2192:
889:(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.
133:
834:
2256:
239:
232:
2284:
392:
at all on the voltage across the load). Thus, efficiency is low (due to power loss in the resistor) and it is usually impractical to construct a 'good' current source this way. Nonetheless, it is often the case that such a circuit will provide adequate performance when the specified current and load resistance are small. For example, a 5 V voltage source in series with a 4.7 kΩ resistor will provide an
1739:
1507:
866:
2342:
657:
36:
215:
208:
256:
2191:
340:. 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.
263:
640:, 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.
2187:
drop (which was 0.6 V at room temperature) to drop to, say, 0.2 V. Now the voltage across the emitter resistor is 0.8 V, twice what it was before the warmup. This means that the collector (load) current is now twice the design value! This is an extreme example of course, but serves to
2158:
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,
391:
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
2316:
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
547:
is ideal only when the voltage across it is zero; so voltage compensation by applying parallel negative feedback might be considered to improve the source. Operational amplifiers with feedback effectively work to minimise the voltage across their inputs. This results in making the inverting input a
387:
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
407:
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
292:
generates a current that is independent of the voltage changes across it. An ideal current source is a mathematical model, which real devices can approach very closely. If the current through an ideal current source can be specified independently of any other variable in a circuit, it is called an
2203:). To see how the circuit works, assume the voltage has just been applied at V+. Current runs through R1 to the base of Q1, turning it on and causing current to begin to flow through the load into the collector of Q1. This same load current then flows out of Q1's emitter and consequently through
2224:
drop of Q2, Q2 begins to turn on. As Q2 turns on it pulls more current through its collector resistor, R1, which diverts some of the injected current in the base of Q1, causing Q1 to conduct less current through the load. This creates a negative feedback loop within the circuit, which keeps the
1184:) 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
2271:
drop. 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
432:
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 their present resistance to compensate current variations. For
1117:
343:
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.
2272:
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
2624:
to voltage or current sources and may work when connected in these manners that are disallowed for actual current or voltage sources. Also, just like voltage sources may be connected in series to add their voltages, current sources may be connected in parallel to add their currents.
2579:
Conversely, a current source provides a constant current, as long as the impedance of the load is sufficiently lower than the current source's parallel impedance (which is preferably very high and ideally infinite). In the case of transistor current sources, impedances of a few
2159:
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.
1753:
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
2619:
Just like how voltage sources should not be connected in parallel to another voltage source with different voltages, a current source also should not be connected in series to another current source. Note, some circuits use elements that are similar
2094:
1720:
1389:
2263:
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 to compensate for the
2111:
Note that this only works well if DZ1 is a reference diode or another stable voltage source. Together with 'normal' Zener diodes especially with lower Zener voltages (<5V) the diode might even worsen overall temperature dependency.
328:, the current source manages the voltage in such a way as to keep the current constant; so in an ideal current source the voltage across the source approaches infinity as the load resistance approaches infinity (an open circuit).
2602:), and hence the power drawn would also approach infinity. The current of a real current source connected to an open circuit would instead flow through the current source's internal parallel impedance (and be wasted as heat).
1615:
1254:
1870:
2584:(at low frequencies) are typical. Because power is current squared times resistance, as a load resistance connected to a current source approaches zero (a short circuit), the current and thus power both approach zero.
2232:
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
991:
1484:
388:
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.
331:
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
1947:
845:
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
810:
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
857:
Due to the large variability in saturation current of JFETs, it is common to also include a source resistor (shown in the adjacent image) which allows the current to be tuned down to a desired value.
2480:
552:, with the current running through the feedback, or load, and the passive current source. The input voltage source, the resistor, and the op-amp constitutes an "ideal" current source with value,
2641:, for instance, uses this linear behavior to measure an unknown voltage by measuring the amount of time it takes a current source to charge a capacitor to that voltage. A voltage source instead
725:
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.
803:(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 (
2594:
open circuit would create the paradox of running a constant, non-zero current (from the current source) through an element with a defined zero current (the open circuit). As the
591:
A typical example are
Howland current source and its derivative Deboo integrator. In the last example (Fig. 1), the Howland current source consists of an input voltage source,
1996:
2247:
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.
473:) 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
2512:
2407:
2375:
2333:
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.
