Editing Schmitt trigger (section)

===== Variations =====
[[Image:Schmitt trigger inverted symbol.svg|thumb|right|200px|Symbol depicting an inverting Schmitt trigger by showing an inverted [[hysteresis]] curve inside a [[buffer amplifier|buffer]]. Other symbols show a hysteresis curve (which may be inverting or non-inverting) embedded in a buffer followed by a bubble, which is similar to the traditional symbol for a [[inverter (logic gate)|digital inverter]] that shows a buffer followed by a bubble. In general, the direction of the Schmitt trigger (inverting or non-inverting) is not necessarily clear from the symbol because multiple conventions are used, even with the same manufacturer. There are several factors leading to such ambiguity,<ref group="nb">One factor contributing to ambiguity is that one simple transistor-based realization of a Schmitt trigger is naturally inverting, with a non-inverting Schmitt trigger sometimes consisting of such an inverting implementation followed by an inverter. An additional inverter may be added for buffering a stand-alone inverting configuration. Consequently, inverting configurations within an integrated circuit may be naturally inverting, while non-inverting configurations are implemented with a single inverter, and stand-alone inverting configurations may be implemented with two inverters. As a result, symbols that combine inverting bubbles and hysteresis curves may be using the hysteresis curve to describe the entire device or the embedded Schmitt trigger only.</ref> These circumstances may warrant a closer investigation of the documentation for each particular Schmitt trigger.]]

'''Non-inverting circuit.''' The classic non-inverting Schmitt trigger can be turned into an inverting trigger by taking V<sub>out</sub> from the emitters instead of from a Q2 collector. In this configuration, the output voltage is equal to the dynamic threshold (the shared emitter voltage) and both the output levels stay away from the supply rails. Another disadvantage is that the load changes the thresholds so, it has to be high enough. The base resistor R<sub>B</sub> is obligatory to prevent the impact of the input voltage through Q1 base-emitter junction on the emitter voltage.

'''Direct-coupled circuit.''' To simplify the circuit, the R<sub>1</sub>&ndash;R<sub>2</sub> voltage divider can be omitted connecting Q1 collector directly to Q2 base. The base resistor R<sub>B</sub> can be omitted as well so that the input voltage source drives directly Q1's base.<ref>[http://www.datasheetcatalog.org/datasheets/400/334439_DS.pdf 7414 datasheet]</ref> In this case, the common emitter voltage and Q1 collector voltage are not suitable for outputs. Only Q2 collector should be used as an output since, when the input voltage exceeds the high threshold and Q1 saturates, its base-emitter junction is forward biased and transfers the input voltage variations directly to the emitters. As a result, the common emitter voltage and Q1 collector voltage follow the input voltage. This situation is typical for over-driven transistor [[Differential amplifier#Long-tailed a nice pair|differential amplifiers]] and [[Emitter-coupled logic#Operation|ECL]] gates.