How Positive Feedback Creates Hysteresis in Schmitt Triggers
Slowly changing analog signals in electronics are highly susceptible to electrical noise, which often causes standard comparators to rapidly switch back and forth, a phenomenon known as output chatter. This article explores how a Schmitt trigger resolves this issue by utilizing positive feedback to introduce hysteresis. You will learn the mechanics of how positive feedback splits a single switching threshold into two distinct upper and lower voltage limits, effectively creating a noise-immune deadband that guarantees clean, single-transition digital outputs even on the slowest signal edges.
The Problem: Noise Chatter on Slow Edges
In a standard comparator circuit, the output switches states whenever an input signal crosses a single, fixed reference voltage. While this works well for fast, clean signals, it fails when dealing with slow-moving inputs, such as those from discharging capacitors, temperature sensors, or mechanical switches.
As a slow edge gradually approaches the reference threshold, any high-frequency noise riding on the signal will cause the voltage to cross the threshold multiple times in rapid succession. Consequently, the comparator's output oscillates wildly between high and low states before settling. In digital logic systems, these false triggers—known as chatter—can corrupt data, cause erratic microcontroller behavior, and burn excess power.
How Positive Feedback Works
A Schmitt trigger eliminates chatter by modifying the basic comparator circuit with a positive feedback loop. This is achieved by connecting a resistor from the comparator's output back to its non-inverting (+) input.
Unlike negative feedback, which stabilizes an amplifier in a linear region, positive feedback reinforces the current state of the output. When the output is high, it feeds a portion of that high voltage back to the non-inverting input, actively pulling the threshold voltage up. Conversely, when the output is low, it pulls the threshold voltage down. Once the input signal finally forces the output to change states, the positive feedback accelerates the transition, snapping the output to its new state almost instantaneously and immediately shifting the threshold away from the input signal's current voltage level.
Introducing Hysteresis: Upper and Lower Thresholds
The dynamic shifting of the reference voltage via positive feedback creates a property called hysteresis. Hysteresis means the circuit's current state depends on its history, resulting in two separate switching thresholds instead of one:
- Upper Threshold Voltage (\(V_{TH}\)): When the output is currently low, the input signal must rise all the way to this higher voltage level to trigger a state change to high.
- Lower Threshold Voltage (\(V_{TL}\)): Once the output is high, the positive feedback drops the threshold, meaning the input signal must fall all the way down to this lower voltage level to switch the output back to low.
The voltage difference between \(V_{TH}\) and \(V_{TL}\) is the hysteresis band (or deadband).
Eliminating Noise on Slow Edges
The hysteresis band is the exact mechanism that stops noise chatter on slow edges. Imagine a slow-rising input signal with 100 millivolts of noise riding on it.
As the signal rises and finally crosses the Upper Threshold Voltage (\(V_{TH}\)), the Schmitt trigger's output snaps to its new state. At that exact microsecond, the positive feedback shifts the active threshold down to the Lower Threshold Voltage (\(V_{TL}\)).
If the hysteresis band is designed to be 500 millivolts wide, the 100-millivolt noise fluctuations on the slow signal are no longer large enough to reach the newly established \(V_{TL}\). The noise is trapped safely within the deadband. The output remains locked in its new state, completely immune to the noise, until the main signal intentionally falls past the lower limit. This ensures a single, crisp logic transition every time.