Editing Schmitt trigger (section)

=== Fundamental idea ===
[[Image:Ideal feedback model.svg|thumb|300px|Block diagram of a Schmitt trigger circuit. It is a system with [[positive feedback]] in which the output signal fed back into the input causes the amplifier ''A'' to switch rapidly from one saturated state to the other when the input crosses a threshold.<br />
'''''A''''' > 1 is the amplifier [[gain (electronics)|gain]]<br />
'''''B''''' < 1 is the feedback [[transfer function]]]]

Circuits with hysteresis are based on positive feedback. Any active circuit can be made to behave as a Schmitt trigger by applying positive feedback so that the [[loop gain]] is more than one. The positive feedback is introduced by adding a part of the output voltage to the input voltage. These circuits contain an [[Attenuator (electronics)|attenuator]] (the B box in the figure on the right) and an [[Analog adder|adder]] (the circle with "+" inside) in addition to an amplifier acting as a comparator. There are three specific techniques for implementing this general idea. The [[#dynamic threshold|first]] two of them are dual versions (series and parallel) of the general positive feedback system. In these configurations, the output voltage increases the effective difference input voltage of the comparator by "decreasing the threshold" or by "increasing the circuit input voltage"; the threshold and memory properties are incorporated in one element. In the [[#two thresholds|third technique]], the threshold and memory properties are separated.

{{anchor|dynamic threshold}}'''Dynamic threshold (series feedback):''' when the input voltage crosses the threshold in either direction, the circuit itself changes its own threshold to the opposite direction. For this purpose, it subtracts a part of its output voltage from the threshold (it is equal to adding voltage to the input voltage). Thus the output affects the threshold and does not affect the input voltage. These circuits are implemented by a differential amplifier with "series positive feedback" where the input is connected to the inverting input and the inverted output to the non-inverting input. In this arrangement, attenuation and summation are separated: a voltage divider acts as an attenuator and the loop acts as a simple [[KVL|series voltage summer]]. Examples are the classic transistor [[#Classic emitter-coupled circuit|emitter-coupled Schmitt trigger]], the [[#Inverting Schmitt trigger|op-amp inverting Schmitt trigger]], etc.

{{anchor|modified input}}'''Modified input voltage (parallel feedback):''' when the input voltage crosses the threshold in either direction the circuit changes its input voltage in the same direction (now it adds a part of its output voltage directly to the input voltage). Thus the output augments the input voltage and does not affect the threshold. These circuits can be implemented by a single-ended non-inverting amplifier with "parallel positive feedback" where the input and the output sources are connected through resistors to the input. The two resistors form a weighted [[Kirchhoff's circuit laws#Kirchhoff's current law (KCL)|parallel summer]] incorporating both the attenuation and summation. Examples are the less familiar [[#Collector-base coupled circuit|collector-base coupled Schmitt trigger]], the [[#Non-inverting Schmitt trigger|op-amp non-inverting Schmitt trigger]], etc.

Some circuits and elements exhibiting [[negative resistance]] can also act in a similar way: [[negative impedance converter]]s (NIC), [[Relaxation oscillator#Pearson–Anson electronic relaxation oscillator|neon lamps]], [[tunnel diode]]s (e.g., a diode with an N-shaped current–voltage characteristic in the first quadrant), etc. In the last case, an oscillating input will cause the diode to move from one rising leg of the "N" to the other and back again as the input crosses the rising and falling switching thresholds.

{{anchor|two thresholds}}'''Two different unidirectional thresholds''' are assigned in this case to two separate open-loop comparators (without hysteresis) driving a [[bistable multivibrator]] (latch) or [[Flip-flop (electronics)|flip-flop]]. The trigger is toggled high when the input voltage crosses down to up the high threshold and low when the input voltage crosses up to down the low threshold. Again, there is a positive feedback, but now it is concentrated only in the memory cell. Examples are the [[555 timer]] and the switch debouncing circuit.<ref>[http://www.ee.nmt.edu/~elosery/fall_2008/ee231L/lab6.pdf Debouncing switches with an SR latch]</ref>

[[Image:Schmitt trigger symbol.svg|thumb|right|200px|The Schmitt trigger symbol shown with a non-inverting [[hysteresis]] curve embedded in a [[buffer amplifier|buffer]]. Schmitt triggers can also be shown with inverting hysteresis curves and may be followed by [[logic gate|bubble]]s. The documentation for the particular Schmitt trigger being used must be consulted to determine whether the device is non-inverting (i.e., where positive output transitions are caused by positive-going inputs) or inverting (i.e., where positive output transitions are caused by negative-going inputs).]]

The symbol for Schmitt triggers in circuit diagrams is a triangle with a symbol inside representing its ideal hysteresis curve.