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.August 2004 issue of the Pavek Museum of Broadcasting Newsletter http://160.94.102.47/Otto_Images/PavekOHSbio.pdf 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.Debouncing switches with an SR latch
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 (the collector resistors and ) and a steady lower leg (). Q1 acts as a comparator with a differential input (Q1 base-emitter junction) consisting of an inverting (Q1 base) and a non-inverting (Q1 emitter) 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 (an emitter follower) 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 (to the input voltage) direction.
This configuration can be considered as a differential amplifier with series positive feedback between its non-inverting input (Q2 base) and output (Q1 collector) 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 ( in the image).
Operation Initial state For the NPN transistors shown on the right, imagine the input voltage is below the shared emitter voltage (high threshold for concreteness) 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.
Crossing up the high threshold When the input voltage (Q1 base voltage) rises slightly above the voltage across the emitter resistor (the high threshold), Q1 begins conducting. Its collector voltage goes down and Q2 starts toward cutoff, because the voltage divider now provides lower Q2 base voltage. The common emitter voltage follows this change and goes down, making Q1 conduct more. The current begins to steer from the right leg of the circuit to the left one. Although Q1 is conducting more, it passes less current through (since ); the emitter voltage continues dropping and the effective Q1 base-emitter voltage continuously increases. This avalanche-like process continues until Q1 becomes completely turned on (saturated) and Q2 turned off. The trigger transitions to the high state and the output (Q2's collector) voltage is close to . Now the two resistors and form a voltage divider that determines the low threshold. Its value is approximately:
Crossing down the low threshold With the trigger now in the high state, if the input voltage drops enough (below the low threshold), Q1 begins cutting off. Its collector current reduces; as a result, the shared emitter voltage drops slightly and Q1's collector voltage rises significantly. The voltage divider conveys this change to the Q2 base voltage and it begins conducting. The voltage across rises, further reducing the Q1 base-emitter potential in the same avalanche-like manner, and Q1 ceases to conduct. Q2 becomes completely turned on (saturated) and the output voltage becomes low again.
Variations
Non-inverting circuit The classic non-inverting Schmitt trigger can be turned into an inverting trigger by taking 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 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 voltage divider can be omitted connecting Q1 collector directly to Q2 base. The base resistor can be omitted as well so that the input voltage source drives directly Q1's base.7414 datasheet 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 amplifiers and ECL gates.
Collector-base coupled circuit
Like every latch, the fundamental collector-base coupled bistable circuit operates with hysteresis. It can be converted to a Schmitt trigger by connecting an additional base resistor R to one of the inputs (Q1's base in the figure). The two resistors and form a parallel voltage summer (the circle in the block diagram above) that sums output (Q2's collector) voltage and the input voltage, and drives the single-ended transistor "comparator" Q1. When the base voltage crosses the threshold ( ≈ ) in either direction, a part of Q2's collector voltage is added in the same direction to the input voltage. Thus the output modifies the input voltage by means of parallel positive feedback and does not affect the threshold (the base-emitter voltage).
Comparison between emitter- and collector-coupled circuit