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A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.
A current source is the dual of a voltage source. The term current sink is sometimes used for sources fed from a negative voltage supply. Figure 1 shows the schematic symbol for an ideal current source driving a electrical load. There are two types. An independent current source (or sink) delivers a constant current. A dependent current source delivers a current which is proportional to some other voltage or current in the circuit.
An ideal current source generates a current that is independent of the voltage changes across it. An ideal current source is a mathematical model, which real devices can approach very closely. If the current through an ideal current source can be specified independently of any other variable in a circuit, it is called an independent current source. Conversely, if the current through an ideal current source is determined by some other voltage or current in a circuit, it is called a dependent or controlled current source. Symbols for these sources are shown in Figure 2.
The internal resistance of an ideal current source is infinite. An independent current source with zero current is identical to an ideal open circuit. The voltage across an ideal current source is completely determined by the circuit it is connected to. When connected to a short circuit, there is zero voltage and thus zero electric power delivered. When connected to a load resistance, the current source manages the voltage in such a way as to keep the current constant; so in an ideal current source the voltage across the source approaches infinity as the load resistance approaches infinity (an open circuit).
No physical current source is ideal. For example, no physical current source can operate when applied to an open circuit. There are two characteristics that define a current source in real life. One is its internal resistance and the other is its compliance voltage. The compliance voltage is the maximum voltage that the current source can supply to a load. Over a given load range, it is possible for some types of real current sources to exhibit nearly infinite internal resistance. However, when the current source reaches its compliance voltage, it abruptly stops being a current source.
In circuit analysis, a current source having finite internal resistance is modeled by placing the value of that resistance across an ideal current source (the Norton equivalent circuit). However, this model is only useful when a current source is operating within its compliance voltage.
For a nearly ideal current source, the value of the resistor should be very large but this implies that, for a specified current, the voltage source must be very large (in the limit as the resistance and the voltage go to infinity, the current source will become ideal and the current will not depend at all on the voltage across the load). Thus, efficiency is low (due to power loss in the resistor) and it is usually impractical to construct a 'good' current source this way. Nonetheless, it is often the case that such a circuit will provide adequate performance when the specified current and load resistance are small. For example, a 5 V voltage source in series with a 4.7 kΩ resistor will provide an approximately constant current of to a load resistance in the range of 50 to 450 Ω.
A Van de Graaff generator is an example of such a high voltage current source. It behaves as an almost constant current source because of its very high output voltage coupled with its very high output resistance and so it supplies the same few microamperes at any output voltage up to hundreds of thousands of volts (or even tens of megavolts) for large laboratory versions.
Active current sources have many important applications in electronic circuits. They are often used in place of ohmic in analog integrated circuits (e.g., a differential amplifier) to generate a current that depends slightly on the voltage across the load.
The common emitter configuration driven by a constant input current or voltage and common source (common cathode) driven by a constant voltage naturally behave as current sources (or sinks) because the output impedance of these devices is naturally high. The output part of the simple current mirror is an example of such a current source widely used in integrated circuits. The common base, common gate and common grid configurations can serve as constant current sources as well.
A JFET can be made to act as a current source by tying its gate to its source. The current then flowing is the of the FET. These can be purchased with this connection already made and in this case the devices are called current regulator diodes or constant current diodes or current limiting diodes (CLD). Alternatively, an depletion-mode NMOS logic
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The floating load is a serious disadvantage of this circuit solution.
The grounded load is an advantage of this circuit solution.
The voltage follower adjusts its output current flowing through the load so that to make the voltage drop across the current sensing resistor R equal to the constant input voltage . Thus the voltage stabilizer keeps up a constant voltage drop across a constant resistor; so, a constant current flows through the resistor and respectively through the load.
If the input voltage varies, this arrangement will act as a voltage-to-current converter (voltage-controlled current source, VCCS); it can be thought as a reversed (by means of negative feedback) current-to-voltage converter. The resistance R determines the transfer ratio (transconductance).
Current sources implemented as circuits with series negative feedback have the disadvantage that the voltage drop across the current sensing resistor decreases the maximal voltage across the load (the compliance voltage).
Due to the large variability in saturation current of JFETs, it is common to also include a source resistor (shown in the adjacent image) which allows the current to be tuned down to a desired value.
A Zener diode, when reverse biased (as shown in the circuit) has a constant voltage drop across it irrespective of the current flowing through it. Thus, as long as the Zener current () is above a certain level (called holding current), the voltage across the Zener diode () will be constant. Resistor, R1, supplies the Zener current and the base current () of NPN transistor (Q1). The constant Zener voltage is applied across the base of Q1 and emitter resistor, R2.
