Current source
Encyclopedia
A current source is an electrical or electronic device that delivers or absorbs electric current. A current source is the dual of a voltage source
. The term constant-current sink is sometimes used for sources fed from a negative voltage supply. Figure 1 shows a schematic for an ideal current source driving a resistor load.
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In circuit theory, an ideal current source is a circuit element where the current through it is independent of the voltage across it. It is a mathematical model
, which real devices can only approach in performance. 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.
An independent current source with zero current is identical to an ideal open circuit
. For this reason, the internal resistance
of an ideal current source is infinite. 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 voltage across the source approaches infinity as the load resistance approaches infinity (an open circuit). Thus, an ideal current source, if such a thing existed in reality, could supply unlimited power and so would represent an unlimited source of energy.
No real current source is ideal (no unlimited energy sources exist) and all have a finite internal resistance (none can supply unlimited voltage). However, the internal resistance of a physical current source is effectively modeled in circuit analysis by combining a non-zero resistance in parallel with an ideal current source (the Norton equivalent
circuit). The connection of an ideal open circuit to an ideal non-zero current source does not represent any physically realizable system.
in series with a resistor. The 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 (a short circuit, an uncharged capacitor, a charged inductor, a virtual ground circuit, etc.) The current delivered to a load with nonzero voltage (drop) across its terminals (a linear or nonlinear resistor with a finite resistance, a charged capacitor, an uncharged inductor, a voltage source, etc.) will always be different. It is given by the ratio of the voltage drop across the resistor (the difference between the exciting voltage and the voltage across the load) 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 (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 kilohm resistor will provide an approximately constant current of 1 mA (±5%) to a load resistance in the range of 50 to 450 ohm.
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 megavolt
s) for large laboratory versions.
.
Active current sources have many important applications in electronic circuit
s. They are often used in place of ohmic resistor
s in analog integrated circuit
s (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 IDSS 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).
An enhancement mode N channel MOSFET can be used in the circuits listed below.
The simple resistor current source will become "ideal" if the voltage across the load is somehow held zero. This idea seems paradoxical since real loads always "create" voltage drops across themselves but it is yet implemented by applying a parallel negative feedback. In these circuits, an op-amp compensates the voltage drop across the load by adding the same voltage to the exciting input voltage. As a result, the op-amp inverting input is held at virtual ground
and the combination of the input voltage source, the resistor and the supplied op-amp constitutes an "ideal" current source with value IOUT = VIN /R. The op-amp voltage-to-current converter in Figure 3, a 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.
The grounded load is an advantage of this circuit solution.
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 IOUT flowing through the load so that to make the voltage drop VR = IOUT.R across the current sensing resistor R equal to the constant input voltage VIN. Thus the voltage stabilizer keeps up a constant voltage drop across a constant resistor; so, a constant current IOUT = VR/R = VIN/R 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).
In this BJT implementation (figure 4) of the general idea above, a Zener voltage stabilizer (R1 and DZ1) drives an emitter follower (Q1) loaded by a constant emitter resistor (R2) sensing the load current. The external (floating) load of this current source is connected to the collector so that almost the same current flows through it and the emitter resistor (they can be thought of as connected in series). The transistor Q1 adjusts the output (collector) 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 (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 (IZ) is above a certain level (called holding current), the voltage across the Zener diode
(VZ) will be constant. Resistor
R1 supplies the Zener current and the base current (IB) of NPN transistor
(Q1). The constant Zener voltage is applied across the base of Q1 and emitter resistor R2.
Voltage across R2 (VR2) is given by VZ - VBE, where VBE is the base-emitter drop of Q1.
The emitter current of Q1 which is also the current through R2 is given by
Since VZ is constant and VBE is also (approximately) constant for a given temperature, it follows that VR2 is constant and hence IE is also constant. Due to transistor
action, emitter current IE is very nearly equal to the collector current IC 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
or
,
since VBE is typically 0.65 V for a silicon device.
