Transcript 9.2 Schmitt Trigger

9.2 Schmitt Trigger
9.2.1 Introduction
数目众多的
多功能性
妨碍, 牵制
The Schmitt trigger has found many
applications in numerous circuits, both
analog and digital. The versatility of a TTL
Schmitt is hampered by its narrow supply
range,
阻抗 limited interface capability, low input
impedance and unbalanced output
characteristics. The Schmitt trigger could
be built from discrete devices to satisfy a
particular parameter, but this is a careful
and sometimes time-consuming design.
耗时的
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9.2 Schmitt Trigger
The CMOS Schmitt trigger has the following
advantages:
• 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
对称的, 均匀的
• 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
变化
免疫性
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9.2 Schmitt Trigger
9.2.2 Schmitt Trigger’s Characteristic





Slowly changing input  fast transition time
at the output
Different switching thresholds for positive
and negative-going inputs
Hysteresis voltage = VT+- VTApplication:

Trun a noisy or slowly varying input signal
into a clean digital output signal
CMOS Schmitt Trigger:

The number of transistors in PUN and PDN
will be changed by feedback  VT also
changed
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9.2 Schmitt Trigger
9.2.3 Schmitt Trigger Consists of CMOS Gates
R2
VI
R1
1
VI1
Vo
1
Vo1
Assume Vth = VDD/ 2 and R1<R2
Input VI is triangle wave
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9.2 Schmitt Trigger
R2
R1
VI 1 
VI 
Vo (1)
R1  R2
R1  R2
VI
VT+
VT-
VI 1  Vth
t
Vo
t
Vo
Vo  VDD
When VI 1  Vth
From (1) we have
R2
VI 1  Vth 
VT 
R1  R2
VT 
VT-
VT+
VI
Vo  0
R1
 (1 
)Vth (2)
R2
VI 1  Vth
Vo  VDD
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9.2 Schmitt Trigger
VI1↓, when
VI 1  Vth
Vo  0
VI  VT 
R2
R1
VI 1  Vth 
VT  
VDD
R1  R2
R1  R2
VT   (1 
R1
)Vth
R2
The hysteresis is
VT  VT   VT 
2 R1

Vth
R2
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9.2 Schmitt Trigger
9.2.3 The Applications of Schmitt Trigger
 Transform of Waveform
VI
VT+
VT-
t
V
o
t
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9.2 Schmitt Trigger
 Pulse Plastic
VI
VT+
VTVo
t
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9.2 Schmitt Trigger
 Pulse Amplitude Detection
VI
VT+
VT-
Vo
t
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9.2 Schmitt Trigger
 Reform a Astable Multivibrator
VDD  VT 
V
 RC ln T 
VDD  VT 
VT 
 VT  VT 

)
 VT  VT 
R
T=T1+T2= RC ln
VDD
 RC ln(
VDD
VI
VT+
VT-
Vo
1
VI
Vo
T1
T2
t
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9.2 Schmitt Trigger
 Reform a Monostable Multivibrator
VR
VI
VR
1
VDD
Tw  RC ln
VT 
VI
VR
VT+
VTVo
Tw
t
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