Current source

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A current source is an electronic circuit that transmits or absorbs an electric current independent of the voltage through it.

Current source is a dual voltage source. The term the current sink is sometimes used for the source fed from the negative voltage supply. Figure 1 shows a schematic symbol for an ideal current source that drives a resistive load. There are two types. An independent current source (or sink) provides a constant current. The current source dependent provides a current proportional to some other voltage or current in the circuit.

Video Current source

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An ideal current source produces a current independent of the voltage change through which it passes. The ideal stream source is a mathematical model, which the real device can approach very closely. If the current through the ideal current source can be determined separately from other variables in the circuit, this is called an independent flow source . Conversely, if the current through the ideal current source is determined by some voltage or other current in the circuit, it is called dependent or controlled current source . The symbols for these sources are shown in Figure 2.

The internal resistance of the ideal current source is unlimited. An independent current source with zero current is identical to the ideal open circuit. The voltage at the ideal current source is fully determined by the connected circuit. When connected to a short circuit, there is a zero voltage and thus zero power is transmitted. When connected to the load resistance, the voltage across the source approaches infinity when the load resistance is close to infinity (open circuit). Thus, the ideal current source, if such a thing exists in reality, can supply unlimited power and will thus represent an infinite source of energy.

There is no ideal physical current source. For example, no physical current source can operate when applied to an open circuit. There are two characteristics that determine the source of the current in real life. One is internal resistance and the other is the voltage of its compliance. Compliance voltage is the maximum voltage that the current source can supply to the load. During the given load range, it is possible for some types of real-current sources to show almost unlimited internal resistance. However, when the current source reaches its compliance voltage, it suddenly ceases to be a current source.

In circuit analysis, a current source having a finite internal resistance is modeled by placing the resistance value at the ideal current source (the Norton equivalent circuit). However, this model is only useful when the current source operates in its compliance voltage.

Maps Current source

Implementations

Passive stream source

The simplest non-ideal current source consists of a voltage source in series with the resistor. The amount of current available from the source is given by the voltage ratio at the source of the voltage to the resistance of the resistor (Ohm's law; I = V/R). This current value will only be sent to the load with a zero voltage drop across the terminals (short circuit, unallocated capacitor, filled inductor, virtual ground circuit etc.) The current is sent to a load with a non-zero voltage across the terminal (resistor linear or nonlinear with limited resistance, loaded capacitors, unallocated inductors, voltage sources, etc.) will always be different. This is given by the ratio of the voltage drop across the resistor (the difference between the pulling voltage and the voltage across the load) for its durability. For an almost ideal current source, the value of the resistor must be very large but this implies that, for the specified current, the voltage source must be very large (within limits as resistance and voltage go to infinity, the current source would be ideal and the current would not be equal once on the voltage across the load). Thus, the efficiency is low (due to loss of power in the resistor) and it is usually not practical to build a 'good' current source in this way. However, it often happens that such a circuit will provide sufficient performance when a specified current and a small load resistance. For example, a 5V voltage source in a circuit with a 4.7 kilohm resistor will give approximately a constant current of 1 mA Ã, Â ± 5% to a load resistance in the range of 50 up to 450Ã, ohm.

Van de Graaff generator is an example of a high-voltage current source. It behaves as an almost constant current source due to its very high output voltage coupled with its very high output resistance and thus supplies the same microamperes at each output voltage up to hundreds of thousands of volts (or even tens of megavolts) for large sizes. laboratory version.

Current active source without negative feedback

In this circuit the output current is not monitored and controlled with negative feedback.

Nonlinear application is currently stable

They are implemented by an active electronic component (transistor) that has a stable non-linear output current characteristic when driven by a stable input quantity (current or voltage). This circuit behaves as a dynamic resistor that changes its current resistance to compensate for the current variation. For example, if the load increases its resistance, the transistor reduces its current output resistance (and otherwise ) to maintain the total constant 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 (eg, differential amplifiers) to produce currents that are slightly dependent on the voltage across the load.

General emitter configurations driven by a constant current source or voltage and common source (common cathode) driven by a constant voltage naturally behave as a current source (or sink) because the output impedance of this device is naturally high. The output part of a simple current mirror is an example of a widely used current source in an integrated circuit. The common base, general gate and general grid configuration can serve as a constant current source as well.