(IR2 is also the emitter current and is assumed to be the same as the collector or required load current, provided hFE is sufficiently large). Resistance R1 at resistor R1 is calculated as
where K = 1.2 to 2 (so that R1 is low enough to ensure adequate IB),
and hFE(min) is the lowest acceptable current gain for the particular transistor type being used.
The Zener diode can be replaced by any other diode, e.g. a light-emitting diode
LED1 as shown in Figure 5. The LED voltage drop (VD) is now used to derive the constant voltage and also has the additional advantage of tracking (compensating) VBE changes due to temperature. R2 is calculated as
and R1 as
, where ID is the LED current.
Temperature changes will change the output current delivered by the circuit of Figure 4 because VBE is sensitive to temperature. Temperature dependence can be compensated using the circuit of Figure 6 that includes a standard diode D (of the same semiconductor material as the transistor) in series with the Zener diode as shown in the image on the left. The diode drop (VD) tracks the VBE changes due to temperature and thus significantly counteracts temperature dependence of the CCS.
Resistance R2 is now calculated as
Since VD = VBE = 0.65 V,
(In practice VD is never exactly equal to VBE and hence it only suppresses the change in VBE rather than nulling it out.)
R1 is calculated as
(the compensating diode's forward voltage drop VD appears in the equation and is typically 0.65 V for silicon devices.)
This method is most effective for Zener diode
s rated at 5.6 V or more. For breakdown diodes of less than 5.6 V, the compensating diode is usually not required because the breakdown
mechanism is not as temperature dependent as it is in breakdown diodes above this voltage.
Series negative feedback is also used in the two-transistor current mirror with emitter degeneration. Negative feedback is a basic feature in some current mirror
s using multiple transistors, such as the Widlar current source
and the Wilson current source
.
on Figure 8). As the bare emitter follower and the precise op-amp follower above, it keeps up a constant voltage drop (1.25 V) across a constant resistor (1.25 Ω); so, a constant current (1 A) flows through the resistor and the load. The LED is on when the voltage across the load exceeds 1.8 V (the indicator circuit introduces some error). The grounded load is an important advantage of this solution.
, a battery
, ...) are best modeled as voltage source
s. Such sources provide constant voltage, which means that as long as the amount of current drawn from the source is within the source's capabilities, its output voltage
stays constant.
An ideal voltage source provides no energy when it is loaded by an open circuit
(i.e. an infinite impedance
), but approaches infinite power and current when the load resistance approaches zero (a short circuit
). Such a theoretical device would have a zero ohm output impedance
in series with the source. A real-world voltage source has a very low, but non-zero output impedance
: often much less than 1 ohm.
Conversely, a current source provides a constant current, as long as the load connected to the source terminals has sufficiently low impedance. An ideal current source would provide no energy to a short circuit and approach infinite energy and voltage as the load resistance approaches infinity (an open circuit
). An ideal current source has an infinite output impedance
in parallel with the source. A real-world current source has a very high, but finite output impedance
. In the case of transistor current sources, impedances of a few megohms (at DC) are typical.
An ideal current source cannot be connected to an ideal open circuit
because this 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
). Also, a current source should not be connected to another current source if their currents differ but this arrangement is frequently used (e.g., in amplifying stages with dynamic load, CMOS
circuits, etc.)
Similarly, an ideal voltage source
cannot be connected to an ideal short circuit
(R=0), since this would result a similar paradox of finite non zero voltage across an element with defined zero voltage (the short circuit). Also, a voltage source should not be connected to another voltage source if their voltages differ but again this arrangement is frequently used (e.g., in common base
and differential amplifying stages).
Contrary, current and voltage sources can be connected to each other without any problems and this technique is widely used in circuitry (e.g., in cascode circuits
, differential amplifier stages with common emitter current source, etc.)
