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High-Speed Switch
All diodes, from vacuum tubes to semiconductor devices, function as one-way switches, allowing current to flow in one direction and blocking it when the voltage across the diode reverses. The 1N4148 has a high switching speed of eight nanoseconds, allowing it to operate efficiently at frequencies up to 125 MHz.
Voltage Source
Silicon diodes, such as the 1N4148, have a constant forward voltage drop of .7 volts. Circuits requiring a voltage reference can use the diode singly or in a series of several diodes for multiples of .7 volts. For example, three 1N4148 diodes wired in series with a 10K ohm resistor, and connected to a nine-volt battery, have a voltage drop of .7 volts each. Across the three diodes, the total drop is 2.1 volts. The resistor limits the current through the diodes but does not affect the voltage drop.
AM Radio Detector
In AM radio, an audio signal modulates the amplitude, or strength, of a high-frequency carrier wave. A single-diode detector circuit, such as one using the 1N4148, recovers the audio from the carrier signal. A radio receiver has a tuner to select one carrier frequency from the AM band, and a diode detector and filter to remove the high-frequency signal, leaving the audio. The diode̵7;s switching action turns the carrier signal̵7;s AC into DC. The amplitude of the DC is simply the audio in the broadcast.
Rectifier
All diodes function as rectifiers, converting alternating current, which reverses direction, into direct current, which flows in one direction. DC power supplies typically use diodes with higher voltage- and current-handling capabilities than the 1N4148 as rectifiers. The 1N4001, for example, has a maximum current of one amp. For applications that do not exceed its 300-milliamp limit, the 1N4148 can act as a rectifier.