Negative Resistance

Definition

 In electronic circuits, negative resistance refers to a relationship where the voltage across a circuit element is inversely proportional to the current passing through it. This means that as the applied voltage increases, the current decreases. Negative resistance causes the circuit element to behave as an energy source rather than a load.

Practical Application

Negative resistance is commonly utilized in the analysis of oscillators. The negative resistance, provided by active circuits such as bipolar junction transistors (BJTs) or metal-oxide-semiconductor field-effect transistors (MOSFETs), injects energy into the LC resonant tank to sustain oscillation. Without the negative resistance, the oscillation would fade away due to the nonzero internal resistance of the circuit.

Another example of negative resistance is observed in certain elements like tunnel diodes. These diodes exhibit a region where the current initially increases with increasing voltage, but then the current drops as the voltage continues to rise. This specific region is known as the negative resistance region, indicating that the overall resistance of the diode suddenly decreases, as if a negative resistor is added in series with the original internal resistance of the diode.

In electronic designs that involve transient voltage suppression (TVS) diodes for electrostatic discharge (ESD) protection circuits, the negative resistance region of the TVS diode is often referred to as the snapback region. The extent of snapback is characterized by the voltage drop. A significant snapback indicates that the TVS diode can protect sensitive circuitry at a lower voltage level. However, excessively deep snapback can lead to issues.

Summary

Negative resistance in electronic circuits refers to a relationship where the voltage across a component decreases as the current passing through it increases. This phenomenon can be observed in elements like tunnel diodes and is utilized in oscillator analysis and ESD protection designs. Negative resistance allows certain active circuits such as oscillator driver to inject energy into LC resonant tanks and sustain oscillations, and it can provide lower voltage protection in snapback region in ESD applications. 

Reference and further reading

For more detailed information on ESD protection design and the dangers associated with deep snapback in ESD circuit protection devices, please refer to the provided reference and further reading link: "The dangers of deep snap-back ESD circuit-protection diodes" at https://e2e.ti.com/blogs_/b/analogwire/archive/2016/09/12/the-dangers-of-deep-snap-back-esd-circuit-protection-devices.