2162:
Imagine in Figure 5, at power up, that the LED has 1 V across it driving the base of the transistor. At room temperature there is about 0.6 V drop across the
1635:
1304:
1180:, 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
2144:
One limitation with the circuits in
Figures 5 and 6 is that the thermal compensation is imperfect. In bipolar transistors, as the junction temperature increases the
2534:) are better modeled using current sources. Sometimes it is easier to view a current source as a voltage source and vice versa (see conversion in Figure 9) using
297:
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
1551:
1190:
1790:
1112:{\displaystyle I_{\text{R2}}(=I_{\text{E}}=I_{\text{C}})={\frac {V_{\text{R2}}}{R_{\text{2}}}}={\frac {V_{\text{Z}}-V_{\text{BE}}}{R_{\text{2}}}}.}
1424:
523:(metal–oxide–semiconductor field-effect transistor) could be used instead of a JFET in the circuits listed below for similar functionality.
182:
is sometimes used for sources fed from a negative voltage supply. Figure 1 shows the schematic symbol for an ideal current source driving a
577:
2950:
2904:
2878:
704:
119:
1898:
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of an ideal current source approaches infinity (an open circuit), the voltage across the load would approach infinity (because
678:
53:
2793:
682:
316:. The voltage across an ideal current source is completely determined by the circuit it is connected to. When connected to a
100:
57:
717:
They are implemented as a voltage follower with series negative feedback driven by a constant input voltage source (i.e., a
2616:
would result a similar paradox of finite non-zero voltage across an element with defined zero voltage (the short circuit).
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72:
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313:
171:
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800:
722:
573:
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as long as the current drawn from the source is within the source's capabilities. An ideal voltage source loaded by an
874:
532:
79:
2732:
667:
622:
and the op-amp). The input voltage source and the resistor R constitute an imperfect current source passing current,
2637:, an ideal constant current source charges a capacitor linearly with time, regardless of any series resistance. The
833:
2255:
765:. Thus the voltage stabilizer keeps up a constant voltage drop across a constant resistor; so, a constant current
686:
671:
132:
46:
2945:
2674:
409:
312:
of an ideal current source is infinite. An independent current source with zero current is identical to an ideal
86:
2940:
2916:
2539:
1527:) is now used to derive the constant voltage and also has the additional advantage of tracking (compensating)
1280:
is also the emitter current and is assumed to be the same as the collector or required load current, provided
2171:
junction and hence 0.4 V across the emitter resistor, giving an approximate collector (load) current of
631:
through the load (Fig. 3 in the source). The INIC acts as a second current source passing "helping" current,
509:
of the FET. These can be purchased with this connection already made and in this case the devices are called
2240:
will remain steady at ≈0.6 V, or one diode drop above ground, regardless of the thermal changes in the
404:
985:
is the base-emitter drop of Q1. The emitter current of Q1 which is also the current through R2 is given by
433:
example, if the load increases its resistance, the transistor decreases its present output resistance (and
68:
2685:
847:
828:
510:
455:
2925:
2710:
2531:
2523:
2133:
2129:
2089:{\displaystyle R_{1}={\frac {V_{\text{S}}-V_{\text{Z}}-V_{\text{D}}}{I_{\text{Z}}+K\cdot I_{\text{B}}}}}
901:
500:
can be made to act as a current source by tying its gate to its source. The current then flowing is the
270:
2873:"Current Sources & Voltage References" Linden T. Harrison; Publ. Elsevier-Newnes 2005; 608-pages;
2700:
2554:
2178:
amps. Now imagine that the power dissipation in the transistor causes it to heat up. This causes the
1515:
2535:
2414:
2410:
478:
309:
2485:
2380:
2348:
1715:{\displaystyle R_{\text{1}}={\frac {V_{\text{S}}-V_{\text{D}}}{I_{\text{D}}+K\cdot I_{\text{B}}}}}
1384:{\displaystyle R_{\text{1}}={\frac {V_{\text{S}}-V_{\text{Z}}}{I_{\text{Z}}+K\cdot I_{\text{B}}}}}
1772:
changes due to temperature and thus significantly counteracts temperature dependence of the CCS.
451:
443:
156:
2151:
drop (voltage drop from base to emitter) decreases. In the two previous circuits, a decrease in
1746:
Temperature changes will change the output current delivered by the circuit of Figure 4 because
2874:
2789:
2785:
2778:
2690:
2519:
721:). The voltage follower is loaded by a constant (current sensing) resistor acting as a simple
421:
361:
in series with a resistor. The amount of current available from such a source is given by the
238:
231:
2659:
2654:
2573:
2569:
2200:
2121:
939:(Q1). The constant Zener voltage is applied across the base of Q1 and emitter resistor, R2.
804:
514:
160:
93:
2920:
2908:
2818:
2736:
2645:, because the charging current from the voltage source decreases exponentially with time.
2595:
2558:
325:
194:
delivers a current which is proportional to some other voltage or current in the circuit.
