Current source
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A Current source is a device that delivers or absorbs electrical energy such that the electrical current is independent of the voltage across its terminals. An ideal current source will produce any voltage necessary to maintain a specified current. Independent current sources are not found in nature, although many electronic devices, such as transistors and vacuum tubes, are modeled as dependent current sources.
Current sources have many important applications in electronic circuits. Current sources are often used in place of resistors in analog integrated circuits. The emitter-collector path of a bipolar transistor, the source-drain path of a field effect transistor, or the filament-plate path of a vacuum tube naturally behave as current sources.
Comparison between a current source and a voltage source
Most of the sources of electrical energy (the mains, a battery, ...) are voltage sources. Such sources provide constant voltage which means that as long as the amount of current drawn from the source is within the source specifications, the source output voltage stays constant.
An ideal voltage source would provide no energy when it is loaded by an infinite impedance (i.e. nothing is connected to the device terminals) and an infinite energy (and an infinite current) when it is short circuited. Such theoretical device would have a zero ohm output impedance. A real-world voltage source has a very low output impedance: often less than 1 ohm.
Conversely, a current source provides a constant current, which means that as long as the load connected to the source terminals has an impedance low enough, the current stays constant. An ideal current source would provide no energy to a short circuit and an infinite energy (and an infinite voltage) to an open circuit. An ideal current source has an infinite output impedance.
Since no ideal sources exist of either variety (all examples have source impedance), it is safe to say that any current source is also a voltage source, and vice versa, through Norton and Thevenin's theorems.
Example of Implementation
One form of current source can be realized with a transistor output configured in such a way that the load is wired from the output of the sensor to the common side of the power source so that when the transistor turns on voltage is sourced to the load. This is a current mirror.
A Van de Graaff generator behaves as a current source; supplying the same few microamperes at any output voltage between zero and hundreds of thousands (or even tens of millions of volts for large laboratory versions.)de:Stromquelle