A Schmitt trigger is a type of comparator circuit that incorporates positive feedback to achieve hysteresis. This means that the circuit has two different threshold voltage levels for switching its output state: one for the rising input voltage and another for the falling input voltage. The difference between these two thresholds is called hysteresis, and it helps to eliminate noise and ensure a clean, stable output even when the input signal is noisy or slowly changing.
Schmitt Trigger with Two BJT Transistors
Follwing circuit diagram shows a
Schmitt Trigger implemented using two 2N3904 Bipolar Junction Transistors
(BJTs).
Here's a basic explanation of how it works:
Circuit Configuration:
The two BJTs Q1 and Q2 are typically arranged in a way that one transistor acts as a switch, and the other provides positive feedback.
The input signal is applied to the base of the first transistor, and the output is taken from the collector of the second transistor.
Resistors are used to set the bias points and control the feedback.
How It Works:
When the input voltage is low, the first transistor (Q1) is off, and the second transistor (Q2) is on. This keeps the output voltage low.
As the input voltage rises, it eventually reaches a threshold where Q1 starts to turn on. This reduces the voltage at the collector of Q1, which in turn reduces the base voltage of Q2, causing Q2 to start turning off. This positive feedback action quickly switches the output to a high state.
Conversely, when the input voltage falls, it must drop below a lower threshold before Q1 turns off and Q2 turns on again, switching the output back to a low state.
Hysteresis:
The key feature of a Schmitt trigger is its hysteresis. The circuit has two distinct threshold voltages:
Upper Threshold Voltage (V_UT): The input voltage at which the output switches from low to high.
Lower Threshold Voltage (V_LT): The input voltage at which the output switches from high to low.
The difference between V_UT and V_LT is the hysteresis voltage (V_HYST). This hysteresis prevents the output from oscillating or chattering when the input signal is near the threshold voltage, which is particularly useful in noisy environments.
Why It Is Called a "Trigger":
The term "trigger" refers to the circuit's ability to quickly switch its output state when the input crosses a certain threshold. This rapid switching action is similar to the triggering mechanism in other electronic devices, such as thyristors or flip-flops.
The Schmitt trigger "triggers" a change in the output state based on the input voltage crossing predefined thresholds, and the hysteresis ensures that this triggering action is stable and noise-resistant.
Applications
Schmitt triggers are widely used in digital circuits, signal conditioning, and noise filtering. Some common applications include:
Debouncing switches: Ensuring a clean transition when a mechanical switch is pressed or released.
Waveform shaping: Converting slow or noisy input signals into clean digital signals.
Threshold detection: Detecting when a signal crosses a certain level, with immunity to noise.
In
summary, your Schmitt trigger circuit using two BJTs is a robust and
reliable way to convert an analog input signal into a clean digital
output, with the added benefit of hysteresis to prevent unwanted
oscillations or noise-induced transitions.
