electronic circuit that generates a periodic waveform on its output without an external signal source. It is used to convert dc to ac.

• Oscillators are circuits that produce a continuous signal of some type without the need of an input.

• These signals serve a variety of purposes. • Communications systems, digital systems (including

computers), and test equipment make use of oscillators

Sub: Basic Electronics Topic: Oscillator’s

•Introduction

• An oscillator is a circuit that produces a repetitive signal from a dc voltage.

• The feedback oscillator relies on a positive feedback of the output to maintain the oscillations.

• The relaxation oscillator makes use of an RC timing circuit to generate a nonsinusoidal signal such as square wave



Oscillators

• Oscillation: an effect that repeatedly and regularly fluctuates about the mean value

• Oscillator: circuit that produces oscillation

• Characteristics: wave-shape, frequency, amplitude, distortion, stability


Types of oscillators

1. RC oscillators– Wien Bridge– Phase-Shift

2. LC oscillators– Hartley– Colpitts– Crystal

3. Unijunction / relaxation oscillators


•Basic principles for oscillation

A

b

V e

V f

V sV o

+(1) fse VVV (2) of βVV (3) osfseo βVVAVVAAVV

•An oscillator is an amplifier with positive feedback



osfs

eo

βVVAVVA

AVV

oso VAAVV b

so AVVA b1

•The closed loop gain is:

Aβ

A

V

VA

s

of

1



sβsA1

sAs

V

VsA

s

of

•In general A and are functions of frequency and thus may be written as;

sβsA is known as loop gain



100 jωβjωA

•Thus, the condition for sinusoidal oscillation of frequency f0 is;

•This is known as Barkhausen criterion.• The frequency of oscillation is solely determined by the phase characteristic of the feedback loop – the loop oscillates at the frequency for which the phase is zero.



In phase

Noninverting amplifier

V f V oA v

Feedback circuit


R L1

L2

C

RC1

C2

L

•Hartley Oscillator •Colpitts Oscillator

T

oLC

1

1

2

RC

Cgm

21

21

CC

CCCT

CLL

o)(

1

21

2

1

RL

Lgm


•Colpitts Oscillator

• The Colpitts oscillator is a type of oscillator that uses an LC circuit in the feed-back loop.

• The feedback network is made up of a pair of tapped capacitors (C1 and C2) and an inductor L to produce a feedback necessary for oscillations.

• The output voltage is developed across C1.

• The feedback voltage is developed across C2.Sub: Basic Electronics Topic: Oscillator’s




011

21

sCsL

VVg

R

V

sC

V ogsm

oo

• KCL at the output node

ogs VsL

sC

sCV

2

2

1

1

•voltage divider produces:



01

1 122

2

sCR

LCssCgV mo

•substitute eq(2) into eq(1):

01

212

2

213

RgCCs

R

LCsCLCs m

•Assume that oscillation has started, then Vo≠0



01

212

212

2

CLCCCj

R

LC

Rgm

• Let s=jω

21

21

1

CCCC

L

o

• Both real & imaginary component must be zero Imaginary component



Rg

R

LCm

122

•both real & imaginary component must be zeroImaginary component

RgC

Cm

1

2

•Combining Eq(3) and Eq(4):

1

2

C

CRgm

•to initiate oscillations spontaneously:


•Hartley Oscillator

• The Hartley oscillator is almost identical to the Colpitts oscillator.

• The primary difference is that the feedback network of the Hartley oscillator uses tapped inductors (L1 and L2) and a single capacitor C.





CLLo

21

1

•the analysis of Hartley oscillator is identical to that Colpitts oscillator.the frequency of oscillation:


Documents

Parul Poltehynic Institute Subject Code : 3320701 Name Of Subject : Basic Electronics Name of Unit : Ch-3 Oscillator’s Topic : Oscillator’s Name of Faculty