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Schmitt trigger

Schmitt trigger is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Schmitt trigger rather than just read about it. In short: 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.

Schmitt trigger — main illustration
Schmitt trigger — illustration

Key takeaways

  • Schmitt trigger belongs to engineering; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Schmitt trigger to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Schmitt trigger from memory before moving on to harder problems.

Reference excerpt

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 cause 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 (lower) 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 (latch or flip-flop). 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 (1937) 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 (the B box in the figure on the right) and an analog 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 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 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 (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 series voltage summer (applying Kirchhoff's voltage law). 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 (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 parallel summer (applying Kirchhoff's current law) 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 (NIC), neon lamps, tunnel diodes (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.

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. 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.

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

… excerpt ends here. Continue reading the full article.

Illustrations

Schmitt trigger: Transfer function of a Schmitt trigger. The horizontal and vertical axes are input voltage and output voltage, respectively. T and −T are the switching thresholds, and M and −M are the output voltage levels.
Transfer function of a Schmitt trigger. The horizontal and vertical axes are input voltage and output voltage, respectively. T and −T are the switching thresholds, and M and −M are the output voltage levels.
Schmitt trigger: Comparison of the action of an ordinary comparator (A) and a Schmitt trigger (B) on a noisy analog input signal (U). The green dotted lines are the circuit's switching thresholds. The Schmitt trigger tends to remove noise from the signal.
Comparison of the action of an ordinary comparator (A) and a Schmitt trigger (B) on a noisy analog input signal (U). The green dotted lines are the circuit's switching thresholds. The Schmitt trigger tends to remove noise from the signal.
Schmitt trigger: 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. A > 1 is the amplifier gain; B < 1 is the feedback transfer function.
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. A > 1 is the amplifier gain; B < 1 is the feedback transfer function.
Schmitt trigger: 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 (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 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 (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).
Schmitt trigger: Schmitt trigger implemented by two emitter-coupled BJTs stages
Schmitt trigger implemented by two emitter-coupled BJTs stages

Worked examples

Example 1 — a first encounter with Schmitt trigger

Start with the simplest possible case. Write down what Schmitt trigger claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Schmitt trigger before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Schmitt trigger ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Schmitt trigger

In research
Schmitt trigger appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Schmitt trigger in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Schmitt trigger is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electronic circuits, Hysteresis, so understanding it makes those chapters shorter.
In everyday life
Look for Schmitt trigger outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Schmitt trigger in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Schmitt trigger means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Schmitt trigger out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Schmitt trigger in simple terms?

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.

Why does Schmitt trigger matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Schmitt trigger?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Schmitt trigger.

Tags

  • Electronic circuits
  • Hysteresis

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