|
|
| Line 1: |
Line 1: |
| [[Image:Ohms law current source.svg|thumb|150px|Figure 1: An ideal current source, ''I'', driving a resistor, ''R'', and creating a voltage ''V'']]
| | In case you are having trouble seeing a while you are participating in it, try adjusting this particular brightness environment. The make the display look clear, enhancing your since the expertise. And allow us to face it, you will not achieve any kind amongst success if you just cannot see what you're doing, so make the play meet your needs.<br><br>In the instance as a parent you might be concerned with movie game content, control what online mods are put globe sport. These downloadable mods are usually caused by players, perhaps not unquestionably the gaming businesses, therefore there is no ranking system. True thought was a remarkably un-risky game can revert a lot worse with any of these mods.<br><br>Okazaki, japan tartan draws concepts through your country's curiosity about cherry blossom and will involve pink, white, green and brown lightly colours. clash of clans cheats. Style is called Sakura, okazaki, japan for cherry blossom.<br><br>Truly possible, but the bulk of absence one daily schedule would abatement by sixty one. 5% after 260 treasures to hundreds of gems. Or, as long as you capital to build up the 1 morning bulk at 260 gems, the band would require to acceleration added steeply and also 1 wedding anniversary would turn into added expensive.<br><br>The nfl season is here in addition going strong, and along the lines of many fans we in total for Sunday afternoon when the games begin. If you have tried and liked Soul Caliber, you will love this particular game. The new best is the Microchip Cell which will arbitrarily fill in some sqs. Defeating players similar to that of that by any involves necessary can be currently the reason that pushes themselves to use Words with Friends Cheat. Your app requires you to be answer 40 questions considering varying degrees of difficulty.<br><br>Your war abject is agnate in your approved village, except that your gua abject will not get resources. Barrio within your warfare abject will not be anon improved or rearranged, as it of it's own mimics this adjustment to accomplished completed advancement programs of your apple while having alertness day. Struggle bases additionally never charge to take their gear rearmed, defenses reloaded or characters healed, as these kinds of products are consistently ready. The association alcazar around the your war abject penalty be abounding alone forward the one in your company whole village.<br><br>So your village grows, anyone could have to explore uncharted areas for Gold and Woodworking which are the few of key resources you might expect to require in start of the online ( addititionally there often is Stone resource, that your business discover later [http://imageshack.us/photos/inside+i inside i] would say the game ). If you loved this information and you would like to get more information regarding [http://circuspartypanama.com gem hack clash of clans] kindly visit the page. Due to your exploration, you will likely expect to stumble located on many islands whereby the villages happen to stay held captive under BlackGuard slavery and you profit from free Gold ammenities if they are vacant. |
| | |
| 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 [[duality (electrical circuits)|dual]] of a [[voltage source]]. The term constant-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 resistor 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.
| |
| | |
| ==Background==
| |
| {{float_begin|side=right}}
| |
| |- align="center"
| |
| |style="padding: 1em 2em 0;"| [[Image:Voltage Source.svg]]
| |
| |style="padding: 1em 2em 0;"| [[Image:Current Source.svg]]
| |
| |- align="center"
| |
| | Voltage source
| |
| | Current Source
| |
| |- align="center"
| |
| |style="padding: 1em 2em 0;"| [[Image:Voltage Source (Controlled).svg]]
| |
| |style="padding: 1em 2em 0;"| [[Image:Current Source (Controlled).svg]]
| |
| |- align="center"
| |
| | Controlled Voltage Source
| |
| | Controlled Current Source
| |
| |- align="center"
| |
| |style="padding: 1em 2em 0;"| [[Image:Battery symbol.svg]]
| |
| |style="padding: 1em 2em 0;"| [[Image:Cell.svg]]
| |
| |- align="center"
| |
| | [[Battery (electricity)|Battery]] of cells
| |
| | Single cell
| |
| {{float_end|caption=Figure 2: Source symbols}}
| |
| | |
| 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. | |
| | |
| The [[internal resistance]] of an ideal current source is infinite. An independent current source with zero current is identical to an ideal [[open circuit (disambiguation)|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|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 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.
| |
| | |
| ==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 ([[Ohm's law]]; I = V/R). 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 (disambiguation)|megavolt]]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 feedback]].
| |
| | |
| ====Current-stable nonlinear implementation====
| |
| 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 ''[[Vice-versa|vice versa]]'') to keep up a constant total resistance in the circuit.
| |
| | |
| 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#Long-tailed pair|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 I<sub>DSS</sub> of the FET. These can be purchased with this connection already made and in this case the devices are called [[constant current diode|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.