JFET can be created as a current source by binding its gate to its source. The current flowing current is I _DSS of the FET. These can be purchased with this connection already made and in this case the device is called current regulator diode or diode current constant or diode current limiting (CLD). Increase mode of N channel MOSFET can be used in the circuit listed below.

Implementation of the following voltage

Example: current source bootstrapped.

The application of voltage compensation

The passive current source of simple resistor is ideal only when the voltage above it is 0; So voltage compensation by applying parallel negative feedback may be considered to increase the source. Operational amplifiers with feedback effectively work to minimize the voltage across their inputs. This results in a virtual ground inverting input, with current flowing through feedback, or load, and a passive current source. The input voltage source, the resistor, and the op-amp are the ideal "current" source with the value, = V _IN / R . The voltage-to-current converter of the op-amp in Figure 3, the transimpedance amplifier and the op-amp's inverting amplifier are typical implementations of this idea.

A floating load is a serious disadvantage of this circuit solution.

Application of current compensation

A typical example is the Howland current source and the Deboo integrator derivative. In the last example (Figure 1), the Howland current source consists of the input voltage source, the positive resistor, R, the load (capacitor , C, acts as an impedance Z ) and an INIC negative impedance converter ( R ₁ = R ₂ = R ₃ = R and op-amp). The input voltage source and resistor R are incomplete current sources through the current, through the load (Fig. 3 in the source). INIC acts as a second current source which passes the "help" current, _-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 increases. However the Howland current source is not widely used because it requires four resistors to be perfectly matched, and the impedance goes down at high frequencies.

Ground load is the advantage of this circuit solution.

Current source with negative feedback

They are implemented as voltage follower with series negative input driven by a constant input voltage source (ie, negative feedback voltage stabilizer ). The voltage follower is encumbered by a constant resistor (current sensing) which acts as a simple current-to-voltage converter that is connected in the feedback loop. The external load of this current source is connected somewhere in the current path supplying the current sensing resistor but out of the feedback loop.

The voltage follower adjusts the output current I _OUT that flows through the load so as to make the voltage drop V _R = R on the current sensing resistor R equal to the constant input voltage V _IN . Thus the voltage stabilizer stabilizes a constant voltage drop through a constant resistor; so, the current is constant I _OUT = V _R / R = V _IN / R flows through the resistor and each through the load.

If the input voltage varies, this setting acts as a voltage-to-current converter (voltage controlled current source, VCCS); can be considered as an inverting (via negative feedback) current-to-voltage converter. Resistance R determines the transfer ratio (transconductance).

A current source implemented as a circuit with a negative feedback circuit has the disadvantage that the voltage drop across the current sensing resistor lowers the maximum voltage across the load (the fulfillment voltage ).

Simple transistor current source

Constant current diode

The simplest constant source or current source is formed from one component: JFET with its gate attached to the source. After the source-drain voltage reaches a certain minimum value, the JFET enters saturation where the current is approximately constant. This configuration is known as a constant current diode, because it behaves like a dual to a constant voltage diode (Zener diode) used in a simple voltage source.

Due to the large variability in the saturated current of the JFET, it is common to also include the source resistor (shown in adjacent drawings) which allows the current to be set to the desired value.

Zener current source diode

In this bipolar junction transistor (BJT) implementation (Figure 4) of the general idea above, a Zener voltage stabilizer (R1 and DZ1) drives the loaded (Q1) follower by the constant emitter resistor (R2) senses the load current. The external load (floating) of the current source is connected to the collector so that an almost equal current flows through it and the emitter resistor (they can be considered as connected in series). Transistor, Q1, adjusts the current output (collector) in order to keep the voltage drop across the constant emitter resistor, R2, almost equal to the constant voltage drop of the Zener diode, DZ1. As a result, the output current is almost constant even if the load and/or voltage resistance varies. Circuit operation is considered in detail below.

