Current source
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 resistive load. There are two types. An independent current source delivers a constant current. A dependent current source delivers a current which is proportional to some other voltage or current in the circuit.
Background
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 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.
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. However, this model is only useful when a current source is operating within its compliance voltage.
Implementations
Passive current source
The simplest non-ideal current source consists of a voltage source in series with a resistor. The amount of current available from such a source is given by the ratio of the voltage across the voltage source to the resistance of the resistor. This value of current will only be delivered to a load with zero voltage drop across its terminals The current delivered to a load with nonzero voltage across its terminals will always be different. It is given by the ratio of the voltage drop across the resistor to its resistance.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. Thus, efficiency is low 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 for large laboratory versions.
Active current sources without negative feedback
In these circuits the output current is not monitored and controlled by means of negative feedback.Current-stable nonlinear implementation
They are implemented by active electronic components having current-stable nonlinear output characteristic when driven by steady input quantity. These circuits behave as dynamic resistors changing their present resistance to compensate current variations. For example, if the load increases its resistance, the transistor decreases its present output resistance to keep up a constant total resistance in the circuit.Active current sources have many important applications in electronic circuits. They are often used in place of ohmic resistors in analog integrated circuits 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 driven by a constant voltage naturally behave as current sources 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. Alternatively, an depletion-mode N-channel MOSFET could be used instead of a JFET in the circuits listed below for similar functionality.
Following voltage implementation
An example: bootstrapped current source.Voltage compensation implementation
The simple [|resistor passive current source] is ideal only when the voltage across it is zero; so voltage compensation by applying parallel negative feedback might be considered to improve the source. Operational amplifiers with feedback effectively work to minimise the voltage across their inputs. This results in making the inverting input a virtual ground, with the current running through the feedback, or load, and the passive current source. The input voltage source, the resistor, and the op-amp constitutes an "ideal" current source with value,. The transimpedance amplifier and an op-amp inverting amplifier are typical implementations of this idea.The floating load is a serious disadvantage of this circuit solution.
Current compensation implementation
A typical example are Howland current source and its derivative Deboo integrator. In the last example, the Howland current source consists of an input voltage source,, a positive resistor, R, a load and a negative impedance converter INIC. The input voltage source and the resistor R constitute an imperfect current source passing current, through the load. The INIC acts as a second current source passing "helping" current,, through the load. As a result, the total current flowing through the load is constant and the circuit impedance seen by the input source is increased. However the Howland current source isn't widely used because it requires the four resistors to be perfectly matched, and its impedance drops at high frequencies.The grounded load is an advantage of this circuit solution.
Current sources with negative feedback
They are implemented as a voltage follower with series negative feedback driven by a constant input voltage source. The voltage follower is loaded by a constant resistor acting as a simple current-to-voltage converter connected in the feedback loop. The external load of this current source is connected somewhere in the path of the current supplying the current sensing resistor but out of the feedback loop.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 ; it can be thought as a reversed current-to-voltage converter. The resistance R determines the transfer ratio.
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.
Simple transistor current sources
Constant current diode
The simplest constant-current source or sink is formed from one component: a JFET with its gate attached to its source. Once the drain-source voltage reaches a certain minimum value, the JFET enters saturation where current is approximately constant. This configuration is known as a constant-current diode, as it behaves much like a dual to the constant voltage diode used in simple voltage sources.Due to the large variability in saturation current of JFETs, it is common to also include a source resistor which allows the current to be tuned down to a desired value.
Zener diode current source
In this bipolar junction transistor implementation of the general idea above, a Zener voltage stabilizer drives an emitter follower loaded by a constant emitter resistor sensing the load current. The external load of this current source is connected to the collector so that almost the same current flows through it and the emitter resistor. The transistor, Q1, adjusts the output current so as to keep the voltage drop across the constant emitter resistor, R2, almost equal to the relatively constant voltage drop across the Zener diode, DZ1. As a result, the output current is almost constant even if the load resistance and/or voltage vary. The operation of the circuit is considered in details below.A Zener diode, when reverse biased 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, the voltage across the Zener diode will be constant. Resistor, R1, supplies the Zener current and the base current of NPN transistor. 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 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. Thus, the load current is constant and the circuit operates as a constant current source. As long as the temperature remains constant, 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.
. Resistance is calculated as
where = 1.2 to 2,
and is the lowest acceptable current gain for the particular transistor type being used.