Schmitt trigger
In electronics, a Schmitt trigger is a comparator circuit with hysteresis implemented by applying positive feedback to the noninverting input of a comparator or differential amplifier. It is an active circuit which converts an analog input signal to a digital output signal. The circuit is named a trigger because the output retains its value until the input changes sufficiently to trigger a change. In the non-inverting configuration, when the input is higher than a chosen threshold, the output is high. When the input is below a different chosen threshold the output is low, and when the input is between the two levels the output retains its value. This dual threshold action is called hysteresis and implies that the Schmitt trigger possesses memory and can act as a bistable multivibrator. There is a close relation between the two kinds of circuits: a Schmitt trigger can be converted into a latch and a latch can be converted into a Schmitt trigger.
Schmitt trigger devices are typically used in signal conditioning applications to remove noise from signals used in digital circuits, particularly mechanical contact bounce in switches. They are also used in closed loop negative feedback configurations to implement relaxation oscillators, used in function generators and switching power supplies.
In signal theory, a schmitt trigger is essentially a one-bit quantizer.
History
The Schmitt trigger was invented by American scientist Otto Schmitt in 1934 while he was a graduate student, later described in his doctoral dissertation as a thermionic trigger. It was a direct result of Schmitt's study of the neural impulse propagation in squid nerves.Implementation
Fundamental idea
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 and an adder in addition to an amplifier acting as a comparator. There are three specific techniques for implementing this general idea. The [|first] two of them are dual versions 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 [|third technique], the threshold and memory properties are separated.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. 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 series voltage summer. Examples are the classic transistor [|emitter-coupled Schmitt trigger], the [|op-amp inverting Schmitt trigger], etc.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. 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 parallel summer incorporating both the attenuation and summation. Examples are the less familiar [|collector-base coupled Schmitt trigger], the [|op-amp non-inverting Schmitt trigger], etc.Some circuits and elements exhibiting negative resistance can also act in a similar way: negative impedance converters, neon lamps, tunnel diodes, 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.
Two different unidirectional thresholds
Are assigned in this case to two separate open-loop comparators driving a bistable multivibrator or 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.File:Schmitt trigger symbol.svg|thumb|The Schmitt trigger symbol shown with a non-inverting hysteresis curve embedded in a buffer. Schmitt triggers can also be shown with inverting hysteresis curves and may be followed by bubbles. The documentation for the particular Schmitt trigger being used must be consulted to determine whether the device is non-inverting or inverting.
The symbol for Schmitt triggers in circuit diagrams is a triangle with a symbol inside representing its ideal hysteresis curve.
Transistor Schmitt triggers
Classic emitter-coupled circuit
The original Schmitt trigger is based on the [|dynamic threshold] idea that is implemented by a voltage divider with a switchable upper leg and a steady lower leg. Q1 acts as a comparator with a differential input consisting of an inverting and a non-inverting inputs. The input voltage is applied to the inverting input; the output voltage of the voltage divider is applied to the non-inverting input thus determining its threshold. The comparator output drives the second common collector stage Q2 through the voltage divider. The emitter-coupled transistors Q1 and Q2 actually compose an electronic double throw switch that switches over the upper legs of the voltage divider and changes the threshold in a different direction.This configuration can be considered as a differential amplifier with series positive feedback between its non-inverting input and output that forces the transition process. There is also a smaller negative feedback introduced by the emitter resistor. To make the positive feedback dominate over the negative one and to obtain a hysteresis, the proportion between the two collector resistors is chosen so that. Thus less current flows through and there is less voltage drop across when Q1 is switched on than in the case when Q2 is switched on. As a result, the circuit has two different thresholds in regard to ground.
Operation
Initial state
For the NPN transistors shown on the right, imagine the input voltage is below the shared emitter voltage so that the Q1 base-emitter junction is reverse-biased and Q1 does not conduct. The Q2 base voltage is determined by the divider described above so that Q2 is conducting and the trigger output is in the low state. The two resistors and form another voltage divider that determines the high threshold. Neglecting, the high threshold value is approximately:The output voltage is low but well above ground. It is approximately equal to the high threshold and may not be low enough to be a logical zero for subsequent digital circuits. This may require an additional level shifting circuit following the trigger circuit.