Implementation

A Schmitt trigger can be implemented with the simple  tunnel diode, a diode with the "N"-shaped current-voltage characteristic in 1st  quadrant. An oscillating input will cause the diode to move from one rising leg of "N" to other and back again as input crosses the rising and  falling  switching  thresholds.  But, the performance of this  Schmitt  trigger  can  be enhanced with transistor-based devices which make explicit use of  positive feedback to implement the switching.

Comparator implementation

Schmitt triggers are commonly implemented using a  comparator connected to have positive feedback (that is instead of the negative feedback used in  operational amplifier circuits). For this circuit, the switching happens near ground, with the amount of  hysteresis controlled by he resistances of R₁ and R₂:

The comparator extracts sign of the difference between its 2 inputs.  When non- inverting (+) input is at the higher voltage than inverting (-) input, comparator output switches to +V_S, which is its high supply voltage. When non-inverting (+) input is at the lower voltage than inverting (-) input, comparator output switches to -V_S, which is its low supply  voltage.  In  this  case,  inverting  (-)  input  is   grounded,  and  so  comparator implements 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.

Due to the resistor network connecting Schmitt  trigger  input, the non-inverting (+) terminal of comparator, and comparator output, the Schmitt trigger behaves like a comparator which switches at a different point depending on whether the output of comparator is high or low. For negative inputs, output will be low, and for positive inputs, the output will be high, and so this is an implementation of a "non-inverting" Schmitt trigger. But, for intermediate inputs, the state of the output depends on the input and  the  output both.  For instance, if the Schmitt trigger is currently in high state, the output will be at 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 Vin should drop below  to get the output to switch. Once the comparator output has switched to -V_S, threshold becomes  to switch back to high.

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

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

A practical Schmitt trigger configuration is shown as follows.

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

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