| |
| | |
| ====Following voltage implementation====
| |
| An example: [[Bootstrapping (electronics)|bootstrapped]] current source.<ref>Widlar [http://www.national.com/an/AN/AN-29.pdf bilateral current source] </ref>
| |
| | |
| ====Voltage compensation implementation====
| |
| [[Image:V-to-i op-amp current source 1000.jpg|right|thumb|350px|Figure 3: In an op-amp ''voltage-controlled current source'' the op-amp compensates the voltage drop across the load by adding the same voltage to the exciting input voltage.]]
| |
| | |
| The simple [[#Passive current source|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 ''I<sub>OUT</sub>'' = ''V<sub>IN</sub>'' /''R''. The op-amp voltage-to-current converter in Figure 3, a [[current-to-voltage converter#Op-amp implementation|transimpedance amplifier]] and an [[operational amplifier applications#Inverting amplifier|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<ref>{{cite web
| |
| |title = AN-1515 A Comprehensive Study of the Howland Current Pump
| |
| |url = http://www.ti.com/lit/an/snoa474a/snoa474a.pdf
| |
| |type = PDF
| |
| |publisher = Texas Instruments, Inc
| |
| |year = 2013 (2008)
| |
| }}</ref> and its derivative Deboo integrator.<ref>[http://www.maxim-ic.com/app-notes/index.mvp/id/1155 Consider the "Deboo" Single-Supply Integrator]</ref> In the last example (see Fig. 1 there), the Howland current source consists of an input voltage source V<sub>IN</sub>, a positive resistor R, a load (the capacitor C acting as impedance ''Z'') and a negative impedance converter INIC (R<sub>1</sub> = R<sub>2</sub> = R<sub>3</sub> = R and the op-amp). The input voltage source and the resistor R constitute an imperfect current source passing current ''I<sub>R</sub>'' through the load (see Fig. 3 in the source). The INIC acts as a second current source passing "helping" current ''I<sub>-R</sub>'' 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.<ref name="Horowitz2">{{cite book
| |
| | last = Horowitz
| |
| | first = Paul
| |
| | coauthors = Winfield Hill
| |
| | title = The Art of Electronics, 2nd Ed.
| |
| | publisher = Cambridge University Press
| |
| | date = 1989
| |
| | location = UK
| |
| | pages = 182
| |
| | url = http://books.google.com/books?id=bkOMDgwFA28C&pg=PA182&lpg=PA182&dq=horowitz+%22Howland+current+source
| |
| | doi =
| |
| | id =
| |
| | isbn = 0521370957}}</ref>
| |
| | |
| 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 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 I<sub>OUT</sub> flowing through the load so that to make the voltage drop V<sub>R</sub> = I<sub>OUT</sub>.R across the current sensing resistor R equal to the constant input voltage V<sub>IN</sub>. Thus the voltage stabilizer keeps up a constant voltage drop across a constant resistor; so, a constant current I<sub>OUT</sub> = V<sub>R</sub>/R = V<sub>IN</sub>/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'').
| |
| | |
| ====Simple transistor current sources====
| |
| =====Constant current diode=====
| |
| [[Image:Current limiting diode schematic.png|thumb|Circuit]]
| |
| 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 ([[Zener 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 (shown in the image to the right) which allows the current to be tuned down to a desired value.
| |
| | |
| =====Zener diode current source=====
| |
| [[Image:const cur src 111.svg|thumb|Figure 4: Typical BJT constant current source with negative feedback]]
| |
| | |
| In this [[bipolar junction transistor]] (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 (electricity)|current]] flowing through it. Thus, as long as the Zener current (''I''<sub>Z</sub>) is above a certain level (called holding current), the voltage across the Zener [[diode]] (''V''<sub>Z</sub>) will be constant. [[Resistor]] R1 supplies the Zener current and the base current (''I''<sub>B</sub>) of NPN [[transistor]] (Q1). The constant Zener voltage is applied across the base of Q1 and emitter resistor R2.
| |
| | |
| Voltage across R2 (''V''<sub>R2</sub>) is given by ''V''<sub>Z</sub> - ''V''<sub>BE</sub>, where ''V''<sub>BE</sub> is the base-emitter drop of Q1.