Because no ideal sources of either variety exist (all real-world examples have finite and non-zero source impedance), any current source can be considered as a voltage source with the same source impedance and vice versa. These concepts are dealt with by Norton's
and Thévenin's theorem
s.
Voltage source
In electric circuit theory, an ideal voltage source is a circuit element where the voltage across it is independent of the current through it. A voltage source is the dual of a current source. In analysis, a voltage source supplies a constant DC or AC potential between its terminals for any current...
. The term constant-current sink is sometimes used for sources fed from a negative voltage supply. Figure 1 shows a schematic for an ideal current source driving a resistor load.
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Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
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In circuit theory, an ideal current source is a circuit element where the current through it is independent of the voltage across it. It is a mathematical model
Mathematical model
A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used not only in the natural sciences and engineering disciplines A mathematical model is a...
, which real devices can only approach in performance. 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.
An independent current source with zero current is identical to an ideal open circuit
Open circuit
The term Open circuit may refer to:*Open-circuit scuba, a type of SCUBA-diving equipment where the user breathes from the set and then exhales to the surroundings without recycling the exhaled air...
. For this reason, the internal resistance
Internal resistance
A practical electrical power source which is a linear electric circuit may, according to Thévenin's theorem, be represented as an ideal voltage source in series with an impedance. This resistance is termed the internal resistance of the source. When the power source delivers current, the measured...
of an ideal current source is infinite. The voltage across an ideal current source is completely determined by the circuit it is connected to. When connected to a short circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
, there is zero voltage and thus zero power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
delivered. When connected to a load resistance, the voltage across the source approaches infinity as the load resistance approaches infinity (an open circuit). Thus, an ideal current source, if such a thing existed in reality, could supply unlimited power and so would represent an unlimited source of energy.
No real current source is ideal (no unlimited energy sources exist) and all have a finite internal resistance (none can supply unlimited voltage). However, the internal resistance of a physical current source is effectively modeled in circuit analysis by combining a non-zero resistance in parallel with an ideal current source (the Norton equivalent
Norton's theorem
Norton's theorem for linear electrical networks, known in Europe as the Mayer–Norton theorem, states that any collection of voltage sources, current sources, and resistors with two terminals is electrically equivalent to an ideal current source, I, in parallel with a single resistor, R...
circuit). The connection of an ideal open circuit to an ideal non-zero current source does not represent any physically realizable system.
Passive current source
The simplest non-ideal current source consists of a voltage sourceVoltage source
In electric circuit theory, an ideal voltage source is a circuit element where the voltage across it is independent of the current through it. A voltage source is the dual of a current source. In analysis, a voltage source supplies a constant DC or AC potential between its terminals for any current...
in series with a resistor. The current available from such a source is given by the ratio
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...
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 (a short circuit, an uncharged capacitor, a charged inductor, a virtual ground circuit, etc.) The current delivered to a load with nonzero voltage (drop) across its terminals (a linear or nonlinear resistor with a finite resistance, a charged capacitor, an uncharged inductor, a voltage source, etc.) will always be different. It is given by the ratio of the voltage drop across the resistor (the difference between the exciting voltage and the voltage across the load) 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 (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 kilohm resistor will provide an approximately constant current of 1 mA (±5%) to a load resistance in the range of 50 to 450 ohm.
A Van de Graaff generator
Van de Graaff generator
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate very high voltages on a hollow metal globe on the top of the stand. It was invented in 1929 by American physicist Robert J. Van de Graaff. The potential differences achieved in modern Van de Graaff...
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 megavolt
Megavolt
*A megavolt is 1 million volts in electronics and physics.*Megavolt is a villain in the Disney animated series Darkwing Duck.*Megavolt is a villain in the Teenage Mutant Ninja Turtles ...
s) 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 feedbackNegative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
.
...with current-stable nonlinear element...