183:
17:
2901:
2565:), the current (and thus power) approach infinity. Such a theoretical device has a zero
2679:
2669:
2527:
2273:
2125:
549:
474:
470:
462:
358:
321:
175:
2843:
2764:
2283:
2934:
2638:
2613:
2562:
1498:
is the lowest acceptable current gain for the particular transistor type being used.
466:
317:
2664:
1181:
926:) will be constant. Resistor, R1, supplies the Zener current and the base current (
897:
384:
2635:
the charge on a capacitor is equal to the integral of current with respect to time
2599:
2341:
913:) is above a certain level (called holding current), the voltage across the Zener
366:
2748:
420:
In these circuits the output current is not monitored and controlled by means of
2704:
1738:
1610:{\displaystyle R_{\text{2}}={\frac {V_{\text{D}}-V_{\text{BE}}}{I_{\text{R2}}}}}
1510:
Figure 5: Typical constant current source (CCS) using LED instead of Zener diode
1506:
1249:{\displaystyle R_{\text{2}}={\frac {V_{\text{Z}}-V_{\text{BE}}}{I_{\text{R2}}}}}
893:
865:
851:
656:
490:
486:
482:
35:
1865:{\displaystyle R_{2}={\frac {V_{\text{Z}}+V_{\text{D}}-V_{BE}}{I_{\text{R2}}}}}
513:
or constant current diodes or current limiting diodes (CLD). Alternatively, an
2642:
2572:
in series with the source. Real-world voltage sources instead have a non-zero
2566:
2330:
1159:
936:
517:
458:) to generate a current that depends slightly on the voltage across the load.
435:
2895:
2108:, appears in the equation and is typically 0.65 V for silicon devices.)
841:
The simplest constant-current source or sink is formed from one component: a
27:
Electronic component delivering stable electric current regardless of voltage
2634:
2326:
365:
of the voltage across the voltage source to the resistance of the resistor (
255:
214:
207:
2729:
1140:
is also (approximately) constant for a given temperature, it follows that
756:
across the current sensing resistor R equal to the constant input voltage
447:
1479:{\displaystyle I_{\text{B}}={\frac {I_{\text{C}}}{h_{FE,{\text{min}}}}}}
600:, a positive resistor, R, a load (the capacitor, C, acting as impedance
2546:
1742:
Figure 6: Typical constant current source (CCS) with diode compensation
164:
583:
The floating load is a serious disadvantage of this circuit solution.
2581:
2217:
to ground is sufficient to cause a voltage drop that is equal to the
520:
869:
Figure 4: Typical BJT constant current source with negative feedback
383:). This value of current will only be delivered to a load with zero
2913:
2340:
2305:
2301:
2288:
2282:
2254:
2190:
1737:
1505:
914:
864:
362:
131:
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2609:
voltage sources don't exist. Hypothetically connecting one to an
2590:
current sources don't exist. Hypothetically connecting one to an
2325:
Nitrogen-filled glass tubes with two electrodes and a calibrated
2199:
The circuit to the left overcomes the thermal problem (see also,
850:, as it behaves much like a dual to the constant voltage diode (
842:
497:
465:
configuration driven by a constant input current or voltage and
2817:
varies logarithmically with current level: for more detail see
896:, when reverse biased (as shown in the circuit) has a constant
262:
796:
flows through the resistor and respectively through the load.
650:
493:
configurations can serve as constant current sources as well.
29:
2643:
charges a capacitor through a resistor non-linearly with time
2576:, which is preferably very low (often much less than 1 ohm).
2557:) will provide no current (and hence no power). But when the
877:(BJT) implementation (Figure 4) of the general idea above, a
1514:
The Zener diode can be replaced by any other diode; e.g., a
799:
If the input voltage varies, this arrangement will act as a
643:
The grounded load is an advantage of this circuit solution.
202:
2749:"AN-1515 A Comprehensive Study of the Howland Current Pump"
1942:{\displaystyle R_{2}={\frac {V_{\text{Z}}}{I_{\text{R2}}}}}
442:
Active current sources have many important applications in
737:
flowing through the load so that to make the voltage drop
2707:, many of which are designed as constant current devices.
439:) to keep up a constant total resistance in the circuit.
2313:
904:
flowing through it. Thus, as long as the Zener current (
2122:
two-transistor current mirror with emitter degeneration
400:
to a load resistance in the range of 50 to 450 Ω.
2309:
477:
is an example of such a current source widely used in
2488:
2475:{\displaystyle V_{\rm {th}}=I_{\rm {no}}R_{\rm {no}}}
2423:
2383:
2351:
1999:
1901:
1793:
1638:
1554:
1427:
1307:
1193:
994:
2225:
voltage at Q1's emitter almost exactly equal to the
357:
The simplest non-ideal current source consists of a
305:. Symbols for these sources are shown in Figure 2.
