Negative resistance
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In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it.
Fluorescent lamp, a device with negative differential resistance. In operation, an increase in current through the fluorescent tube causes a drop in voltage across it. If the tube were connected directly to the power line, the falling tube voltage would cause more and more current to flow, causing it to arc flash and destroy itself. To prevent this, fluorescent tubes are connected to the power line through a ballast. The ballast adds positive impedance (AC resistance) to the circuit to counteract the negative resistance of the tube, limiting the current.
A Gunn diode, a semiconductor device with negative differential resistance used in electronic oscillators to generate microwaves
An oscillator consisting of a Gunn diode inside a cavity resonator. The negative resistance of the diode excites microwave oscillations in the cavity, which radiate through the aperture into a waveguide (not shown).
8–12 GHz microwave amplifier consisting of two cascaded tunnel diode reflection amplifiers
Electronic oscillator
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An electronic oscillator is an electronic circuit that produces a periodic, oscillating or alternating current (AC) signal, usually a sine wave, square wave or a triangle wave, powered by a direct current (DC) source. Oscillators are found in many electronic devices, such as radio receivers, television sets, radio and television broadcast transmitters, computers, computer peripherals, cellphones, radar, and many other devices.
Crystal oscillator
A 120 MHz oscillator from 1938 using a parallel rod transmission line resonator (Lecher line). Transmission lines are widely used for UHF oscillators.