They are implemented by active electronic components (transistors) having current-stable nonlinear output characteristic when driven by steady input quantity (current or voltage). These circuits behave as dynamic resistors changing its present resistance to compensate current variations. For example, if the load increases its resistance, the transistor decreases its present output resistance (and v.v.) to keep up a constant total resistance in the circuit.Active current sources have many important applications in electronic circuit
Electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
s. They are often used in place of ohmic resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
s in analog integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s (e.g., a differential amplifier) to generate a current that depends slightly on the voltage across the load.
The common emitter
Common emitter
In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor amplifier topologies, typically used as a voltage amplifier...
configuration driven by a constant input current or voltage and common source
Common source
In electronics, a common-source amplifier is one of three basic single-stage field-effect transistor amplifier topologies, typically used as a voltage or transconductance amplifier. The easiest way to tell if a FET is common source, common drain, or common gate is to examine where the signal...
(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
Current mirror
A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being 'copied' can be, and sometimes is, a varying signal current...
is an example of such a current source widely used in integrated circuits. The common base
Common base
In electronics, a common-base amplifier is one of three basic single-stage bipolar junction transistor amplifier topologies, typically used as a current buffer or voltage amplifier...
, common gate
Common gate
In electronics, a common-gate amplifier is one of three basic single-stage field-effect transistor amplifier topologies, typically used as a current buffer or voltage amplifier...
and common grid configurations can serve as constant current sources as well.
A JFET
JFET
The junction gate field-effect transistor is the simplest type of field-effect transistor. It can be used as an electronically-controlled switch or as a voltage-controlled resistance. Electric charge flows through a semiconducting channel between "source" and "drain" terminals...
can be made to act as a current source by tying its gate to its source. The current then flowing is the IDSS of the FET. These can be purchased with this connection already made and in this case the devices are called current regulator diodes
Constant current diode
Constant current diode consists of a JFET with the gate shorted to the source, and it functions like a two-terminal current limiter or current source...
or constant current diodes or current limiting diodes (CLD).
An enhancement mode N channel MOSFET can be used in the circuits listed below.
...with following voltage...
An example: Bootstrapped current source (Widlar bilateral current source)....with voltage compensation...
Virtual ground
Virtual ground is a node of the circuit that is maintained at a steady reference potential, without being connected directly to the reference potential...
and the combination of the input voltage source, the resistor and the supplied op-amp constitutes an "ideal" current source with value IOUT = VIN /R. The op-amp voltage-to-current converter in Figure 3, a 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.
...with current compensation
A typical example are Howland current source and its derivative Deboo integrator. In the last example (see Fig. 1 there), the Howland current source consists of an input voltage source VIN, a positive resistor R, a load (the capacitor C acting as impedance Z) and a negative impedance converter INIC (R1 = R2 = R3 = R and the op-amp). The input voltage source and the resistor R constitute an imperfect current source passing current IR through the load (see Fig. 3 in the source). The INIC acts as a second current source passing "helping" current I-R 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.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 (i.e., a negative feedback voltage stabilizer). The voltage follower is loaded by a constant (current sensing) resistor acting as a simple current-to-voltage converterCurrent-to-voltage converter
In electronics, a transimpedance amplifier is an amplifier that converts current to voltage. Its input ideally has zero impedance, and the input signal is a current...
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 IOUT flowing through the load so that to make the voltage drop VR = IOUT.R across the current sensing resistor R equal to the constant input voltage VIN. Thus the voltage stabilizer keeps up a constant voltage drop across a constant resistor; so, a constant current IOUT = VR/R = VIN/R 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
Transconductance
Transconductance, also known as mutual conductance, is a property of certain electronic components. Conductance is the reciprocal of resistance; transconductance, meanwhile, is the ratio of the current change at the output port to the voltage change at the input port. It is written as gm...
).
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).