60:. Unsourced material may be challenged and removed.
2777:
2545:Voltage sources provide an almost-constant output
2506:
2474:
2401:
2369:
2088:
1941:
1864:
1714:
1609:
1478:
1383:
1248:
1111:
837:The internal structure of a current limiting diode
2833:See above note on logarithmic current dependence.
2140:Constant current source with thermal compensation
1734:Transistor current source with diode compensation
1518:LED1 as shown in Figure 5. The LED voltage drop (
1171:, is very nearly equal to the collector current,
2914:Using Current Sources / Sinks / Mirrors In Audio
2099:(the compensating diode's forward voltage drop,
728:The voltage follower adjusts its output current
416:Active current sources without negative feedback
2304:can be implemented by an IC voltage regulator (
2124:. Negative feedback is a basic feature in some
1268:is typically 0.65 V for a silicon device.
2765:Consider the "Deboo" Single-Supply Integrator
2276:discusses another example of these so-called
2120:Series negative feedback is also used in the
8:
2317:is an important advantage of this solution.
2287:Figure 8: Constant current source using the
2926:Differential Amplifiers and Current Sources
2784:. UK: Cambridge University Press. pp.
1970:and hence it only suppresses the change in
685:. Unsourced material may be challenged and
606:) and a negative impedance converter INIC (
544:
2829:
2827:
580:are typical implementations of this idea.
2902:JFET Current Source and pSpice Simulation
2514:. The conversion also works in reverse.
2494:
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1015:
999:
993:
705:Learn how and when to remove this message
190:(or sink) delivers a constant current. A
120:Learn how and when to remove this message
2259:Figure 7: Typical op-amp current source.
2128:using multiple transistors, such as the
2116:Current mirror with emitter degeneration
832:
446:. They are often used in place of ohmic
2721:
2600:voltage equals current times resistance
428:Current-stable nonlinear implementation
2776:Horowitz, Paul; Winfield Hill (1989).
647:Current sources with negative feedback
320:, there is zero voltage and thus zero
2639:Wilkinson analog-to-digital converter
2337:Current and voltage source comparison
2302:general negative feedback arrangement
2210:to ground. When this current through
7:
2754:(PDF). Texas Instruments, Inc. 2013.
2195:Current limiter with NPN transistors
1289:is sufficiently large). Resistance
719:negative feedback voltage stabilizer
683:adding citations to reliable sources
58:adding citations to reliable sources
2896:FET Constant-Current Source/Limiter
2891:Current Sources and Current Mirrors
2846:Curpistor, minute current regulator
2518:Most sources of electrical energy (
587:Current compensation implementation
539:Voltage compensation implementation
136:Figure 1: An ideal current source,
2498:
2495:
2482:with equivalent series resistance
2466:
2463:
2451:
2448:
2433:
2430:
2393:
2390:
2361:
2358:
854:) used in simple voltage sources.
25:
2296:Voltage regulator current sources
1409:is low enough to ensure adequate
818:Simple transistor current sources
412:) for large laboratory versions.
2377:with parallel source resistance
655:
527:Following voltage implementation
261:
254:
237:
230:
213:
206:
34:
2308:on Figure 8). As with the bare
545:resistor passive current source
324:delivered. When connected to a
45:needs additional citations for
2780:The Art of Electronics, 2nd Ed
2682:, a compensated current source
2329:(decays per second) amount of
1034:
1005:
900:across it irrespective of the
1:
1979:rather than nulling it out.)
2696:Voltage-to-current converter
2526:, etc.) are best modeled as
2507:{\displaystyle R_{\rm {th}}}
2402:{\displaystyle R_{\rm {no}}}
2370:{\displaystyle I_{\rm {no}}}
1536:changes due to temperature.
801:voltage-to-current converter
723:current-to-voltage converter
268:
252:
244:
228:
220:
204:
163:which is independent of the
159:that delivers or absorbs an
2345:Figure 9: A current source
875:bipolar junction transistor
2967:
1961:is never exactly equal to
861:Zener diode current source
826:
578:op-amp inverting amplifier
249:Controlled current source
246:Controlled voltage source
188:independent current source
186:. There are two types. An
18:Constant current regulator
2675:Current sources and sinks
1162:action, emitter current,
1158:is also constant. Due to
887:constant emitter resistor
303:controlled current source
144:, and creating a voltage
2951:Electrical power control
2730:bilateral current source
2530:, however some (notably
879:Zener voltage stabilizer
574:transimpedance amplifier
511:current regulator diodes
283:Figure 2: Source symbols
192:dependent current source
170:A current source is the
2306:LM317 voltage regulator
881:(R1 and DZ1) drives an
405:Van de Graaff generator
2686:Iron-hydrogen resistor
2515:
2508:
2476:
2403:
2371:
2292:
2260:
2251:Op-amp current sources
2196:
2188:illustrate the issue.