The Zener diode, when the reverse bias (as shown in the circuit) has a constant voltage drop above it irrespective of the current flowing through it. Thus, as long as the Zener current ( I _Z ) is above a certain level (called hold current), Zener diode voltage ( > V _Z ) will be constant. Resistor, R1, supplies the Zener current and base current (B) of the NPN transistor (Q1). The constant Zener voltage is applied across the base of Q1 and the emitter resistor, R2.

Tegangan di R2 ( V _R2 ) diberikan oleh V _Z - V _BE , di mana V _BE adalah basis- setetes emiter Q1. Arus emitor Q1 yang juga arus melalui R2 diberikan oleh

${\ displaystyle I _ {\ text {R2}} (= I _ {\ text {E}} = I _ {\ text {C}}) = {\ frac {V_ { \ text {R2}}} {R _ {\ text {R2}}}} = {\ frac {V_ {\ text {Z}} - V_ {\ text {BE}}} {R _ {\ text {R2}} }}.}  ÂÂ$

Since V _Z is a constant and V _BE also (roughly) constant for a certain temperature, then V _R2 is constant and hence I _E is also constant. Because of the action of the transistor, the emitter current, I _E , is very similar to the collector current, I < sub> C , of the transistor (which, in turn, is the current through the load). Thus, the load current is constant (ignoring the output resistance of the transistor due to the initial effect) and the circuit operating as a constant current source. As the temperature remains constant (or does not vary much), the load current will be independent of the supply voltage, R1 and the transistor gain. R2 allows the load current to be set at any desired value and calculated by

$Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â}$ _{         R                      R2                          =                                  Â    Â Â <Â> V                 <<<<<<<<<<<<<<<<<<<<<<<<<<<        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,       Â      Â     Â    Â Â <Â> V                         ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ, the        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,       Â                  Â    Â         I                             R2        ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ,          Â                                {\ displaystyle R _ {\ text {R2}} = {\ frac {V_ {\ text {Z}} - V_ {\ text {BE}} } {I _ {\ text {R2}}}}}  Â}

where V _BE is usually 0.65 V for silicon devices.

( Saya _R2 juga merupakan arus emitor dan diasumsikan sama dengan kolektor atau arus beban yang diperlukan, dengan ketentuan h _FE cukup besar). Perlawanan, R _R1 , pada resistor, R1, dihitung sebagai

${\ displaystyle R _ {\ text {R1}} = {\ frac {V_ {\ text {S}} - V_ {\ text {Z}}} {I_ {\ teks {Z}} K \ cdot I _ {\ text {B}}}}}  ÂÂ$

di mana K = 1,2 hingga 2 (sehingga R _R1 cukup rendah untuk pastikan cukup I _B ),

${\ displaystyle I _ {\ text {B}} = {\ frac {I_ {\ text {C}}} {h_ {FE, {\ text {min}}} }}}  ÂÂ$

and h _{FE, min} is the acceptable lowest current gain for a particular transistor type used.

LED current source

Zener diodes can be replaced by other diodes; for example, the LED1 light emitting diode as shown in Figure 5. The decrease in LED voltage ( V _D ) is now used to obtain the voltage constant and also has the added benefit of tracking (compensation) V _BE changes due to temperature. R _R2 is calculated as ${\ displaystyle R _ {\ text {R2}} = {\ frac {V_ {\ text {D}} - V_ {\ text {BE}} } {I _ {\ text {R2}}}}}$

dan R ₁ sebagai

${\ displaystyle R _ {\ text {R1}} = {\ frac {V_ {\ text {S}} - V_ {\ text {D}}} {I _ {\ teks {D}} K \ cdot I _ {\ text {B}}}}}  ÂÂ$ , di mana I _D adalah arus LED

Sumber arus transistor dengan kompensasi dioda

The temperature change will change the output current delivered by the circuit of Figure 4 because V _BE is temperature sensitive. The temperature dependence can be compensated using the circuit of Figure 6 which includes the standard diode, D, (from the same semiconductor material as the transistor) in series with the Zener diode as shown in the figure on the left. Decreased diodes ( V _D ) track V _BE change due to temperature and thus significantly neutralize temperature dependence of CCS.