Zener diode current source
In this BJT implementation (figure 4) of the general idea above, a Zener voltage stabilizer (R1 and DZ1) drives an emitter follower (Q1) loaded by a constant emitter resistor (R2) sensing the load current. The external (floating) load of this current source is connected to the collector so that almost the same current flows through it and the emitter resistor (they can be thought of as connected in series). The transistor Q1 adjusts the output (collector) 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
Zener diode
A Zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "Zener knee voltage" or "Zener...
, when reverse biased (as shown in the circuit) has a constant voltage drop
Voltage drop
Voltage drop is the reduction in voltage in the passive elements of an electrical circuit. Voltage drops across conductors, contacts, connectors and source internal resistances are undesired as they reduce the supplied voltage while voltage drops across loads and other electrical and electronic...
across it irrespective of the current flowing through it. Thus, as long as the Zener current (IZ) is above a certain level (called holding current), the voltage across the Zener diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
(VZ) will be constant. Resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
R1 supplies the Zener current and the base current (IB) of NPN transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
(Q1). The constant Zener voltage is applied across the base of Q1 and emitter resistor R2.
Voltage across R2 (VR2) is given by VZ - VBE, where VBE is the base-emitter drop of Q1.
The emitter current of Q1 which is also the current through R2 is given by
Since VZ is constant and VBE is also (approximately) constant for a given temperature, it follows that VR2 is constant and hence IE is also constant. Due to transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
action, emitter current IE is very nearly equal to the collector current IC 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
Early Effect
The Early effect is the variation in the width of the base in a bipolar junction transistor due to a variation in the applied base-to-collector voltage, named after its discoverer James M. Early...
) 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
or
,since VBE is typically 0.65 V for a silicon device.
(IR2 is also the emitter current and is assumed to be the same as the collector or required load current, provided hFE is sufficiently large). Resistance R1 at resistor R1 is calculated as
where K = 1.2 to 2 (so that R1 is low enough to ensure adequate IB),
and hFE(min) is the lowest acceptable current gain for the particular transistor type being used.
LED current source
The Zener diode can be replaced by any other diode, e.g. a light-emitting diode
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
LED1 as shown in Figure 5. The LED voltage drop (VD) is now used to derive the constant voltage and also has the additional advantage of tracking (compensating) VBE changes due to temperature. R2 is calculated as
and R1 as
, where ID is the LED current.Transistor current source with diode compensation
Temperature changes will change the output current delivered by the circuit of Figure 4 because VBE is sensitive to temperature. Temperature dependence can be compensated using the circuit of Figure 6 that includes a standard diode D (of the same semiconductor material as the transistor) in series with the Zener diode as shown in the image on the left. The diode drop (VD) tracks the VBE changes due to temperature and thus significantly counteracts temperature dependence of the CCS.
Resistance R2 is now calculated as
Since VD = VBE = 0.65 V,
(In practice VD is never exactly equal to VBE and hence it only suppresses the change in VBE rather than nulling it out.)
R1 is calculated as
(the compensating diode's forward voltage drop VD appears in the equation and is typically 0.65 V for silicon devices.)This method is most effective for Zener diode
Zener diode
A Zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "Zener knee voltage" or "Zener...
s rated at 5.6 V or more. For breakdown diodes of less than 5.6 V, the compensating diode is usually not required because the breakdown
Electrical breakdown
The term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....
mechanism is not as temperature dependent as it is in breakdown diodes above this voltage.
Current mirror with emitter degeneration
Series negative feedback is also used in the two-transistor current mirror with emitter degeneration. Negative feedback is a basic feature in some current mirror
Current mirror
A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being 'copied' can be, and sometimes is, a varying signal current...
s using multiple transistors, such as the Widlar current source
Widlar current source
A Widlar current source is a modification of the basic two-transistor current mirror that incorporates an emitter degeneration resistor for only the output transistor, enabling the current source to generate low currents using only moderate resistor values....
and the Wilson current source
Wilson current source
A Wilson current mirror or Wilson current source is a circuit configuration designed to provide a constant current source or sink. It is named after George Wilson, an integrated circuit design engineer working for Tektronix...