2090:
1943:
1866:
1743:
1716:
1611:
1511:
1480:
1385:
1250:
1149:is constant and hence
1113:
870:
848:constant-current diode
838:
829:Constant-current diode
823:Constant current diode
456:differential amplifier
353:Passive current source
148:
140:, driving a resistor,
2711:Widlar current source
2509:
2477:
2404:
2372:
2344:
2286:
2258:
2194:
2134:Wilson current source
2130:Widlar current source
2091:
1944:
1867:
1784:is now calculated as
1741:
1717:
1612:
1509:
1481:
1386:
1251:
1114:
868:
836:
336:and the other is its
135:
2703:, a device used for
2701:Welding power supply
2629:Charging a capacitor
2486:
2421:
2417:of a voltage source
2381:
2349:
1997:
1899:
1791:
1636:
1552:
1516:light-emitting diode
1425:
1400:= 1.2 to 2 (so that
1305:
1191:
992:
679:improve this section
396:constant current of
290:ideal current source
54:improve this article
2561:approaches zero (a
2553:(i.e., an infinite
2540:Thévenin's theorems
2415:Thevenin Equivalent
479:integrated circuits
452:integrated circuits
444:electronic circuits
334:internal resistance
310:internal resistance
2919:2019-07-17 at the
2907:2016-11-15 at the
2735:2011-06-07 at the
2516:
2504:
2472:
2399:
2367:
2293:
2261:
2197:
2086:
1939:
1862:
1744:
1729:is the LED current
1712:
1607:
1512:
1502:LED current source
1476:
1381:
1246:
1109:
871:
839:
812:compliance voltage
338:compliance voltage
157:electronic circuit
149:
2691:Saturable reactor
2622:but not identical
2520:mains electricity
2291:voltage regulator
2280:current mirrors.
2084:
2080:
2061:
2049:
2036:
2023:
1990:is calculated as
1937:
1934:
1924:
1860:
1857:
1830:
1817:
1710:
1706:
1687:
1675:
1662:
1646:
1605:
1602:
1591:
1578:
1562:
1545:is calculated as
1474:
1470:
1450:
1435:
1379:
1375:
1356:
1344:
1331:
1315:
1298:is calculated as
1244:
1241:
1230:
1217:
1201:
1104:
1101:
1090:
1077:
1062:
1059:
1049:
1031:
1018:
1002:
885:(Q1) loaded by a
715:
714:
707:
422:negative feedback
280:
279:
130:
129:
122:
104:
16:(Redirected from
2958:
2946:Electric current
2862:
2861:
2859:
2857:
2852:
2840:
2834:
2831:
2822:
2816:
2806:
2800:
2799:
2783:
2773:
2767:
2762:
2756:
2755:
2753:
2745:
2739:
2726:
2660:Current limiting
2655:Constant current
2574:output impedance
2570:output impedance
2513:
2511:
2510:
2505:
2503:
2502:
2501:
2481:
2479:
2478:
2473:
2471:
2470:
2469:
2456:
2455:
2454:
2438:
2437:
2436:
2408:
2406:
2405:
2400:
2398:
2397:
2396:
2376:
2374:
2373:
2368:
2366:
2365:
2364:
2312:and the precise
2310:emitter follower
2270:
2246:
2239:
2231:
2223:
2216:
2209:
2201:current limiting
2186:
2177:
2170:
2157:
2150:
2107:
2095:
2093:
2092:
2087:
2085:
2083:
2082:
2081:
2078:
2063:
2062:
2059:
2052:
2051:
2050:
2047:
2038:
2037:
2034:
2025:
2024:
2021:
2014:
2009:
2008:
1989:
1978:
1969:
1960:
1948:
1946:
1945:
1940:
1938:
1936:
1935:
1932:
1926:
1925:
1922:
1916:
1911:
1910:
1891:
1871:
1869:
1868:
1863:
1861:
1859:
1858:
1855:
1849:
1848:
1847:
1832:
1831:
1828:
1819:
1818:
1815:
1808:
1803:
1802:
1783:
1771:
1762:
1752:
1721:
1719:
1718:
1713:
1711:
1709:
1708:
1707:
1704:
1689:
1688:
1685:
1678:
1677:
1676:
1673:
1664:
1663:
1660:
1653:
1648:
1647:
1644:
1628:
1616:
1614:
1613:
1608:
1606:
1604:
1603:
1600:
1594:
1593:
1592:
1589:
1580:
1579:
1576:
1569:
1564:
1563:
1560:
1544:
1535:
1526:
1497:
1485:
1483:
1482:
1477:
1475:
1473:
1472:
1471:
1468:
1452:
1451:
1448:
1442:
1437:
1436:
1433:
1417:
1408:
1399:
1390:
1388:
1387:
1382:
1380:
1378:
1377:
1376:
1373:
1358:
1357:
1354:
1347:
1346:
1345:
1342:
1333:
1332:
1329:
1322:
1317:
1316:
1313:
1297:
1288:
1279:
1267:
1255:
1253:
1252:
1247:
1245:
1243:
1242:
1239:
1233:
1232:
1231:
1228:
1219:
1218:
1215:
1208:
1203:
1202:
1199:
1179:
1170:
1157:
1148:
1139:
1131:is constant and
1130:
1118:
1116:
1115:
1110:
1105:
1103:
1102:
1099:
1093:
1092:
1091:
1088:
1079:
1078:
1075:
1068:
1063:
1061:
1060:
1057:
1051:
1050:
1047:
1041:
1033:
1032:
1029:
1020:
1019:
1016:
1004:
1003:
1000:
984:
975:
959:
950:
934:
925:
912:
883:emitter follower
805:transconductance
795:
764:
755:
736:
710:
703:
699:
696:
690:
659:
651:
639:
630:
621:
605:
599:
571:
535:current source.
515:enhancement-mode
508:
399:
382:
265:
258:
241:
234:
217:
210:
203:
161:electric current
125:
118:
114:
111:
105:
103:
69:"Current source"
62:
38:
30:
21:
2966:
2965:
2961:
2960:
2959:
2957:
2956:
2955:
2941:Analog circuits
2931:
2930:
2921:Wayback Machine
2909:Wayback Machine
2887:
2870:
2868:Further reading
2865:
2855:
2853:
2850:
2842:
2841:
2837:
2832:
2825:
2819:diode modelling
2814:
2809:
2807:
2803:
2796:
2775:
2774:
2770:
2763:
2759:
2751:
2747:
2746:
2742:
2737:Wayback Machine
2727:
2723:
2719:
2651:
2631:
2596:load resistance
2559:load resistance
2528:voltage sources
2489:
2484:
2483:
2457:
2442:
2424:
2419:
2418:
2384:
2379:
2378:
2352:
2347:
2346:
2339:
2323:
2321:Curpistor tubes
2314:op-amp follower
2298:
2269:
2265:
2253:
2245:
2241:
2238:
2234:
2230:
2226:
2222:
2218:
2215:
2211:
2208:
2204:
2185:
2179:
2176:
2172:
2169:
2163:
2156:
2152:
2149:
2145:
2142:
2126:current mirrors
2118:
2106:
2100:
2073:
2054:
2053:
2042:
2029:
2016:
2015:
2000:
1995:
1994:
1988:
1982:
1977:
1971:
1968:
1962:
1959:
1953:
1927:
1917:
1902:
1897:
1896:
1889:
1882:
1876:
1850:
1836:
1823:
1810:
1809:
1794:
1789:
1788:
1782:
1776:
1770:
1764:
1761:
1755:
1751:
1747:
1736:
1728:
1699:
1680:
1679:
1668:
1655:
1654:
1639:
1634:
1633:
1627:
1621:
1595:
1584:
1571:
1570:
1555:
1550:
1549:
1543:
1537:
1534:
1528:
1525:
1519:
1504:
1496:
1490:
1453:
1443:
1428:
1423:
1422:
1416:
1410:
1407:
1401:
1395:
1368:
1349:
1348:
1337:
1324:
1323:
1308:
1303:
1302:
1296:
1290:
1287:
1281:
1278:
1272:
1266:
1260:
1234:
1223:
1210:
1209:
1194:
1189:
1188:
1178:
1172:
1169:
1163:
1156:
1150:
1147:
1141:
1138:
1132:
1129:
1123:
1094:
1083:
1070:
1069:
1052:
1042:
1024:
1011:
995:
990:
989:
983:
977:
974:
967:
961:
958:
952:
949:
943:
942:Voltage across
933:
927:
924:
918:
911:
905:
863:
831:
825:
820:
790:
779:
772:
766:
763:
757:
751:
744:
738:
735:
729:
711:
700:
694:
691:
676:
660:
649:
638:
632:
629:
623:
619:
615:
611:
607:
601:
598:
592:
589:
566:
559:
553:
541:
529:
507:
501:
430:
418:
397:
370:
355:
350:
348:Implementations
326:load resistance
286:
285:
284:
225:Current source
222:Voltage source
200:
126:
115:
109:
106:
63:
61:
51:
39:
28:
23:
22:
15:
12:
11:
5:
2964:
2962:
2954:
2953:
2948:
2943:
2933:
2932:
2929:
2928:
2923:
2911:
2899:
2893:
2886:
2885:External links
2883:
2882:
2881:
2869:
2866:
2864:
2863:
2835:
2823:
2812:
2808:The value for
2801:
2794:
2768:
2757:
2740:
2720:
2718:
2715:
2714:
2713:
2708:
2698:
2693:
2688:
2683:
2680:Fontana bridge
2677:
2672:
2670:Current mirror
2667:
2662:
2657:
2650:
2647:
2630:
2627:
2500:
2497:
2492:
2468:
2465:
2460:
2453:
2450:
2445:
2441:
2435:
2432:
2427:
2395:
2392:
2387:
2363:
2360:
2355:
2338:
2335:
2322:
2319:
2297:
2294:
2274:current mirror
2267:
2252:
2249:
2243:
2236:
2228:
2220:
2213:
2206:
2183:
2174:
2167:
2154:
2147:
2141:
2138:
2117:
2114:
2104:
2097:
2096:
2076:
2072:
2069:
2066:
2057:
2045:
2041:
2032:
2028:
2019:
2012:
2007:
2003:
1986:
1975:
1966:
1957:
1952:(In practice,
1950:
1949:
1930:
1920:
1914:
1909:
1905:
1887:
1880:
1873:
1872:
1853:
1846:
1843:
1839:
1835:
1826:
1822:
1813:
1806:
1801:
1797:
1780:
1768:
1759:
1749:
1735:
1732:
1731:
1730:
1726:
1702:
1698:
1695:
1692:
1683:
1671:
1667:
1658:
1651:
1642:
1625:
1618:
1617:
1598:
1587:
1583:
1574:
1567:
1558:
1541:
1532:
1523:
1503:
1500:
1494:
1487:
1486:
1466:
1463:
1460:
1456:
1446:
1440:
1431:
1414:
1405:
1392:
1391:
1371:
1367:
1364:
1361:
1352:
1340:
1336:
1327:
1320:
1311:
1294:
1285:
1276:
1264:
1257:
1256:
1237:
1226:
1222:
1213:
1206:
1197:
1176:
1167:
1154:
1145:
1136:
1127:
1120:
1119:
1108:
1097:
1086:
1082:
1073:
1066:
1055:
1045:
1039:
1036:
1027:
1023:
1014:
1010:
1007:
998:
981:
972:
965:
960:) is given by
956:
947:
931:
922:
909:
862:
859:
827:Main article:
824:
821:
819:
816:
788:
777:
770:
761:
749:
742:
733:
713:
712:
663:
661:
654:
648:
645:
636:
627:
617:
613:
609:
596:
588:
585:
564:
557:
550:virtual ground
540:
537:
528:
525:
505:
475:current mirror
471:common cathode
463:common emitter
429:
426:
417:
414:
359:voltage source
354:
351:
349:
346:
282:
281:
278:
277:
274:
267:
266:
259:
251:
250:
247:
243:
242:
235:
227:
226:
223:
219:
218:
211:
201:
199:
196:
184:resistive load
176:voltage source
153:current source
128:
127:
42:
40:
33:
26:
24:
14:
13:
10:
9:
6:
4:
3:
2:
2963:
2952:
2949:
2947:
2944:
2942:
2939:
2938:
2936:
2927:
2924:
2922:
2918:
2915:
2912:
2910:
2906:
2903:
2900:
2897:
2894:
2892:
2889:
2888:
2884:
2880:
2879:0-7506-7752-X
2876:
2872:
2871:
2867:
2849:
2847:
2839:
2836:
2830:
2828:
2824:
2820:
2815:
2805:
2802:
2797:
2791:
2787:
2782:
2781:
2772:
2769:
2766:
2761:
2758:
2750:
2744:
2741:
2738:
2734:
2731:
2725:
2722:
2716:
2712:
2709:
2706:
2702:
2699:
2697:
2694:
2692:
2689:
2687:
2684:
2681:
2678:
2676:
2673:
2671:
2668:
2666:
2663:
2661:
2658:
2656:
2653:
2652:
2648:
2646:
2644:
2640:
2636:
2628:
2626:
2623:
2617:
2615:
2614:short circuit
2612:
2608:
2603:
2601:
2597:
2593:
2589:
2585:
2583:
2577:
2575:
2571:
2568:
2564:
2563:short circuit
2560:
2556:
2552:
2548:
2543:
2541:
