Schmitt trigger with two transistors

In positive-feedback configuration used in implementation of the Schmitt trigger, most of the complexity of comparator's own implementation is unused. Therefore, it can be replaced with 2 cross-coupled transistors (that is the transistors which would otherwise implement the input stage of comparator). An example of 2-transistor-based configuration is illustrated below. The chain R_K1 R₁ R₂ fixes the base voltage for the transistor T₂. This divider, is affected by transistor T₁, providing higher voltage if T₁ is open. Thus the threshold voltage for switching between the states depends on present state of trigger.

For NPN transistors, when the input voltage is below the shared emitter voltage, T₁ does not conduct. The base voltage of the transistor T₂ can be determined by the mentioned divider. Because of negative feedback, the voltage at shared emitters should be almost as high as that set by divider so that T₂ is conducting, and trigger output is in low state. T₁ will conduct when the input voltage (T₁ base voltage) increases slightly above the voltage across the resistor R_E (emitter voltage). When T₁ starts to conduct, T₂ ceases to conduct, as the voltage divider now provides lower T₂ base voltage while emitter voltage does not drop because T₁ is drawing current across RE now. With T₂ not conducting the trigger has transitioned to the high state now.

With the trigger in high state now, if input voltage lowers enough, current through T₁ reduces, lowering the shared emitter voltage and raising base voltage for T₂. As T₂ starts to conduct, the voltage across R_E rises, further reducing T₁ base-emitter potential and T₁ stops to conduct.

In high state, the output voltage is close to V₊, but in low state it is still above V_-. This may not  be low to  be a "logical zero  " for digital circuits. This may need additional amplifiers following trigger circuit.

The circuit can be simplified: R₁ can be replaced by a short circuit connection, connecting T₂ base directly to the T₁ collector, and R₂ can be taken out and can be replaced with an open circuit. The key to its operation is that less current flows through R_E when T₁ is switched on (as a result of input current into its base) than when T₁ is off, because turning T₁ on turns T₂ off, and T₂, when on, provides more current through RE  than does T₁. With less current entering R_E, the voltage across it will be lower, and so as the current gets going into T₁ once, the input voltage must go lower to turn T₁ back off as now its emitter voltage has been lowered. This Schmitt trigger buffer can also be turned into a Schmitt trigger inverter and another resistor saved in the process, by replacing R_K2 with a short connection, and connecting Vout to the emitter of T₂ instead of its collector. In this case, a larger value o f resistance should be used for RE as it now serves as the pull-down resistor on the output, lowering the voltage on the output when the output should be low, in spite of a serving as a small resistance only which is only intended to develop a small voltage across it that actually adds to the output voltage when it should be at a digital low.

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