Create circuit from current–voltage characteristic 2
Both voltage sources and current sources are idealized abstractions of two possible kinds of devices which can bring energy into a circuit. They have different behaviors. An ideal current source supplies a fixed amount of current into any load, from zero ohms (the easiest load for a current source) to an arbitrarily large, finite resistance (a difficult load). It develops whatever voltage is necessary across the load so that that fixed amount of current flows. A voltage source maintains the same voltage across any load from an open circuit (the easiest load for a voltage source), down to almost zero (a difficult load). It develops the amount of current necessary to produce the voltage across the load. Also, a current source looks like an open circuit: it has a very high impedance, whereas a voltage source has zero impedance.

Power supplies usually look more like ideal voltage sources than current sources. Sources of voltage are easy to identify in many kinds of circuits. Some section of the schematics for a device is often dedicated to the power supply, and there we often find networks labeled with voltages.

However, some circuits require a current source in order to work properly. In a high level, abstract design for these circuits, we can show the current source symbol. For instance, in a diagram of a differential amplifier, we might show that there is some kind of current source for the tail current by using the schematic symbol of a circle with an arrow, or the Euro-style overlapping circles.

In a real engineering circuit depicting a device that will be built, current sources are usually not obvious because in their place are drawn the circuits that implement the current sourcing behavior. To identify the current source, you have to recognize a pattern, such as a pair of transistors acting as a mirror. Nevertheless, the current source is real.

Current sources have valuable properties. They may be exploited for their very high impedance. (For instance when used as "active loads" in amplifier stages).

Current sources can control the distribution of current in a circuit. If two currents come into a node, and the exit from the node is via a current source, then the current source forces the sum of those two currents to be constant. So a current source can be used to force more current through one branch of a circuit when less current is flowing in another and vice versa.

Current sources are useful for driving devices which respond poorly to voltage: low impedances, or devices whose current is very sensitive to small changes in voltage.

Key properties in point form:


Both voltage and current are present in circuits built around voltage and current sources.
Current sources maintain constant current (adjusting voltage as needed)
Voltage sources maintain constant voltage (adjusting current as needed)
Current sources "like" short circuit loads; voltage sources "hate" them.
Voltage sources "like" open circuit loads; current sources hate open circuits.
Voltage sources are zero impedances; current sources are open circuits. 
Voltage sources "hate" being connected in parallel to other voltage sources (of a different voltage)
Current sources "hate" being in series with other current sources (of a different current value)