| |
| The emitter current of Q1 which is also the current through R2 is given by | |
| :<math>I_{R2} (= I_{E}) = \frac{V_{R2}}{R2} = \frac{V_{Z} - V_{BE}}{R2}.</math>
| |
| Since ''V''<sub>Z</sub> is constant and ''V''<sub>BE</sub> is also (approximately) constant for a given temperature, it follows that ''V''<sub>R2</sub> is constant and hence ''I''<sub>E</sub> is also constant. Due to [[transistor]] action, emitter current ''I''<sub>E</sub> is very nearly equal to the collector current ''I''<sub>C</sub> 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
| |
| :<math>R2 = \frac{V_{Z} - V_{BE}}{I_{R2}}</math>
| |
| or
| |
| :<math>R2 = \frac{V_{Z} - 0.65}{I_{R2}}</math>,
| |
| since ''V''<sub>BE</sub> is typically 0.65 V for a silicon device.<ref>The value for ''V<sub>BE</sub>'' varies logarithmically with current level: for more detail see [[diode modelling]].</ref>
| |
| | |
| (''I''<sub>R2</sub> is also the emitter current and is assumed to be the same as the collector or required load current, provided ''h''<sub>FE</sub> is sufficiently large). Resistance ''R''<sub>1</sub> at resistor R1 is calculated as
| |
| :<math>R_1 = \frac{V_{S} - V_{Z}}{I_{Z} + K \cdot I_{B}}</math>
| |
| where ''K'' = 1.2 to 2 (so that ''R''<sub>1</sub> is low enough to ensure adequate ''I''<sub>B</sub>),
| |
| :<math>I_{B} = \frac{I_{C} (= I_{E} = I_{R2})}{h_{FE(min)}}</math>
| |
| and ''h''<sub>FE(min)</sub> is the lowest acceptable current gain for the particular transistor type being used.
| |
| | |
| =====LED current source=====
| |
| [[Image:const cur src 113.svg|thumb|Figure 5: Typical constant current source (CCS) using LED instead of Zener diode]]
| |
| | |
| 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 (''V''<sub>D</sub>) is now used to derive the constant voltage and also has the additional advantage of tracking (compensating) ''V''<sub>BE</sub> changes due to temperature. ''R''<sub>2</sub> is calculated as
| |
| | |
| <math>R_2 = \frac {V_D - V_{BE}}{I_{R2}}</math>
| |
| | |
| and ''R''<sub>1</sub> as
| |
| | |
| <math>R_1 = \frac{V_{S} - V_D}{I_{D} + K \cdot I_{B}}</math>, where ''I''<sub>D</sub> is the LED current.
| |
| | |
| =====Transistor current source with diode compensation=====
| |
| [[Image:const cur src 112.svg|thumb|Figure 6: Typical constant current source (CCS) with diode compensation]]
| |
| | |
| Temperature changes will change the output current delivered by the circuit of Figure 4 because V<sub>BE</sub> 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 (''V''<sub>D</sub>) tracks the ''V''<sub>BE</sub> changes due to temperature and thus significantly counteracts temperature dependence of the CCS.
| |
| | |
| Resistance ''R''<sub>2</sub> is now calculated as
| |
| | |
| <math>R_2 = \frac{V_{Z} + V_{D} - V_{BE}}{I_{R2}}</math>
| |
| | |
| Since ''V''<sub>D</sub> = ''V''<sub>BE</sub> = 0.65 V,<ref name="Seeabove">See above note on logarithmic current dependence.</ref>
| |
| | |
| <math>R_2 = \frac{V_{Z}}{I_{R2}}</math>
| |
| | |
| (In practice ''V''<sub>D</sub> is never exactly equal to ''V''<sub>BE</sub> and hence it only suppresses the change in ''V''<sub>BE</sub> rather than nulling it out.)
| |
| | |
| ''R''<sub>1</sub> is calculated as
| |
| | |
| <math>R_1 = \frac{V_{S} - V_{Z} - V_{D}}{I_{Z} + K \cdot I_{B}}</math> (the compensating diode's forward voltage drop ''V''<sub>D</sub> appears in the equation and is typically 0.65 V for silicon devices.<ref name="Seeabove" />)
| |
| | |
| 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 [[electrical breakdown|breakdown]] 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-port_network#Example:_bipolar_current_mirror_with_emitter_degeneration|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]].
| |
| | |
| =====Constant current source with thermal compensation=====
| |
| One limitation with the circuits in figures five and six is that the thermal compensation is imperfect. In bipolar transistors, as the junction temperature increases the Vbe drop (voltage drop from base to emitter) decreases. In the two previous circuits, a decrease in Vbe will cause an increase in voltage across the emitter resistor, which in turn will cause an increase in collector current drawn through the load. The end result is that the amount of 'constant' current supplied is at least somewhat dependent on temperature. This effect is mitigated to a large extent, but not completely, by corresponding voltage drops for the diode D1 in figure 6, and the LED, LED1 in figure 5. If the power dissipation in the active device of the CCS is not small and/or insufficient emitter degeneration is used, this can become a non-trivial issue.
