Schmitt trigger

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The effect of using a Schmitt trigger (B) instead of a comparator (A).

In electronics, a Schmitt trigger is a comparator circuit that incorporates positive feedback.

When the input is higher than a certain chosen threshold, the output is high; when the input is below another (lower) chosen threshold, the output is low; when the input is between the two, the output retains its value. The trigger is so named because the output retains its value until the input changes sufficiently to trigger a change. This dual threshold action is called hysteresis, and implies that the Schmitt trigger has some memory. In fact, the Schmitt trigger is a bistable multivibrator.

Schmitt trigger devices are typically used in open loop configurations for noise immunity and closed loop positive feedback configurations to implement multivibrators.

Invention

The Schmitt trigger was invented by US scientist Otto H. Schmitt in 1934 while he was still a graduate student,^[1] later described in his doctoral dissertation (1937) as a "thermionic trigger".^[2] It was a direct result of Schmitt's study of the neural impulse propagation in squid nerves.^[2]

Symbol

The symbol for Schmitt triggers in circuit diagrams is a triangle with an inverting or non-inverting hysteresis symbol. The symbol depicts the corresponding ideal hysteresis curve.

Standard Schmitt trigger

Inverting Schmitt trigger

Implementation

A Schmitt trigger can be implemented with a simple tunnel diode, a diode with an "N"-shaped current–voltage characteristic in the first quadrant. 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. However, the performance of this Schmitt trigger can be improved with transistor-based devices that make explicit use of positive feedback to implement the switching.

Comparator implementation

Schmitt triggers are commonly implemented using a comparator^{[nb 1]} connected to have positive feedback (i.e., instead of the usual negative feedback used in operational amplifier circuits). For this circuit, the switching occurs near ground, with the amount of hysteresis controlled by the resistances of R₁ and R₂:

The comparator extracts the sign of the difference between its two inputs. When the non-inverting (+) input is at a higher voltage than the inverting (-) input, the comparator output switches to +V_S, which is its high supply voltage. When the non-inverting (+) input is at a lower voltage than the inverting (-) input, the comparator output switches to -V_S, which is its low supply voltage. In this case, the inverting (-) input is grounded, and so the comparator implements the sign function – its 2-state output (i.e., either high or low) always has the same sign as the continuous input at its non-inverting (+) terminal.

Because of the resistor network connecting the Schmitt trigger input, the non-inverting (+) terminal of the comparator, and the comparator output, the Schmitt trigger acts like a comparator that switches at a different point depending on whether the output of the comparator is high or low. For very negative inputs, the output will be low, and for very positive inputs, the output will be high, and so this is an implementation of a "non-inverting" Schmitt trigger. However, for intermediate inputs, the state of the output depends on both the input and the output. For instance, if the Schmitt trigger is currently in the high state, the output will be at the positive power supply rail (+V_S). V₊ is then a voltage divider between V_in and +V_S. The comparator will switch when V₊=0 (ground). Current conservation shows that this requires

and so V_in must drop below to get the output to switch. Once the comparator output has switched to −V_S, the threshold becomes to switch back to high.

Typical hysteresis curve (which matches the curve shown on a Schmitt trigger symbol)

So this circuit creates a switching band centered around zero, with trigger levels . The input voltage must rise above the top of the band, and then below the bottom of the band, for the output to switch on and then back off. If R₁ is zero or R₂ is infinity (i.e., an open circuit), the band collapses to zero width, and it behaves as a standard comparator. The output characteristic is shown in the picture on the right. The value of the threshold T is given by and the maximum value of the output M is the power supply rail.

A practical Schmitt trigger configuration is shown below.

The output characteristic has exactly the same shape of the previous basic configuration, and the threshold values are the same as well. On the other hand, in the previous case, the output voltage was depending on the power supply, while now it is defined by the Zener diodes (which could also be replaced with a single double-anode Zener diode). In this configuration, the output levels can be modified by appropriate choice of Zener diode, and these levels are resistant to power supply fluctuations (i.e., they increase the PSRR of the comparator). The resistor R₃ is there to limit the current through the diodes, and the resistor R₄ minimizes the input voltage offset caused by the comparator's input leakage currents (see Limitations of real op-amps).

Schmitt trigger with two transistors

In the positive-feedback configuration used in the implementation of a Schmitt trigger, most of the complexity of the comparator's own implementation is unused. Hence, it can be replaced with two cross-coupled transistors (i.e., the transistors that would otherwise implement the input stage of the comparator). An example of such a 2-transistor-based configuration is shown below. The chain R_K1 R₁ R₂ sets the base voltage for transistor T2. This divider, however, is affected by transistor T1, providing higher voltage if T1 is open. Hence the threshold voltage for switching between the states depends on the present state of the trigger.

For NPN transistors as shown, when the input voltage is well below the shared emitter voltage, T1 does not conduct. The base voltage of transistor T2 is determined by the mentioned divider. Due to negative feedback, the voltage at the shared emitters must be almost as high as that set by the divider so that T2 is conducting, and the trigger output is in the low state. T1 will conduct when the input voltage (T1 base voltage) rises slightly above the voltage across resistor R_E (emitter voltage). When T1 begins to conduct, T2 ceases to conduct, because the voltage divider now provides lower T2 base voltage while the emitter voltage does not drop because T1 is now drawing current across R_E. With T2 now not conducting the trigger has transitioned to the high state.