Perlawanan R ₂ sekarang dihitung sebagai

${\ displaystyle R_ {2} = {\ frac {V_ {\ text {Z}} V_ {\ text {D}} - V_ {BE}} {I_ { \ text {R2}}}}}  ÂÂ$

Karena V _D = V _BE = 0,65 V ,

${\ displaystyle R_ {2} = {\ frac {V _ {\ text {Z}}} {I _ {\ text {R2}}}}}  ÂÂ$

(In practice, V _D is never exactly the same as V _BE and therefore only suppress changes in V _BE instead of canceling them.)

R ₁ dihitung sebagai

${\ displaystyle R_ {1} = {\ frac {V_ {\ text {S}} - V_ {\ text {Z}} - V_ {\ text {D}} } {I _ {\ text {Z}} K \ cdot I _ {\ text {B}}}}}  ÂÂ$

(decrease in the forward voltage of the compensation diode, V _D , appear in the equation and usually 0.65 V for the silicon device.)

This method is most effective for Zener diodes rated at 5.6 V or more. For diode interference of less than 5.6 V, compensation diodes are usually not required because the damage mechanism is independent of temperature as in the breakdown diode above this voltage.

Current mirror with emitter degeneration

Series negative feedback is also used in the mirror of two current transistors with emitter degeneration. Negative feedback is a basic feature in some current mirrors using multiple transistors, such as Widlar current sources and Wilson current sources.

Constant current source with thermal compensation

One limitation with the circuits in Figures 5 and 6 is that thermal compensation is not perfect. In bipolar transistors, since the connection temperature increases the drop V <> sub (decrease the voltage from base to emitter) decreases. In the previous two circuits, a decrease of V <> sub will cause an increase in voltage across the emitter resistor, which in turn will cause an increase in the collector current being pulled through the load. The end result is that the amount of 'constant' current given is at least somewhat dependent on temperature. This effect is reduced to a large extent, but not completely, by the appropriate voltage drops for diodes, D1, in FIG. 6, and LED, LED1, in FIG. 5. If the power dissipation on the active device of CCS is not minor emitter degeneration and/or not enough to use, this can be a problem that is not trivial.

Imagine in Figure 5, when turned on, that the LED has 1 V on it that drives the base of the transistor. At room temperature there is about 0.6 V drop at the _to junction and hence 0.4V at the emitter resistor, gives the estimated collector (load ) current 0.4 â € <â € e amp. Now imagine that the power dissipation in the transistor causes it to heat up. This causes drop V _to (which is 0.6Ã, V at room temperature) to descend to, say, 0.2Ã, V. Now the voltage across the emitter resistor is 0.8 V, twice what it was before heating. This means that the collector current (load) is now twice that of the design value! This is an extreme example of course, but serves to illustrate this problem.

The circuit to the left overcomes the thermal problem (see also, limiting the current). To see how the circuit works, assume the voltage was just applied to V. The current goes through R1 to base Q1, turns it on and causes the current to start flowing through the load to the collector Q1. This same load current then flows out of the emitter Q1 and consequently through R _sense to ground. When this current goes through R _sense to ground enough to cause the same voltage drop as V _to drop of Q2, Q2 start to light up. When Q2 is lit, it draws more current through its collector resistor R1, which diverts some of the injected currents at the base of Q1, causing Q1 to conduct less current through the load. This creates a negative feedback loop in the circuit, which keeps the voltage at emitter Q1 almost exactly equal to the V _be Q2. Since Q2 discharges very little power compared to Q1 (since all load currents pass Q1 instead of Q2), Q2 will not heat up a significant amount and reference (voltage regulation) across R _sense will remain stable at ~ 0.6 V, or one drop diode above ground, regardless of thermal change in V _to drop of Q1. This circuit is still sensitive to changes in ambient temperature where the device operates as the BE voltage drop in Q2 is slightly different from the temperature.

Current source op-amp

The simple transistor current source of FIG. 4 can be increased by inserting the emitter base-base of the transistor in the feedback loop of the op-amp (Figure 7). Now the op-amp increases its output voltage to compensate for the decrease of V _BE . This circuit is actually a non-inverting buffer amplifier driven by a constant input voltage. It keeps up this constant voltage across the sense resistor constant. Consequently, the current flowing through the load is also constant; it's 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 cases it is floating as in all the above circuits. The transistor is not required if the required current does not exceed the procurement capability of the op-amp. The article in the current mirror discusses another example of what is called a gain-boosted current mirror.