Voltage across () is given by , where is the base-emitter drop of Q1. The emitter current of Q1 which is also the current through R2 is given by
Since is constant and is also (approximately) constant for a given temperature, it follows that is constant and hence is also constant. Due to transistor action, emitter current, , is very nearly equal to the collector current, , of the transistor (which in turn, is the current through the load). Thus, the load current is constant (neglecting the output resistance of the transistor due to the Early effect) and the circuit operates as a constant current source. As long as the temperature remains constant (or doesn't vary much), the load current will be independent of the supply voltage, R1 and the transistor's gain. R2 allows the load current to be set at any desirable value and is calculated by
where is typically 0.65 V for a silicon device.The value for varies logarithmically with current level: for more detail see diode modelling.
( is also the emitter current and is assumed to be the same as the collector or required load current, provided is sufficiently large). Resistance is calculated as
where = 1.2 to 2 (so that is low enough to ensure adequate ),
and is the lowest acceptable current gain for the particular transistor type being used.
and as
Resistance is now calculated as
Since ,See above note on logarithmic current dependence.
(In practice, is never exactly equal to and hence it only suppresses the change in rather than nulling it out.)
is calculated as
(the compensating diode's forward voltage drop, , appears in the equation and is typically 0.65 V for silicon devices.)
Note that this only works well if DZ1 is a reference diode or another stable voltage source. Together with 'normal' Zener diodes especially with lower Zener voltages (<5V) the diode might even worsen overall temperature dependency.
Imagine in Figure 5, at power up, that the LED has 1 V across it driving the base of the transistor. At room temperature there is about 0.6 V drop across the junction and hence 0.4 V across the emitter resistor, giving an approximate collector (load) current of amps. Now imagine that the power dissipation in the transistor causes it to heat up. This causes the drop (which was 0.6 V at room temperature) to drop to, say, 0.2 V. Now the voltage across the emitter resistor is 0.8 V, twice what it was before the warmup. This means that the collector (load) current is now twice the design value! This is an extreme example of course, but serves to illustrate the issue.
The circuit to the left overcomes the thermal problem (see also, current limiting). To see how the circuit works, assume the voltage has just been applied at V+. Current runs through R1 to the base of Q1, turning it on and causing current to begin to flow through the load into the collector of Q1. This same load current then flows out of Q1's emitter and consequently through to ground. When this current through to ground is sufficient to cause a voltage drop that is equal to the drop of Q2, Q2 begins to turn on. As Q2 turns on it pulls more current through its collector resistor, R1, which diverts some of the injected current in the base of Q1, causing Q1 to conduct less current through the load. This creates a negative feedback loop within the circuit, which keeps the voltage at Q1's emitter almost exactly equal to the drop of Q2. Since Q2 is dissipating very little power compared to Q1 (since all the load current goes through Q1, not Q2), Q2 will not heat up any significant amount and the reference (current setting) voltage across will remain steady at ≈0.6 V, or one diode drop above ground, regardless of the thermal changes in the drop of Q1. The circuit is still sensitive to changes in the ambient temperature in which the device operates as the BE voltage drop in Q2 varies slightly with temperature.
]] Most sources of electrical energy (mains electricity, a battery, etc.) are best modeled as , however some (notably solar cells) are better modeled using current sources. Sometimes it is easier to view a current source as a voltage source and vice versa (see conversion in Figure 9) using Norton's and Thévenin's theorems.
Voltage sources provide an almost-constant output voltage as long as the current drawn from the source is within the source's capabilities. An ideal voltage source loaded by an open circuit (i.e., an infinite impedance) will provide no current (and hence no power). But when the load resistance approaches zero (a short circuit), the current (and thus power) approach infinity. Such a theoretical device has a zero ohm output impedance in series with the source. Real-world voltage sources instead have a non-zero output impedance, which is preferably very low (often much less than 1 ohm).
Conversely, a current source provides a constant current, as long as the impedance of the load is sufficiently lower than the current source's parallel impedance (which is preferably very high and ideally infinite). In the case of transistor current sources, impedances of a few (at low frequencies) are typical. Because power is current squared times resistance, as a load resistance connected to a current source approaches zero (a short circuit), the current and thus power both approach zero.
Ideal current sources don't exist. Hypothetically connecting one to an ideal open circuit would create the paradox of running a constant, non-zero current (from the current source) through an element with a defined zero current (the open circuit). As the load resistance of an ideal current source approaches infinity (an open circuit), the voltage across the load would approach infinity (because voltage equals current times resistance), and hence the power drawn would also approach infinity. The current of a real current source connected to an open circuit would instead flow through the current source's internal parallel impedance (and be wasted as heat).
Similarly, ideal voltage sources don't exist. Hypothetically connecting one to an ideal short circuit would result a similar paradox of finite non-zero voltage across an element with defined zero voltage (the short circuit).
Just like how voltage sources should not be connected in parallel to another voltage source with different voltages, a current source also should not be connected in series to another current source. Note, some circuits use elements that are similar but not identical to voltage or current sources and may work when connected in these manners that are disallowed for actual current or voltage sources. Also, just like voltage sources may be connected in series to add their voltages, current sources may be connected in parallel to add their currents.
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