With the trigger now in the high state, if the input voltage lowers enough, the current through T1 reduces, lowering the shared emitter voltage and raising the base voltage for T2. As T2 begins to conduct, the voltage across R_E rises, further reducing the T1 base-emitter potential and T1 ceases to conduct.

In the high state, the output voltage is close to V+, but in the low state it is still well above V−. This may not be low enough to be a "logical zero " for digital circuits. This may require additional amplifiers following the trigger circuit.

The circuit can be simplified: R1 can be omitted, connecting the T2 base directly to the T1 collector, and the connection of the T2 base to V- via R2 can be completely omitted. When T1 conducts, it connects the T2 base to the T2 emitter so that T2 does not conduct. When T1 does not conduct, R_K1 pulls up the T2 base and T

Applications

Schmitt triggers are typically used in open loop configurations for noise immunity and closed loop positive feedback configurations to implement multivibrators.

Noise immunity

One application of a Schmitt trigger is to increase the noise immunity in a circuit with only a single input threshold. With only one input threshold, a noisy input signal near that threshold could cause the output to switch rapidly back and forth from noise alone. A noisy Schmitt Trigger input signal near one threshold can cause only one switch in output value, after which it would have to move beyond the other threshold in order to cause another switch.

For example, in Fairchild Semiconductor's QSE15x family of infrared photosensors^[3], an amplified infrared photodiode generates an electric signal that switches frequently between its absolute lowest value and its absolute highest value. This signal is then low-pass filtered to form a smooth signal that rises and falls corresponding to the relative amount of time the switching signal is on and off. That filtered output passes to the input of a Schmitt trigger. The net effect is that the output of the Schmitt trigger only passes from low to high after a received infrared signal excites the photodiode for longer than some known delay, and once the Schmitt trigger is high, it only moves low after the infrared signal ceases to excite the photodiode for longer than a similar known delay. Whereas the photodiode is prone to spurious switching due to noise from the environment, the delay added by the filter and Schmitt trigger ensures that the output only switches when there is certainly an input stimulating the device.

Devices that include a built-in Schmitt trigger

As discussed in the example above, the Fairchild Semiconductor QSE15x family of photosensors use a Schmitt trigger internally for noise immunity. Schmitt triggers are common in many switching circuits for similar reasons (e.g., for switch debouncing).

The following 7400 series devices include a Schmitt trigger on their input or on each of their inputs:

• 7413: Dual Schmitt trigger 4-input NAND Gate
• 7414: Hex Schmitt trigger Inverter
• 7419: Hex Schmitt trigger Inverter
• 74121: Monostable Multivibrator with Schmitt Trigger Inputs
• 74132: Quad 2-input NAND Schmitt Trigger
• 74221: Dual Monostable Multivibrator with Schmitt Trigger Input
• 74232: Quad NOR Schmitt Trigger
• 74240: Octal Buffer with Schmitt Trigger Inputs and Three-State Inverted Outputs
• 74241: Octal Buffer with Schmitt Trigger Inputs and Three-State Noninverted Outputs
• 74244: Octal Buffer with Schmitt Trigger Inputs and Three-State Noninverted Outputs
• 74310: Octal Buffer with Schmitt Trigger Inputs
• 74540: Octal Buffer with Schmitt Trigger Inputs and Three-State Inverted Outputs
• 74541: Octal Buffer with Schmitt Trigger Inputs and Three-State Noninverted Outputs

A number of 4000 series devices include a Schmitt trigger on inputs, for example:

• 14093: Quad 2-Input NAND
• 40106: Hex Inverter
• 14538: Dual Monostable Multivibrator
• 4020: 14-Stage Binary Ripple Counter
• 4024: 7-Stage Binary Ripple Counter
• 4040: 12-Stage Binary Ripple Counter
• 4017: Decade Counter with Decoded Outputs
• 4022: Octal Counter with Decoded Outputs

Dual Schmitt input configurable single-gate CMOS logic, AND, OR, XOR, NAND, NOR, XNOR

• NC7SZ57 Fairchild
• NC7SZ58 Fairchild
• SN74LVC1G57 Texas Instruments
• SN74LVC1G58 Texas Instruments

Use as an oscillator

A Schmitt trigger is a bistable multivibrator, and it can be used to implement another type of multivibrator, the relaxation oscillator. This is achieved by connecting a single resistor–capacitor network to an inverting Schmitt trigger – the capacitor connects between the input and ground and the resistor connects between the output and the input. The output will be a continuous square wave whose frequency depends on the values of R and C, and the threshold points of the Schmitt trigger. Since multiple Schmitt trigger circuits can be provided by a single integrated circuit (e.g. the 4000 series CMOS device type 40106 contains 6 of them), a spare section of the IC can be quickly pressed into service as a simple and reliable oscillator with only two external components.

Output and capacitor waveforms for comparator-based relaxation oscillator.

For example, the comparator-based implementation of a relaxation oscillator is shown below.

Here, a comparator-based Schmitt trigger is used in its inverting configuration. That is, the input and ground are swapped from the Schmitt trigger shown above, and so very negative signals correspond to a positive output and very positive signals correspond to a negative output. Additionally, slow negative feedback is added with an RC network. The result, which is shown on the right, is that the output automatically oscillates from V_SS to V_DD as the capacitor charges from one Schmitt trigger threshold to the other.

See also

• Relaxation oscillator
• Bistable multivibrator

Notes

References

External links

• Hyperphysics description and equations for an alternative, more general circuit
• Calculator which determines the resistor values required for given thresholds