Current source voltage regulator

General negative feedback settings can be implemented by IC voltage regulator (LM317 voltage regulator in Figure 8). As with the naked emitter follower and the op-amp follower right above, it continues to rise and fall a constant voltage (1.25 V) across a constant resistor (1.25?); so, the constant current (1 A) flows through the resistor and the load. The LED lights up when the voltage across the load exceeds 1.8 V (the indicator circuit introduces some errors). A grounded load is an important advantage of this solution.

Curpistor Tubes

The nitrogen-containing glass tube with two electrodes and the number of Becquerel (fission per second) calibrated from ²²⁶ Ra offers a constant load of carriers per second for conduction, which determines the maximum current of the tube to pass through the voltage range from 25 to 500 Â ° V.

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Comparison of current and voltage sources

Most electrical energy sources (main electricity, batteries, etc.) are best modeled as a voltage source. These sources provide a constant voltage, which means that as long as the current taken from the source is within the source capability, the output voltage remains constant. The ideal voltage source does not provide energy when it is loaded by an open circuit (ie, infinite impedance), but approaches infinite power and current when load resistance approaches zero (short circuit). Such a theoretical device would have a zero ohm output impedance in series with its source. Real-world voltage sources have very low output impedances, but not zero: often less than 1 ohm.

In contrast, the current source provides a constant current, as long as the load connected to the source terminals has a low enough impedance. The ideal current source will not provide energy for short circuits and approximate infinite energy and voltage when load resistance approaches infinity (open circuit). The ideal source currently has an unlimited output impedance in parallel with the source. The real-world sources currently have very high output impedances, but are limited. In the case of a transistor current source, the impedance of several megohms (in DC) is typical.

The ideal source currently can not connect to the ideal open circuit because this will create a paradox running a constant current, not zero (from current source) via elements with specified zero current ( open circuit). In addition, the current source should not be connected to other current sources if the current is different but these settings are often used (for example, in reinforcing stages with dynamic loads, CMOS circuits, etc.)

Similarly, the ideal voltage source can not be connected to an ideal short circuit (R = 0), since this will produce a similar paradox of finite non-zero stress across elements specified zero voltage (short circuit). Also, the voltage source should not be connected to another voltage source if the voltage is different but again this arrangement is often used (for example, in the general base stage and differential amplification).

Different voltage sources, currents and voltages can be connected to each other without problems, and this technique is widely used in circuits (for example, in cascode circuits, differential amplifier stages with common emitter current sources, etc.)

Since there is no ideal source of the various existing ones (all real-world examples have finite and non-zero source impedance), any current source can be regarded as a voltage source with the same impedance source and vice versa. These concepts are handled by Norton's theorem and ThÃÆ' Â © venin.

Charging capacitor by constant current source and different voltage source. Linearity is maintained for charging constant current source capacitor with time, while charging capacitor voltage source is exponential with time. The special property of this constant current source helps for precise signal conditioning with almost zero reflections of the load.

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See also

• Constant current
• Current limitations
• Current circle
• Current mirror
• Current sources and sinks
• Fontana Bridge, compensation current source
• Iron-hydrogen resistance
• Groupable reactors
• Current-to-current converter
• The welding power supply, a tool used for arc welding, many of which are designed as constant current devices.
• Current widening source

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References

src: www.allaboutcircuits.com

Further reading

• "Current Source & Reference Voltage" Linden T. Harrison; Publ. Elsevier-Newnes 2005; 608 pages; ISBNÃ, 0-7506-7752-X

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External links

• Current Source and Current Mirror
• FET Current Constant Sources/Limiters - Vishay
• JFET Current Source and pSpice Simulation
• Using Current Resources/Sinks/Mirrors In Audio
• Differential Amplifier and Current Source
• Passive Voltage-to-Pass Converter
• The ideal simulation, a constant current source initiated by an ideal AM receiver whose current can only be enlarged while under low load conditions.

Source of the article : Wikipedia