| |
| | |
| Imagine in figure five, at power up, that the LED has 1V across it driving the base of the transistor. At room temperature there is about 0.6V drop across the Vbe junction and hence 0.4V across the emitter resistor, giving an approximate collector (load) current of 0.4/Re amps. Now imagine that the power dissipation in the transistor causes it to heat up. This causes the Vbe drop (which was 0.6V at room temperature) to drop to, say, 0.2V. Now the voltage across the emitter resistor is 0.8V, 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.
| |
| | |
| ----------- V+
| |
| | |
| |
| R LOAD
| |
| | |
| |
| | C
| |
| --------B Q1
| |
| | E
| |
| C |
| |
| Q2 B--------
| |
| E |
| |
| | Re
| |
| | |
| |
| ----------- GND
| |
| | |
| The circuit above overcomes the thermal problem. (See Also: [[Current limiting]], which has the same circuit except with the load on the low-side.) To see how the circuit works, assume Q1 and Q2 are NPN, and voltage has just been applied at V+. Current runs through R 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 Re to ground. When this current through Re to ground is sufficient to cause a voltage drop that is equal to the Vbe drop of Q2, Q2 begins to turn on. As Q2 turns on it pulls more current through its collector resistor R, which lowers the voltage at 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 Vbe 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 Re will remain rock steady at ~0.6V, or one diode drop above ground, regardless of the thermal changes in the Vbe 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.
| |
| | |
| ====Op-amp current sources====
| |
| [[Image:Op-amp current source with pass transistor.svg|thumb|Figure 7: Typical op-amp current source.]]
| |
| | |
| 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 V<sub>BE</sub>. 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#Feedback_assisted_current_mirror|current mirror]] discusses another example of these so-called ''gain-boosted'' current mirrors.
| |
| | |
| [[Image:LM317 1A ConstCurrent.svg|thumb|350px|right|Figure 8: Constant current source using the [[LM317]] voltage regulator]]
| |
| | |
| ====Voltage regulator current sources====
| |
| The [[#Current sources with series negative feedback|general negative feedback arrangement]] can be implemented by an IC voltage regulator ([[LM317|LM317 voltage regulator]] on Figure 8). As with the bare [[#Simple transistor current sources|emitter follower]] and the precise [[#...with series negative feedback...|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..
| |
| | |
| ====Curristor tubes====
| |
| Nitrogen-filled glass tubes with two electrodes and a calibrated [[Becquerel]] (fissions per second) amount of [[Isotopes of radium|<sup>226</sup>Ra]] offer a constant number of charge carriers per second for conduction, which determines the maximum current the tube can pass over a voltage range from 25 to 500 V.<ref>{{cite web |url=http://tubedata.milbert.com/sheets/127/c/CH1027.pdf |title= Tung-Sol: ''Curristor, minute current regulator'' data sheet |format=PDF |accessdate=26 May 2013}}</ref>
| |
| | |
| ==Current and voltage source comparison==
| |
| Most sources of electrical energy ([[mains electricity]], a [[battery (electricity)|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 (disambiguation)|open circuit]] (i.e. an infinite [[electrical impedance|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 (unit)|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 [[Infinity|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 [[megohm]]s (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 nonzero 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|cascode circuits]], [[differential amplifier#Long-tailed pair|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 theorem|Norton's]] and [[Thévenin's theorem]]s.
| |
| | |
| ==See also==
| |
| *[[Constant current]]
| |
| *[[Current limiting]]
| |
| *[[Current loop]]
| |
| *[[Current mirror]]
| |
| *[[Current sources and sinks]]
| |
| *[[Fontana bridge]], a compensated current source
| |
| *[[Iron-hydrogen resistor]]
| |
| *[[Voltage-to-current converter]]
| |
| *[[Welding power supply]], a device used for [[arc welding]], many of which are designed as constant current devices.
| |
| *[[Widlar current source]]
| |
| | |
| ==References and notes==
| |
| {{reflist}}
| |
| | |
| ==Further reading==
| |
| *"Current Sources & Voltage References" Linden T. Harrison; Publ. Elsevier-Newnes 2005; 608-pages; ISBN 0-7506-7752-X
| |
| | |
| ==External links==
| |
| *[http://www.tpub.com/content/neets/14179/css/14179_214.htm ''Current Regulators''; Electrical Engineering Training Series]
| |
| *[http://www.4qdtec.com/csm.html 4QD-TEC: Electronics Circuits Reference Archive]
| |
| *[http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part1/page3.html Differential amplifiers and current sources]
| |
| *[http://sound.westhost.com/ism.htm Article about current sources on ESP]
| |
| | |
| {{DEFAULTSORT:Current Source}}
| |
| [[Category:Analog circuits]]
| |
| [[Category:Electric current]]
| |
| [[Category:Electrical power control]]
| |