Hysteresis Circuits, Schmitt Trigger

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In electronics technology, there's an input-output mechanism that exhibits a type of "memory" or hysteresis behavior. This hysteresis circuit is known as the Schmitt Trigger (S/T), named in honor of its inventor, Otto H. Schmitt, a bio-engineer and electrical engineer, who worked at the University of Washington in the mid-1930s. (Dr. Schmitt also a co-inventor of the differential amplifier and the chopper stabilized amplifier .)


This circuit falls into the category of logic or digital circuits. A Schmitt Trigger is essentially a Finite State Machine (FSM) with two distinct states. It can even be classified as a type of bistable multivibrator. Over time, the S/T circuit has become critically important because "almost all" (though not all) modern ICs, including processors, incorporate this circuit on their logic input pins. Its primary function is to :

"clean up an input signal that's dirty with noise or has ambiguous high-to-low or low-to-high transitions, turning it into a clean, well-defined signal with clear, noise-free logic transitions."



Simply put, its operation can be explained using a black-box model with an output vs. input signal diagram as shown in Figure 1.




1. Initial State: = High (State 1)

  • As the input moves from 0 to its maximum value, the output becomes HIGH when it reaches the upper threshold voltage, VH.

  • As the input moves from its maximum value back towards 0, the output becomes LOW when it reaches the lower threshold voltage, VL.

  • The threshold voltages for generating a HIGH output (VH​) and a LOW output (VL​) are not the same, where VL < VH.

  • The difference between VH and VL is called the hysteresis voltage.


Analyzing the Schmitt Trigger Circuit

Figure 2A shows one of many ways to implement an S/T, which can be built using discrete components or as an integrated circuit (IC). To simplify the analysis, the circuit in Figure 2A can be modeled as the equivalent circuit in Figure 2B, where the transistors are replaced by switches.

  • When Vin​ is HIGH, switch Q1 closes. As a result, the voltage at point Q (VQ​) is lower than the voltage at point R (VR​), causing Q2 to open (turn OFF).

  • Since Q2 is OFF, the output voltage (Vout​) will be in a HIGH state.

  • In this condition (State 1), the current flowing through R3 comes only from R2. The voltage at point R (VR1​) becomes the lower threshold voltage (VL​) and is calculated as:

2. Transition to the Second State : Vin  Decreases

  • As Vin decreases, as soon as its value drops slightly below V, switch Q1 will open (turn OFF).

  • Consequently, the voltage at point Q goes HIGH, causing Q2 to close (turn ON) and the output voltage to go LOW. This is State 2.

  • In this condition (State 2), switch Q1 is open and switch Q2 is closed. The current flowing through R3 now comes only from R4 through switch Q2. The voltage at point R (VR2​) becomes the upper threshold voltage (VH​) and is calculated as:




​

3. Transition Back to the First State: Vin Increases

  • As Vin increases, as soon as its value is slightly greater than V, switch Q1 will close (turn ON).

  • This will cause switch Q2 to open (turn OFF), and the circuit returns to State 1.

  • By adjusting the values of R2, R3, and R4 (with a slight influence from R1), we can set the desired hysteresis voltage. Figure 1 shows the hysteresis produced by the chosen values of R2, R3, and R4 in this circuit.


Signal Cleanup from Noise

Figure 3 illustrates a clean input signal with a slow slope, which can lead to logic ambiguity. After passing through a Schmitt Trigger - the output signal becomes sharp with clear transitions from LOW to HIGH and vice versa. The difference between the threshold voltages VL and VH is clearly visible.



Figure 4 shows a dirty signal corrupted by noise. When this signal passes through a black-box without hysteresis, it results in a series of unwanted pulses during both the LOW-to-HIGH and HIGH-to-LOW transitions.



In contrast, as shown in Figure 5, when the same dirty signal passes through a black-box equipped with a hysteresis circuit, the resulting output is clean and free from unwanted pulses during both the LOW-to-HIGH and HIGH-to-LOW transitions.

I hope this explanation helps those who are new to the Schmitt Trigger concept or wish to refresh their memory.



(This post is parallel to the status on the FaceBookGroup The Art of Electronics with the same Topic)


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