Schmitt-Trigger

The Schmitt trigger, named after its inventor Otto Schmitt, who in 1934 invented this as a student, is an electronic comparator circuit in which do not coincide with the one- or switch-off, but to a certain voltage, the switching hysteresis, offset from each other are. A Schmitt trigger is used to gain for generating binary signals or unique switching states from an analog input waveform. Examples of application are (in combination with an RC network ) debouncing of switches or the vibration generating ( relaxation oscillator ).

Function

The Schmitt trigger is an analog comparator having positive feedback. He works as a comparator for two analog voltages and functions as a threshold switch: For a given set in the Schmitt trigger threshold is exceeded, the output of the maximum possible output voltage (logic 1) assumes at the non-inverting design, in the other case, the minimum possible output voltage (logic 0). In the inverting version, the output behaves vice versa. Output voltages between the maximum and minimum values ​​do not occur in static mode. The positive feedback he has, in contrast to the pure comparator different input and switch-off, the apart -mentioned value for the hysteresis.

Exceeds the input voltage at an inverting Schmitt - trigger the upper switching threshold of the Schmitt trigger flips its output voltage from the maximum voltage to the minimum voltage value. Lower than the input voltage, then the lower threshold, the output voltage tilts back to the maximum output voltage. The small differential voltage between the two threshold voltages to the input side of the hysteresis and is in the discrete version of a Schmitt trigger set by means of resistors in the construction. In Schmitt triggers, which are designed as an integrated circuit, a change in these thresholds is usually not possible.

The hysteresis of the Schmitt trigger (the voltage between the two switching thresholds) may be from a few millivolts up to a value of the supply voltage also. Which is determined by the dimensioning of the resistors in the positive feedback of the comparator contained.

Inverting Schmitt trigger

The adjacent diagram shows a constructed with an operational amplifier inverting Schmitt trigger with adjustable threshold in the form of voltage Ur dar. In circuits only the input, here denoted Ue, and the output is usually drawn with Ua. The positive feedback is achieved in that a part of the output voltage is fed from the resistor network back to the positive (non - inverting) input of the operational amplifier.

The exact thresholds U1 and U2 for the on or off soft from the externally prescribed threshold from Ur, due to hysteresis and can be determined using the following equations:

The adjacent characteristic of a Schmitt trigger inverter, shown in the input to output voltage level, illustrates the different threshold voltages.

The voltage Vs corresponds to the model of the symmetrical power supply voltage of the operational amplifier. In a real operational amplifier, however, the saturation voltages of the output for the calculation of parameters should be used which are slightly lower than the supply voltage.

It is also possible to use an operational amplifier as a Schmitt trigger with an asymmetrical power supply. In this case, the negative supply voltage U is 0 V and the output switches between 0 V, which corresponds to logic 0, and Uv, which corresponds to logic 1. The threshold voltage Ur is then usually chosen equal to half the supply voltage Vs - but the exact voltage levels may vary depending on the logic family. This type of asymmetric supply is mostly preferred in digital circuits. A widely used type in CMOS technology is the CD4093, which combines four trigger in a case. So you can build a very simple oscillator circuits.

Instead of the reference voltage Ur Are both thresholds given U1 and U2, so is independent of the resistors R1 and R2, the first reference voltage Ur to choose as follows:

R2 can be chosen freely and R1 is then obtained from:

A non-inverting Schmitt trigger

Opposite, the circuit of a non-inverting Schmitt trigger is shown. The behavior of the output voltage is exactly inversely compared to the above circuit. The input resistance of the sample circuit is much smaller than that of the inverting Schmitt - trigger circuit and corresponds to the dynamic resistance of R1 R2. Due to the switching action of a return occurs to the input. Therefore, this circuit is rarely applied - the inversion of the output voltage can also be made by an inverter and then nachgeschaltenen avoids the disadvantage of the low input resistance of the non-inverting Schmitt - trigger.

At the given circuit, the threshold voltage can be switched fixed to 0 V by the negative input of the operational amplifier is set to 0 V (ground). In digital circuits, placing the center between the switching thresholds to half the supply voltage ( 2.5 V with 5 V supply voltage) or in the middle of the " forbidden area " of the logic level ( 1.4 V).

Precision Schmitt trigger

With a precision Schmitt trigger is in contrast to the previously shown feedforward circuits possible to independently adjust the thresholds exactly. It can be composed of two comparators and a flip-flop RS nachgeschaltenen. The switching thresholds can be set for example by means of a voltage divider. One comparator compares the input voltage to the upper switching threshold, and switches the flip-flop when exceeding this threshold in the set state; the other comparator compares the input voltage to the lower switching threshold and sets the flip-flop at the back below the threshold. To build a precision Schmitt trigger, for example, the integrated IC NE521 containing two comparators and two additional NAND gate, which can be connected to form a flip-flop is.

Discrete circuit configuration

A Schmitt trigger may also be implemented as a discrete circuit using MOSFETs or bipolar transistors. The following figures show circuit examples of which are given.

The four stacked MOS transistors in the left diagram realize an ordinary inverter. One can imagine the same respective transistors as a transistor with a center of the gate -source path. The two right transistors, switched from the output signal change by " train shortening " of the gate -source path of the switching threshold UGS, thus resulting in the desired behavior. The setting of trigger levels is done constructively by the dimensioning of the participating MOS transistor structures.

The right diagram is similar to a two-stage, direct coupled, a total non-inverting analog amplifier. The difference is the common emitter resistor RE. This realizes the positive feedback and thus the desired Schmitt trigger behavior. The switching threshold is carried out by the dimensioning of the resistors.

Application

If digital signals through long cables transmit the signal edges are smoothed with mismatch because of the low-pass characteristic and even with correct adjustment due to the dispersion. A Schmitt trigger producing for subsequent circuit elements again steep signal edges and precise signal level.

For an inverting Schmitt trigger, a resistor as feedback and a capacitor in parallel with the input ( RC element), a simple oscillator can be built ( relaxation oscillator), which can generate oscillations at a sufficiently fast semiconductor elements to the FM band into it.

Schmitt trigger with an upstream low-pass ( RC element) can be used to debounce keys and other contacts. The time constant of the RC element must be higher than the maximum bounce time ( usually a few milliseconds). Debouncing is required if a digital circuit by means of a mechanical key is to run a command for each key pressed. If the key is not debounced, several commands are executed, since the logical level nor changes several times in the switching moment when pressed. Debouncing instead of hardware possible with software.