In electronics, a Schmitt trigger is a comparator circuit with positive feedback and are known as schmitt triggers or regenerative comparators. Schmitt trigger outputs two stable states +Vsat and -Vsat and all intermediate states are unstable. It has the tendency to saturate either in the two stable state. It is a type of digital circuit that converts an analog input signal into a digital output signal. It is named after Otto H. Schmitt, who invented it in 1938. A Schmitt trigger is commonly used in electronics to improve the noise immunity of digital circuits by converting an erratic analog input signal into a clean digital signal. It is also useful electronics part in voltage controlled oscillator(VCO).
There are two types of Schmitt trigger circuits: inverting Schmitt trigger and non-inverting Schmitt trigger. In this blog post, we will discuss the differences between the two types of circuits and their applications.
Inverting Schmitt Trigger
The inverting Schmitt trigger circuit has an input signal applied to the inverting input of an operational amplifier. The output of the operational amplifier is fed back to the non-inverting input through a voltage divider network composed of two resistors R1 and R2 as shown in the circuit diagram below. The voltage at the non-inverting input of the operational amplifier is set to a reference voltage, called the threshold voltage(Vth) which can either be upper threshold voltage(Vut) and lower threshold voltage(Vlt).
The following shows inverting Schmitt trigger circuit implemented using LM358 operational amplifier.
The inverting Schmitt trigger works by comparing the input voltage with the threshold voltage (Vth). This threshold voltage which can be either upper threshold voltage \(V_{UT}\) or lower threshold voltage \(V_{LT}\). These threshold voltage is the voltage across resistor R1 which is feedback to the non-inverting input.
The upper threshold voltage \(V_{UT}\) is the voltage across R1 when the output voltage Vout = +Vsat.
\(V_{UT} = \frac{+V_{sat}\times R_1}{R_1+R_2} \)
Based on the above circuit diagram it is calculated as follows,
\(V_{UT} = \frac{+5V \times 1k\Omega}{1k\Omega+10k\Omega} = 0.45V \)
If the input voltage Vin is higher than the upper threshold voltage \(V_{UT}\), the output voltage of the operational amplifier will be low, that is output goes from \(+V_{sat}\) to \(-V_{sat}\).
The lower threshold voltage \(V_{LT}\) is the voltage across R1 when the output voltage Vout = -Vsat.
\(V_{LT} = \frac{-V_{sat}\times R_1}{R_1+R_2}\)
Again based on the above Schmitt circuit diagram we have,
\(V_{LT} = \frac{-5V \times 1k\Omega}{1k\Omega+10k\Omega} = -0.45V \)
If the input voltage Vin is lower than the lower threshold voltage \(V_{LT}\), the output voltage of the operational amplifier will be high, that is output goes from \(-V_{sat}\) to \(+V_{sat}\).
The following signal waveform graph helps to understand what was described above, that is how the output is triggered by comparing the input and the threshold voltage.
Hysteresis
If the input voltage is lower than the upper threshold voltage and higher than the lower threshold voltage, the output voltage of the operational amplifier will depend on the previous output until either the input is greater or lower than the two threshold voltage. This phenomenon in Schmitt trigger circuit(comparator with positive feedback) is called a hysteresis loop. It means that the output voltage will remain high or low until the input voltage crosses the threshold voltage in the opposite direction.
The hysteresis condition is expressed as,
\( V_{LT} < V_{out} < V_{UT} \)
This voltage range is called dead band or dead zone since the variation of output voltage(Vout) within this band yields no change on the output. Stated in another way, this dead band condition is called hysteresis. For the above Schmitt circuit diagram the dead zone range is,
\( -0.45V < V_{out} < +0.45V \)
The hysteresis band width is defined as,
\(V_{hy} = V_{UT} - V_{LT}\)
or, \(V_{hy} = \frac{+V_{sat}\times R_1}{R_1+R_2} - \frac{-V_{sat}\times R_1}{R_1+R_2} \)
Again for the above Schmitt trigger circuit the hysteresis bandwidth is,
\(V_{hy} = 0.45 - (-0.45) = 0.9V\)
Applications of inverting Schmitt trigger
Oscillator circuits: The inverting Schmitt trigger is commonly used in oscillator circuits, which generate a periodic signal with a specific frequency. The Schmitt trigger acts as a feedback element that controls the frequency of the oscillator circuit.
Noise reduction circuits: The inverting Schmitt trigger is used in noise reduction circuits to clean up noisy input signals. The hysteresis loop created by the Schmitt trigger ensures that the output signal is free from noise and fluctuations.
Signal conditioning circuits: The inverting Schmitt trigger is used in signal conditioning circuits to convert analog signals into digital signals. The Schmitt trigger ensures that the output signal is a clean and stable digital signal.
Non-inverting Schmitt Trigger
The non-inverting Schmitt trigger circuit has an input signal applied to the non-inverting input of an operational amplifier. The output of the operational amplifier is fed back to the non-inverting input through a voltage divider network composed of two resistors. The inverting input of the operational amplifier grounded.
The circuit diagram of non-inverting Schmitt trigger circuit implemented using LM358 operational amplifier is shown below.
The non-inverting Schmitt trigger works by comparing the input voltage with the threshold voltage (Vth). If the input voltage is higher than the threshold voltage, the output voltage of the operational amplifier will be high. If the input voltage is lower than the threshold voltage, the output voltage of the operational amplifier will be low. This creates a hysteresis loop, which means that the output voltage will remain high or low until the input voltage crosses the threshold voltage in the opposite direction.
Applications of non-inverting Schmitt trigger
Digital signal processing circuits: The non-inverting Schmitt trigger is used in digital signal processing circuits to convert analog signals into digital signals. The Schmitt trigger ensures that the output signal is a clean and stable digital signal.
Comparator circuits: The non-inverting Schmitt trigger is used in comparator circuits to compare two input signals and produce an output signal that indicates the relative magnitudes of the input signals.
Level shifter circuits: The non-inverting Schmitt trigger is used in level shifter circuits to shift the voltage level of an input signal up or down. The Schmitt trigger ensures that the output signal is a clean and stable signal with the desired voltage level.
