Hysteresis in Thyristors: How Positive Feedback Enables Latching

Thyristors are semiconductor devices that exhibit hysteresis—a behavior where the device remains in its current state even after the triggering stimulus is removed. A classic illustration is a toggle switch: once you push the lever, it snaps to one of two stable positions and stays there until you move it again. In contrast, a momentary push‑button returns to its original state immediately when you lift your hand, showing no latching behavior.

Most modern transistors—bipolar junction transistors (BJTs), junction field‑effect transistors (JFETs), and insulated‑gate field‑effect transistors (MOSFETs)—are non‑hysteric. They produce a predictable output for any given input and do not latch into a state. Thyristors, however, stay “on” once triggered and remain “off” once turned off. Because of this latching property, they function only as on/off switches and cannot be used as analog amplifiers.

Thyristors are built from the same underlying technology as BJTs and can be modeled as a pair of transistors connected in a feedback loop. The key to creating hysteresis from inherently non‑hysteric components is positive (regenerative) feedback. While negative feedback smooths an amplifier’s gain and improves stability, positive feedback drives the output toward saturation—exactly the behavior that makes a thyristor “want” to stay in its current state.

In the following sections we’ll examine several types of thyristors that all share a common two‑transistor core circuit. Before diving into those, it’s useful to look at the technology that preceded them: gas‑discharge tubes, which also display latching behavior.

Related Worksheet

• Thyristors Worksheet