Schmitt Trigger: Robust Comparator Design with Hysteresis

3 days ago

施密特触发器是一种具有内置滞回的比较器，它使用两个切换阈值（V_TH 和 V_TL）来消除噪声并确保稳定的输出跳变。
反相运算放大器施密特触发器阈值：V_TH = (R1/(R1+R2)) × V(sat)，V_TL = (R1/(R1+R2)) × (-V(sat))，滞回宽度 V_H = (2×R1/(R1+R2)) × |V(sat)|。
同相运算放大器施密特触发器阈值：V_UT = +(R1/Rf) × V(sat)，V_LT = -(R1/Rf) × V(sat)，滞回 = 2×(R1/Rf)×|V(sat)|。
分立BJT施密特触发器使用两个晶体管和公共发射极电阻；滞回由发射极电压偏移产生，通常约为0.4 V。
集成CMOS施密特触发反相器（例如SN74HC14、SN74LVC1G14、CD40106）提供固定的滞回和快速切换，非常适合数字接口。
像LM393这样的比较器IC需要外部反馈来实现滞回，而TLV3201包含1.2 mV的内部滞回，延迟为40 ns。
应用包括开关去抖动、过零检测、信号整形、弛张振荡器和传感器信号调理。
常见错误：省略正反馈、滞回设置过小、未能偏置单电源电路、使用运算放大器作为比较器、以及忽略开集电极输出上的上拉电阻。
滞回宽度应超过预期噪声幅度（例如至少为噪声的2倍）以防止抖动，同时平衡灵敏度和传播延迟。

The article explains reverse engineering of analog chips using the Philips TDA7000 FM radio receiver as an example, a 1977 chip with over 100 transistors and single-layer metal.
Key components identified on the die include NPN transistors (rectangular, vertical structure), PNP transistors (circular, lateral), resistors (zig-zag P-type silicon), and junction capacitors (comb-shaped reverse-biased diodes).
Common analog circuits are described: emitter follower (voltage follower with current amplification), current mirror (clones reference current), current source/sink, differential pair amplifier (two transistors with connected emitters), and Gilbert cell multiplier (four-quadrant multiplication).

The author built a physical 17x17 matrix of switches with integrated LEDs to simulate Conway's Game of Life, using a microcontroller (AVR128DA64) to control rows and columns.
The LED display uses a multiplexing scheme where each row is active for 1/17th of the time, requiring higher current (150 mA per LED) to maintain brightness, with MOSFETs for row and column switching.
User input is detected by reusing row select lines to ground and sensing switch closures via GPIO pins, with pull-up resistors integrated on the MCU.

The article explains reverse engineering of analog chips using the Philips TDA7000 FM radio receiver as an example, a 1977 chip with over 100 transistors and single-layer metal.
Key components identified on the die include NPN transistors (rectangular, vertical structure), PNP transistors (circular, lateral), resistors (zig-zag P-type silicon), and junction capacitors (comb-shaped reverse-biased diodes).
Common analog circuits are described: emitter follower (voltage follower with current amplification), current mirror (clones reference current), current source/sink, differential pair amplifier (two transistors with connected emitters), and Gilbert cell multiplier (four-quadrant multiplication).

The author built a physical 17x17 matrix of switches with integrated LEDs to simulate Conway's Game of Life, using a microcontroller (AVR128DA64) to control rows and columns.
The LED display uses a multiplexing scheme where each row is active for 1/17th of the time, requiring higher current (150 mA per LED) to maintain brightness, with MOSFETs for row and column switching.
User input is detected by reusing row select lines to ground and sensing switch closures via GPIO pins, with pull-up resistors integrated on the MCU.

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