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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 Return Next 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 Return Back Next 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 Return Back Next 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 Return Back Next 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 Return Back Next 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 Return Back Next 9.2 Schmitt Trigger 9.2.3 The Applications of Schmitt Trigger Transform of Waveform VI VT+ VT- t V o t Return Back Next 9.2 Schmitt Trigger Pulse Plastic VI VT+ VTVo t Return Back Next 9.2 Schmitt Trigger Pulse Amplitude Detection VI VT+ VT- Vo t Return Back Next 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 Return Back Next 9.2 Schmitt Trigger Reform a Monostable Multivibrator VR VI VR 1 VDD Tw RC ln VT VI VR VT+ VTVo Tw t Return Back