9.2 Schmitt Trigger.ppt

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9.2 Schmitt Trigger

The Schmitt trigger has found manyapplications in numerous circuits, bothanalog and digital. The versatility of a TTL

Schmitt is hampered by its narrow supplyrange, limited interface capability, low inputimpedance and unbalanced outputcharacteristics. The Schmitt trigger couldbe built from discrete devices to satisfy aparticular parameter, but this is a carefuland sometimes time-consuming design.

9.2.1 Introduction 数目众多的多功能性妨碍

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阻抗

耗时的

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9.2 Schmitt Trigger

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The CMOS Schmitt trigger has the followingadvantages:

• High impedance input

• Balanced input and output characteristics

— Thresholds are typically symmetrical to 1⁄2 VCC

— Outputs source and sink equal currents— Outputs drive to supply rails

• Positive and negative-going thresholds show low

variation with respect to temperature

• Wide supply range (3V–15V), split supplies possible• Low power consumption, even during transitions

• High noise immunity, 0.70 VCC typical

免疫性

对称的

,均匀的

变化



Slowly changing input fast transition timeat the output

Different switching thresholds for positiveand negative-going inputs

Hysteresis voltage = V T+- V T-

Application:

Trun a noisy or slowly varying input signalinto a clean digital output signal

CMOS Schmitt Trigger: The number of transistors in PUN and PDN

will be changed by feedback V T alsochanged

9.2 Schmitt Trigger

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9.2.2 Schmitt Trigger’s Characteristic



9.2 Schmitt Trigger

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9.2.3 Schmitt Trigger Consists of CMOS Gates

1 1 VI R 1

R 2

VI1 Vo1

Vo

Assume Vth

= VDD

/ 2 and R1<R2

Input VI is triangle wave



9.2 Schmitt Trigger

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VI

VT+

VT-

t

t

VI

Vo

Vo

VT- VT+

o I I V

R R

RV

R R

RV

21

1

21

2

1

(1)

01

oth I

V V V

When DDoth I V V V V

1

From (1) we have

T th I

V R R

RV V

21

2

1

thT V R

R

V )1(2

1

(2)

DDoth I V V V V

1



9.2 Schmitt Trigger

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VI1↓, when

T I oth I

V V V V V 01

DDT th I V

R R

RV

R R

RV V

21

1

21

2

1

thT V

R

RV )1(

2

1

The hysteresis is

thT T T V

R

RV V V

2

12



9.2 Schmitt Trigger

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Transform of Waveform

VT+

VT-

VI

V

o

t

t

9.2.3 The Applications of Schmitt Trigger



9.2 Schmitt Trigger

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Pulse Plastic

VT+

VT-

VI

Vo

t



9.2 Schmitt Trigger

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Pulse Amplitude Detection

Vo

t

VT+

VT-

VI



9.2 Schmitt Trigger

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Reform a Astable Multivibrator

1

R

VI Vo

Vo

VT+

VT-

VI

t T1 T2

T=T1+T2=

T

T

T DD

T DD

V

V RC

V V

V V C lnln

)ln(

T

T

T DD

T DD

V

V

V V

V V RC



9.2 Schmitt Trigger

Reform a Monostable Multivibrator

1 VI

VR VR

T

DD

w

V

V RC T ln

Vo

t

VI

Tw

VT+

VT-

VR

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9.2 Schmitt Trigger.ppt

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