.
Op-amp current sources
The simple transistor current source from Figure 4 can be improved by inserting the base-emitter junction of the transistor in the feedback loop of an op-amp (Figure 7). Now the op-amp increases its output voltage so that to compensate VBE. The circuit is actually a buffered non-inverting amplifier driven by a constant input voltage. It keeps up this constant voltage across the constant sense resistor. As a result, the current flowing through the load is constant as well; it is exactly the Zener voltage divided by the sense resistor. The load can be connected either in the emitter (Figure 7) or in the collector (Figure 4) but in both the cases it is floating as in all the circuits above. The transistor is not needed if the required current doesn't exceed the sourcing ability of the op-amp. The article on current mirror discusses another example of these so-called gain-boosted current mirrors.Voltage regulator current sources
The general negative feedback arrangement can be implemented by an IC voltage regulator (LM317 voltage regulatorLM317
The LM317 is a popular variable voltage regulator. It was invented by Robert C. Dobkin and Robert J. Widlar in 1970 while they worked at National Semiconductor.-Specifications:-See also:...
on Figure 8). As the bare emitter follower and the precise op-amp follower above, it keeps up a constant voltage drop (1.25 V) across a constant resistor (1.25 Ω); so, a constant current (1 A) flows through the resistor and the load. The LED is on when the voltage across the load exceeds 1.8 V (the indicator circuit introduces some error). The grounded load is an important advantage of this solution.
Current and voltage source comparison
Most sources of electrical energy (mains electricityMains electricity
Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...
, a battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
, ...) are best modeled as voltage source
Voltage source
In electric circuit theory, an ideal voltage source is a circuit element where the voltage across it is independent of the current through it. A voltage source is the dual of a current source. In analysis, a voltage source supplies a constant DC or AC potential between its terminals for any current...
s. Such sources provide constant voltage, which means that as long as the amount of current drawn from the source is within the source's capabilities, its output voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
stays constant.
An ideal voltage source provides no energy when it is loaded by an open circuit
Open circuit
The term Open circuit may refer to:*Open-circuit scuba, a type of SCUBA-diving equipment where the user breathes from the set and then exhales to the surroundings without recycling the exhaled air...
(i.e. an infinite impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
), but approaches infinite power and current when the load resistance approaches zero (a short circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
). Such a theoretical device would have a zero ohm output impedance
Output impedance
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
in series with the source. A real-world voltage source has a very low, but non-zero output impedance
Output impedance
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
: often much less than 1 ohm.
Conversely, a current source provides a constant current, as long as the load connected to the source terminals has sufficiently low impedance. An ideal current source would provide no energy to a short circuit and approach infinite energy and voltage as the load resistance approaches infinity (an open circuit
Open circuit
The term Open circuit may refer to:*Open-circuit scuba, a type of SCUBA-diving equipment where the user breathes from the set and then exhales to the surroundings without recycling the exhaled air...
). An ideal current source has an infinite output impedance
Output impedance
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
in parallel with the source. A real-world current source has a very high, but finite output impedance
Output impedance
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
. In the case of transistor current sources, impedances of a few megohms (at DC) are typical.
An ideal current source cannot be connected to an ideal open circuit
Open circuit
The term Open circuit may refer to:*Open-circuit scuba, a type of SCUBA-diving equipment where the user breathes from the set and then exhales to the surroundings without recycling the exhaled air...
because this 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
Open circuit
The term Open circuit may refer to:*Open-circuit scuba, a type of SCUBA-diving equipment where the user breathes from the set and then exhales to the surroundings without recycling the exhaled air...
). Also, a current source should not be connected to another current source if their currents differ but this arrangement is frequently used (e.g., in amplifying stages with dynamic load, CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
circuits, etc.)
