Ch23 Oscillator Circuits1

8/13/2019 Ch23 Oscillator Circuits1

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Chapter 23 Oscillator Circuits

Applications of Electronic Devices

225

Andrew E. Milks PE


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Positive Feedback and Oscillators

Phase Shift Oscillator

Wien Bridge Oscillator

Tuned Oscillator

Crystal Oscillator

555 Timer

Voltage Controlled Oscillator


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Positive Feedback


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Positive Feedback

Causes the system to become unstable (oscillate)

and increase the size of the output


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Gain must be high enough to cause oscillation


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Gain must be high enough to cause oscillation

The RC components cause a total shift of 180 degrees

62

1

RCf


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Practical FET and BJT phase shift oscillators

62

1

RCf


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Practical FET and BJT phase shift oscillators

62

1

RCf


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If and

22112

1

CRCR

fo

2121 , CCRR 24

3

R

R

RCfo

2

1


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OP AMP Wien Bridge Oscillator


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Tuned Oscillator

Based on the resonant frequency

eqeq

oCL

f2

1


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Colpitts FET Tuned Oscillator


C1 and C2 can be considered to be in series since no currentflows in to the gate

21

212

1

CC

CCL

fo


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Colpitts BJT Tuned Oscillator


C1 and C2 are in series

21

212

1

CC

CCL

fo


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Colpitts Op Amp Tuned Oscillator


C1 and C2 are in series

21

212

1

CC

CCL

fo


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Hartley Tuned Oscillator


Similar to Colpitts exceptL1 and L2 are in series

The combination is parallel with C

M is the mutual inductance

CMLLCLf

eq

o)2(2

1

2

1

21


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Crystal Oscillator

Applying voltage to a crystal causes oscillations

Each crystal oscillates at 2 frequencies f1 when connected inseries f2 when connected in parallel which can be amplified in an

oscillator


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Crystal Oscillator

Series square wave oscillator circuit (f1)


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Crystal Oscillator

Parallel oscillator circuit (f2)


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Crystal Oscillator

Miller parallel oscillator circuit (f2)


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IC Timers 555

Modes of operation

Astable

square wave oscillator Monostable one shot pulse generator

Both modes are based on capacitor charging and

discharging


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IC Timers 555

Function blocks

2 comparators RS flip flop

Capacitor

discharging BJT

switch


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IC Timers 555

Astable Oscillator

Output is on (high) whencapacitor is charging through 2

resistors

Output is off (low) when

capacitor is discharging

through 1 resistor

Note the duty cycle cannot be

50%


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IC Timers 555

Astable Oscillator

CRTT

CRRTT

BlowOFF

BAhighON

)(7.0

)(7.0


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IC Timers 555

Mono stable One Shot

One output pulse for each timethe circuit is triggered

CRTTT PhighON )(1.1


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Voltage Controlled Oscillator

VCO 556

The oscillator output frequency

depends on the input

modulation voltage

Square and triangular wave outputs

available

V

VV

CRf Co

11

2

CV


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Voltage Controlled

Oscillator VCO 556

V

VV

CRf Co

11

2C

V


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Voltage Controlled

Oscillator VCO 556

HzV

VV

CRf Co 8.31

2

11

CV


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Phase Locked Loop PLL 565

Used in communication

circuits

Square wave output

Closed loop operation

The output signal is

compared to a reference and

adjusted to get the same zero

crossing


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Phase Locked Loop PLL 565

Used in communication

circuits

Frequency Demodulation

FM

Frequency Synthesis

multiples of the fundamentalinput locked in phase

Ch23 Oscillator Circuits1

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