electronic oscillators

7/29/2019 Electronic Oscillators

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Oscillators


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Objectives State the function of the oscillator.

Describe positive feedback used in the oscillator

Explain the Barkhausen criterion

List the three requirements for proper oscillatoroperation.

Describe the operating characteristics of thephase-shift and Wien-bridge RC oscillators.

Describe the operation of the Colpitts and Hartley

oscillators. Describe the operating principles of crystals and

crystal-controlled oscillators.


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Positive feedback.

Av

v

RL

vin

vf

Amplifier(=180)

Feedback network(=180)

voutIn-phase


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How to generate a signal.

Regenerative

feedback

Av

v

vin

vf

vout

Av

v

vin

vf

vout

Amplifier

Oscillator

1v vA


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Effects ofvAv on oscillator

operation.1v vA Barkhausen criterion:

1v vA

1v vA

1v vA


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Phase-shift oscillator.

IfR1 = R2 = R3 = Rand C1 = C2 = C3 = C, thenosc

1 1and

292 6vf

RC

R1

R2

R3

C1

C2

C3

Feedback network

Av

(Av

< 0)


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Wien-bridge oscillator.

R

R

C

C

Rf

Rh

osc

1 1and

2 3vf

RC

1 3

2

f

v

h

f

h

RAR

R

R


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Wien-Bridge Oscillator (Lab.)

741

R

R

C

C

R2

R1

vo

RST

vi1

vi2

R = 10k

C= 15nF

R1 = 8.2 kR2 = 3.9 k

RST

= 120 k

2x1N4148


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Colpitts oscillator (CEconfiguration.)

1 2

1 2

T

C CC

C C

1

2r

T

fLC

2 1

1 2

Cv

C

X C

X C

R1

R2

RE

RFC

C1

C2

L

VCC

vout


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ExampleR

1

R2

RE

RFC

C1

3.3nFC

2

0.1F

L

47H

VCC

vout

1 2

1 2

3.3nF 0.1F3.19nF

3.3nF 0.1FT

C CC

C C

1 1411kHz

2 2 47H 3.19nF

r

T

f

LC

2 1

1 2

3.3nF

0.1F

0.033

C

vC

X C

X C


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R1

R2

RE

RFC

C1

LP

VCC

vout

R1

R2

RE

RFC

C1

L1

VCC

vout

L2

Hartley oscillator.

1 2

1

1

2

T

r

T

L L L

fL C

1

1

2r

P

fL C


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Other Oscillator Circuits Clapp oscillator

Armstrong oscillator

Etc


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Quartz crystal.x

x

y

y

z

z

CC L R

CM

XL

XC

fs

fp

f

Symbol

Equivalent circuitFrequency response

s p

f f


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Crystal-controlled Colpittsoscillators.

RE

RFCR1

R2

C2

C1

vout

Y1

VCC


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Pierce oscillator.

XTAL

CF CCv

out

RG

RFC

+VDD

C1

C2


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Typical Crystal Oscillator Circuits usedin a Microprocessor-Based System

Rf

C1 C2

R1

vout

C1 = C2 = 22 pF or 30 pF


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Overtone Mode Normal crystal frequency 25 MHz.

Higher frequency is obtained by tuning the

frequency to crystal harmonics (overtones).

Used in digital and communicationselectronics.


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Summary Oscillator principle

Phase-shift oscillators

Wien-bridge oscillator Colpitts oscillator

Other LC oscillators

Crystal-controlled oscillators

electronic oscillators

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