9.2 Schmitt Trigger 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, 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 Introduction 数目众多的 多功能性 妨碍, 牵制 阻抗 耗时的 ReturnNext

9.2 Schmitt Trigger NextBackReturn 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 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 time at the output Different switching thresholds for positive and negative-going inputs Hysteresis voltage = V T+ - V T- Application: 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  V T also changed 9.2 Schmitt Trigger NextBackReturn Schmitt Trigger’s Characteristic

9.2 Schmitt Trigger NextBackReturn Schmitt Trigger Consists of CMOS Gates 1 1 VIVI R1R1 R2R2 V I1 V o1 VoVo Assume V th = V DD / 2 and R1<R2 Input V I is triangle wave

9.2 Schmitt Trigger NextBackReturn VIVI V T+ V T- t t VIVI VoVo VoVo V T+ (1) When From (1) we have (2)

9.2 Schmitt Trigger NextBackReturn V I1 ↓, when The hysteresis is

9.2 Schmitt Trigger NextBackReturn Transform of Waveform V T+ V T- VIVI VoVo t t The Applications of Schmitt Trigger

9.2 Schmitt Trigger NextBackReturn  Pulse Plastic V T+ V T- VIVI VoVo t

9.2 Schmitt Trigger NextBackReturn  Pulse Amplitude Detection VoVo t V T+ V T- VIVI

9.2 Schmitt Trigger NextBackReturn  Reform a Astable Multivibrator 1 R VIVI VoVo VoVo V T+ V T- VIVI t T1T1 T2T2 T=T1+T2=

9.2 Schmitt Trigger  Reform a Monostable Multivibrator 1 VIVI VRVR VRVR VoVo t VIVI TwTw V T+ V T- VRVR BackReturn