2537:
2533:
2529:
2525:
2521:
2490:
2458:
2443:
2439:
2425:
2416:
2412:
2385:
2353:
2343:
2336:
2334:
2332:
2328:
2320:
2318:
2315:
2311:
2307:
2303:
2295:
2290:
2285:
2281:
2279:
2275:
2257:
2250:
2248:
2202:
2193:
2189:
2182:
2166:
2160:
2139:
2137:
2135:
2131:
2127:
2123:
2115:
2113:
2109:
2103:
2074:
2070:
2067:
2064:
2055:
2043:
2039:
2030:
2026:
2017:
2010:
2005:
2001:
1993:
1992:
1991:
1985:
1980:
1974:
1965:
1956:
1928:
1918:
1912:
1907:
1903:
1895:
1894:
1893:
1886:
1879:
1851:
1844:
1841:
1837:
1833:
1824:
1820:
1811:
1804:
1799:
1795:
1787:
1786:
1785:
1779:
1773:
1767:
1763:) tracks the
1758:
1740:
1733:
1725:
1700:
1696:
1693:
1690:
1681:
1669:
1665:
1656:
1649:
1640:
1632:
1631:
1630:
1624:
1596:
1585:
1581:
1572:
1565:
1556:
1548:
1547:
1546:
1540:
1531:
1522:
1517:
1508:
1501:
1499:
1493:
1464:
1461:
1458:
1454:
1444:
1438:
1429:
1421:
1420:
1419:
1413:
1404:
1398:
1369:
1365:
1362:
1359:
1350:
1338:
1334:
1325:
1318:
1309:
1301:
1300:
1299:
1293:
1284:
1275:
1269:
1263:
1235:
1224:
1220:
1211:
1204:
1195:
1187:
1186:
1185:
1183:
1175:
1166:
1161:
1153:
1144:
1135:
1126:
1106:
1095:
1084:
1080:
1071:
1064:
1053:
1043:
1037:
1025:
1021:
1012:
1008:
996:
988:
987:
986:
980:
971:
964:
955:
946:
940:
938:
930:
921:
916:
908:
903:
899:
895:
890:
888:
884:
880:
876:
867:
860:
858:
855:
853:
849:
844:
835:
830:
822:
817:
815:
813:
808:
806:
802:
797:
794:
787:
783:
776:
769:
760:
754:
748:
741:
732:
726:
724:
720:
709:
706:
698:
688:
684:
680:
674:
673:
669:
664:This section
662:
658:
653:
652:
646:
644:
641:
635:
626:
604:
595:
586:
584:
581:
579:
575:
570:
563:
556:
551:
546:
538:
536:
534:
526:
524:
522:
519:
516:
512:
504:
499:
494:
492:
488:
484:
480:
476:
472:
468:
467:common source
464:
459:
457:
453:
449:
445:
440:
438:
437:
427:
425:
423:
415:
413:
411:
406:
401:
395:
394:approximately
389:
386:
381:
377:
373:
368:
364:
360:
352:
347:
345:
341:
339:
335:
329:
327:
323:
319:
318:short circuit
315:
311:
306:
304:
300:
296:
291:
275:
272:
269:
264:
260:
257:
253:
248:
245:
240:
236:
233:
229:
224:
221:
216:
212:
209:
205:
197:
195:
193:
189:
185:
181:
177:
173:
168:
166:
162:
158:
154:
147:
143:
139:
134:
124:
121:
113:
102:
99:
95:
92:
88:
85:
81:
78:
74:
71: –
70:
66:
65:Find sources:
59:
55:
49:
48:
43:This article
41:
37:
32:
31:
19:
2854:. Retrieved
2845:
2838:
2810:
2804:
2779:
2771:
2760:
2743:
2724:
2665:Current loop
2632:
2621:
2618:
2610:
2606:
2604:
2591:
2587:
2586:
2578:
2551:open circuit
2544:
2517:
2324:
2299:
2278:gain-boosted
2277:
2262:
2198:
2180:
2164:
2161:
2143:
2119:
2110:
2101:
2098:
1983:
1981:
1972:
1963:
1954:
1951:
1884:
1877:
1874:
1777:
1774:
1765:
1756:
1745:
1723:
1622:
1619:
1538:
1529:
1520:
1513:
1491:
1488:
1411:
1402:
1396:
1393:
1291:
1282:
1273:
1270:
1261:
1258:
1182:Early effect
1173:
1164:
1151:
1142:
1133:
1124:
1121:
978:
969:
962:
953:
944:
941:
928:
919:
906:
898:voltage drop
891:
886:
882:
878:
872:
856:
840:
811:
809:
798:
792:
785:
781:
774:
767:
758:
752:
746:
739:
730:
727:
718:
716:
701:
695:October 2014
692:
677:