Similarly, an ideal voltage source
Voltage source
In electric circuit theory, an ideal voltage source is a circuit element where the voltage across it is independent of the current through it. A voltage source is the dual of a current source. In analysis, a voltage source supplies a constant DC or AC potential between its terminals for any current...
cannot be connected to an ideal short circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
(R=0), since this would result a similar paradox of finite non zero voltage across an element with defined zero voltage (the short circuit). Also, a voltage source should not be connected to another voltage source if their voltages differ but again this arrangement is frequently used (e.g., in common base
Common base
In electronics, a common-base amplifier is one of three basic single-stage bipolar junction transistor amplifier topologies, typically used as a current buffer or voltage amplifier...
and differential amplifying stages).
Contrary, current and voltage sources can be connected to each other without any problems and this technique is widely used in circuitry (e.g., in cascode circuits
Cascode
The cascode is a two-stage amplifier composed of a transconductance amplifier followed by a current buffer. Compared to a single amplifier stage, this combination may have one or more of the following characteristics: higher input-output isolation, higher input impedance, high output impedance,...
, differential amplifier stages with common emitter current source, etc.)
Because no ideal sources of either variety exist (all real-world examples have finite and non-zero source impedance), any current source can be considered as a voltage source with the same source impedance and vice versa. These concepts are dealt with by Norton's
Norton's theorem
Norton's theorem for linear electrical networks, known in Europe as the Mayer–Norton theorem, states that any collection of voltage sources, current sources, and resistors with two terminals is electrically equivalent to an ideal current source, I, in parallel with a single resistor, R...
and Thévenin's theorem
Thévenin's theorem
In circuit theory, Thévenin's theorem for linear electrical networks states that any combination of voltage sources, current sources, and resistors with two terminals is electrically equivalent to a single voltage source V and a single series resistor R. For single frequency AC systems the theorem...
s.
See also
- Constant currentConstant currentConstant current is a term most often used in electronics to describe a system that can vary the voltage across an electronic circuit to maintain a constant electric current....
- Voltage-to-current converter
- Current mirrorCurrent mirrorA current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being 'copied' can be, and sometimes is, a varying signal current...
- Iron-hydrogen resistorIron-hydrogen resistorAn iron-hydrogen resistor consists of a hydrogen-filled glass bulb like a light bulb, in which an iron wire is located. This resistor has a positive temperature coefficient of resistance. This characteristic made it useful for stabilizing circuits against fluctuations in power supply voltages...
- Widlar current sourceWidlar current sourceA Widlar current source is a modification of the basic two-transistor current mirror that incorporates an emitter degeneration resistor for only the output transistor, enabling the current source to generate low currents using only moderate resistor values....
- Wilson current sourceWilson current sourceA Wilson current mirror or Wilson current source is a circuit configuration designed to provide a constant current source or sink. It is named after George Wilson, an integrated circuit design engineer working for Tektronix...
- Current loopCurrent loopA current loop describes two different electrical signalling schemes.- Digital :For digital serial communications, a current loop is a communication interface that uses current instead of voltage for signaling...
- Welding power supplyWelding power supplyA welding power supply is a device that provides an electric current to perform welding. Welding usually requires high current and it can need above 12,000 amperes in spot welding. Low current can also be used; welding two razor blades together at 5 amps with gas tungsten arc welding is a good...
, a device used for arc weldingArc weldingArc welding is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct or alternating current, and consumable or non-consumable electrodes...
, many of which are designed as constant current devices. - Current sources and sinksCurrent sources and sinksCurrent sources and sinks are analysis formalisms which distinguish points, areas, or volumes through which current enters or exits a system. While current sources or sinks are abstract elements used for analysis, generally they have physical counterparts in real-world applications; e.g. the anode...
Further reading
- "Current Sources & Voltage References" Linden T. Harrison; Publ. Elsevier-Newnes 2005; 608-pages; ISBN 0-7506-7752-X