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MALVINO & BATES

SEVENTH EDITION

Electronic

PRINCIPLES

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Oscillators

ChapterChapter2323

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Topics Covered in Chapter 23

• Theory of sinusoidal oscillation

• The Wien-bridge oscillator

• Other RC oscillators

• The Colpitts oscillator

• Other LC oscillators

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Topics Covered in Chapter 23

(Continued)

• Quartz crystals

• The 555 timer

• Astable operation of the 555 timer

• 555 circuits

• The phase-locked loop

• Function generator ICs

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voutAV

B

Sinusoidal oscillation requires

both the correct phase and loop gain.

AVB < 1

AVB = 1

AVB > 1

φφφφ = 0° = positive feedback..

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Sinusoidal oscillators

• The starting signal is thermal noise.

• AVB > 1 at startup (AVB is the loop gain).

• The feedback network determines B and

the phase of the feedback.

• Only one frequency arrives at the input

as an in-phase signal (positive feedback).

• Either AV or B is eventually decreased so

that AVB = 1.

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Wien-bridge oscillator

• Used for low to moderate frequencies (5

Hz to 1 MHz)

• Produces an almost perfect sine wave

• Nonlinear resistance is used to decrease

loop gain to 1

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R

vout

C

Wien-Bridge oscillator circuit

RL2R’

Tungsten

lamp

C R

R’

2ππππRC

1fr =

vout t

Resistance of lamp

increases until

equilibrium is reached

R’

.

.. .

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Other RC oscillators

• Twin-T uses an amplifier and RC circuits

to produce the required loop gain and

phase shift

• Twin-T works well at only one frequency

• Phase shift uses an amplifier and RC

circuits

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Colpitts oscillator

• LC oscillators are preferred for

frequencies between 1 MHz to 500 MHz

• One of the most widely used oscillators

• Recognized by the capacitive voltage

divider

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+VCCColpitts oscillator circuit

2ππππ LC

1fr =

R2

R1

RFC

RE

C2

C1

CE

L

C3

vout

C1 C2

C1 +C2

C =

C1

C2

B =

C2

C1

AVmin =

..

.

.

.

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Other LC oscillators

• Armstrong uses a transformer to produce

a feedback signal

• Hartley uses an inductive voltage divider

to produce a feedback signal

• Clapp has a small series capacitor in the

inductive branch of the resonant circuit

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+VCCColpitts CB oscillator

2ππππ LC

1fr =

R2

R1

RFC

RE

C2

C1

LC3

C1 C2

C1 +C2

C =C1

C1 +C2

B =

C1

AVmin =C1 +C2

RL

C4.

.

.

. ..

.

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+VCCHartley oscillator

2ππππ LC

1fr =

R2

R1

RFC

R3

C L1

vout

L = L1 + L2

L2

L1

B =

L1

L2

AVmin =

L2

.

.

.

.

. .

.

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Quartz crystal Slab cut from

crystal

Electrodes

and leads

Schematic

symbol

Crystal-controlled oscillators are used

when frequency stability is important.

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+VCCCrystal oscillator

R2

R1

RFC

R3

C2

C1

CE

vout

Xtal

.

.

.

.

.

..

.

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Crystals

• The fundamental frequency (series

resonance) is controlled by the slab

thickness.

• Higher multiples of the fundamental are

called overtones.

• The electrode capacitance creates a

parallel resonant frequency which is

slightly higher.

• Typical frequency accuracy is measured

in parts per million (ppm).

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The 555 timer

• Contains two comparators, an RS flip-

flop, and an npn transistor

• Has an upper trip point (UTP) and a

lower trip point (LTP)

• In the monostable mode, the input

triggers must fall below the LTP to start

the action

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Monostable operation of the 555 timer IC

555 vout

+VCC

T

8

3

1

2

Wvout

T T = trigger

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Astable operation of the 555 timer IC

555 vout

+VCC

8

3

1

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5 kΩΩΩΩ

R

S Q

Q5 kΩΩΩΩ

5 kΩΩΩΩ

Gnd1

Out

3

+VCC8Discharge

Threshold

Control

Trigger

7

6

5

2

555

Reset4

UTP2VCC /3

LTPVCC /3

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RC RC

RB RB

RS RR

QQ

+VCC

S R

A discrete RS flip-flop

S

R

Q

Q

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1

3

+VCC

7

6

2

8

555 IC configured for monostable operation

R

C W

W = 1.1RC

. .

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Astable operation of the 555

• Produces a rectangular output whose

duty cycle can be set between 50 and 100

percent

• A control voltage is sometimes used to

determine the frequency

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1

3

+VCC

7

6

2

8

555 IC configured for astable operation

R2

CW

W = 0.693(R1 + R2)CR1

TT = 0.693(R1 + 2R2)C

R1 + 2R2

R1 + R2D =

.

.

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1

3

+VCC

7

6

2

8

555 voltage-controlled oscillator

R2

C

R1

W = -(R1 + R2)C lnVCC - Vcon

VCC - 0.5Vcon

W + 0.693R2C

1f =

5

Vcon applied to pin 5

+Vcon

+Vcon /2

.

.

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555 circuits

• Time delays

• Alarms

• Ramp outputs

• Pulse-width modulators

• Pulse-position modulators

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+VCC

8

R

6 5

4

3

2 1

7

C

555

Pulse-width modulation with the 555 timer IC

Clock in

Modulating

signal in

A

B

PWM

out

UTP = 2VCC /3 + vmod

W = -RC ln 1 -UTP

VCC( )

W

T

T

The output frequency is

established by the input clock.

. .

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Pulse width is variable

+VCC

8

R1

6 5

4

3

2 1

7

C

555

Pulse-position modulation with the 555 timer IC

Modulating

signal in

A

B

PPM

outR2Space is constant

The leading edge of each pulse

is a function of the modulation.

Space = 0.693R2C

.

.

.

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Phase-locked loops

• It is possible to phase-lock an oscillator to

a signal by using a phase detector and

negative feedback.

• PLLs can be used to remove noise from a

signal.

• PLLs can be used to demodulate an FM

signal.

• PLLs are available as monolithic ICs.

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Function generator ICs

• Ability to produce sine, square, triangle, pulse, and sawtooth waveforms

• By connecting external resistors and capacitors, output waveforms can be made to vary in frequency and amplitude

• AM/FM, voltage-to-frequency conversion, and FSK can also be performed