Schmitt Trigger: What is it And How Does it Work? (Circuit Diagrams Included)

Contents

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Key learnings:

Schmitt Trigger Definition: A Schmitt Trigger is a comparator circuit that uses hysteresis through two threshold voltages to stabilize signal transitions.
Functionality: The device switches its output between high and low states only when the input crosses defined threshold levels, enhancing signal integrity.
Circuit Design: Schmitt Triggers can be designed using operational amplifiers or transistors, and are available in inverting and non-inverting forms.
Noise Handling: They are particularly effective in environments with noisy signals, ensuring reliable operation by filtering out unwanted noise.
Practical Applications: Schmitt Triggers are essential in electronic devices for waveform shaping and eliminating signal disturbances, demonstrating their versatility.

What is a Schmitt Trigger?

A Schmitt Trigger is a comparator with hysteresis, made by applying positive feedback to the non-inverting input of a comparator or differential amplifier. It uses two threshold voltage levels so small noise around one trip point does not chatter the output. That dual-threshold action is hysteresis.

Otto H. Schmitt described the circuit in 1934. The name Otto H Schmitt trigger came later from that thermionic trigger.

A plain comparator has one threshold. It compares the input with that level. If the input chatters around the threshold, the output chatters too.

Noise Effect On Output — Noise effect on output

In the figure, noise at A and B takes V1 across the single reference V2. While V1 is below V2 the comparator output goes low, then returns when the noise ends.

So the comparator output follows the noise. A single threshold does not reject it.

The word trigger in the name means the output holds its state until the input moves far enough to force a change.

How does a Schmitt Trigger work?

The Schmitt trigger uses two thresholds: an upper threshold (VUT) and a lower threshold (VLT). Noise smaller than that gap does not change the output.

For the non-inverting sense used in this example, the output stays low until the input rises above VUT. It then stays high until the input falls below VLT.

The figure below starts with the input at zero and then rising.

Noise Effect With Schmitt Trigger — Noise effect with Schmitt Trigger

The input starts at zero and rises, as drawn above.

The output stays low until point A. At A the input crosses VUT and the output goes high.

The output stays high until point B. At B the input crosses below VLT and the output goes low.

At point C the input again crosses VUT and the output goes high.

The input is still noisy. The output does not follow those small crossings.

Schmitt Trigger Circuit

A Schmitt trigger can be built with an Op-Amp or with Transistor pairs.

Op-amp based Schmitt trigger
Transistor based Schmitt trigger

Op-Amp based Schmitt Trigger

An op-amp version is inverting when the signal goes to the inverting input, and non-inverting when the signal goes to the non-inverting input.

Inverting Schmitt Trigger

In an inverting Schmitt Trigger, the input goes to the inverting terminal. Positive feedback still goes to the non-inverting terminal, which sets the two thresholds.

At the non-inverting node A the voltage is V. The applied input is Vin.

If Vin is greater than V, the inverting output is low. If Vin is less than V, the output is high.

$V_{in} > V \quad V_{out} = V_L$

$V_{in} < V \quad V_{out} = V_H$

Find V from the resistor divider.

Apply Kirchhoff’s Current Law (KCL) at that node:

$\frac{V-0}{R_1} + \frac{V-V_{out}}{R_2} = 0$

$\frac{V}{R_1} + \frac{V}{R_2} - \frac{V_{out}}{R_2} = 0$

$V(\frac{1}{R_1} + \frac{1}{R_2}) = \frac{V_{out}}{R_2}$

$V (\frac{R_1 + R_2}{R_1 R_2}) = \frac{V_{out}}{R_2}$

$V = \frac{R_1}{R_1 + R_2} \times V_{out}$

If the output is already high,

$V_{out} = V_H \quad and \quad V=V_1$

the divider gives

$V_1 = \frac{R_1}{R_1 + R_2} \times V_{H}$

When Vin rises above V₁, the inverting output switches low. That V₁ is the upper threshold (V_UT).

$V_{UT} = \frac{R_1}{R_1 + R_2} \times V_{H}$

The output then stays low until Vin falls below the new V set by VL. With the output low,

$V_{out} = V_L \quad and \quad V=V_2$

$V_2 = \frac{R_1}{R_1 + R_2} \times V_{L}$

The output stays low until Vin falls below V₂. That V₂ is the lower threshold (V_LT).

$V_{LT} = \frac{R_1}{R_1 + R_2} \times V_{L}$

Non-Inverting Schmitt Trigger

In a non-inverting Schmitt Trigger, the input and the feedback both meet at the non-inverting terminal. The inverting terminal is grounded.

The output is high when node V is greater than zero. The output is low when V is less than zero.

$V>0 , V_{out} = V_H$

$V<0 , V_{out} = V_L$

Apply KCL at that node to find V.

$\frac{V-V_{in}}{R_1} + \frac{V-V_{out}}{R_2} = 0$

$\frac{V}{R_1} - \frac{V_{in}}{R_1} + \frac{V}{R_2} - \frac{V_{out}}{R_2} = 0$

$V \left(\frac{R_1 + R_2}{R_1 R_2} \right) = \frac{V_{in}}{R_1} + \frac{V_{out}}{R_2}$

$V = \frac{R_2}{R_1 + R_2} V_{in} + \frac{R_1}{R_1 + R_2} V_{out}$

Take the output low, so Vout is V_L and call that node voltage V₁.

In this condition,

$V_{out} = V_L \quad and \quad V = V_1$

From the above equation,

$V_1 = \frac{R_2}{R_1 + R_2} V_{in} + \frac{R_1}{R_1 + R_2} V_{L}$

The output goes high when V₁ is greater than zero. That inequality is

$\frac{R_2}{R_1 + R_2} V_{in} > - \frac{R_1}{R_1 + R_2} V_{L}$

$V_{in} > -\frac{R_1}{R_2} V_L$

The Vin that satisfies it is the upper threshold (V_UT) for this non-inverting circuit, provided VL is negative so that VUT is positive.

$V_{UT} = - \frac{R_1}{R_2} V_L$

Now take the output high and call the node voltage V₂.

$V_{out} = V_H \quad and \quad V = V_2$

From the equation of voltage V,

$V2 = \frac{R_2}{R_1 + R_2} V_{in} + \frac{R_1}{R_1 + R_2} V_{H}$

The output goes low when V₂ is less than zero:

$\frac{R_2}{R_1 + R_2} V_{in} < - \frac{R_1}{R_1 + R_2} V_{H}$

$\[ V_{in} < -\frac{R_1}{R_2} V_H$

That Vin is the lower threshold (V_LT) when VH is positive, so VLT is negative.

$V_{LT} = -\frac{R_1}{R_2} V_H$

Transistor based Schmitt Trigger

A two-transistor Schmitt trigger is shown below. The voltages that follow are example values for the drawn 5 V divider, not a universal design rule.

V_in = input voltage
V_ref = Reference voltage = 5V

Start with Vin = 0 at the base of T₁. T₁ is then cut off.

V_a and V_b are the drawn divider nodes. With a 5 V reference those nodes can be found from the resistor ratio. The page then uses V_a and V_b values from that figure.

V_b in the example is 1.98 V at the base of T₂, so T₂ is on and the collector output is low. An emitter drop of about 0.7 V then puts the common emitters near 1.28 V.

The emitters of T₂ and T₁ are common, so both sit at that 1.28 V emitter voltage while T2 is on.

T₁ then needs about 0.7 V more than 1.28 V at its base, or about 1.98 V, before it turns on.

When Vin rises above about 1.98 V, T₁ turns on. That lowers the base of T₂, T₂ cuts off, and the collector output goes high.

On the way down, the page takes T₁ off at about 1.28 V (0.7 V below 1.98 V). T₂ then turns on again from the 5 V divider and the output goes low. Real thresholds also depend on the collector and emitter resistors, so treat 1.28 V and 1.98 V as the worked example.

So this example has a lower threshold near 1.28 V and an upper threshold near 1.98 V.

Schmitt Trigger Oscillator

A Schmitt Trigger becomes an oscil l ator when a single RC network is tied from the output back to the input. The circuit below is that relaxation oscillator.

The output is a continuous square wave. Its frequency depends on R, C and the two thresholds.

$f = \frac{k}{RC}$

k is a factor set by the threshold ratio. The stored range 0.2 to 1 is a rough span, not a datasheet constant.

CMOS Schmitt Trigger

A plain CMOS inverter flips the input: high in gives low out. If the input has spikes around the trip point, the output follows those spikes. A CMOS Schmitt-trigger inverter adds hysteresis so those spikes do not appear on the output.

Waveform Of Simple Signal Inverter Circuit — Waveform of Simple Signal Inverter Circuit

The first waveform is a clean input and a clean inverted output. The second input is noisy and a plain inverter copies that noise. The CMOS Schmitt-trigger inverter is used to stop that.

The CMOS Schmitt-trigger inverter below uses six MOSFETs: PMOS and NMOS devices.

PMOS and NMOS symbols used on that drawing are shown below.

Pmos And Nmos Transistors — PMOS and NMOS transistors

An NMOS device conducts when its gate-source voltage is above threshold. A PMOS device conducts when its gate is sufficiently below its source. The extra PMOS and NMOS in a Schmitt inverter steer the trip points.

When the input is high, the P_N transistor is ON and the N_N transistor is OFF on the stored drawing, and node-A has a path toward ground. The output is then zero.

When the input is low, the N_N device is off and the P_N path can pull node-B toward V_DD (High). The output is then high.

Schmitt Trigger Applications

Common uses:

A Schmitt trigger can turn a sine wave or a triangle wave into a square wave.
It is widely used to clean noisy analog edges before a digital input.
With extra timing parts it can form a simple function generator.
With one RC network it implements a relaxation oscillator.
A Schmitt trigger plus an RC circuit is a common switch-debounce method.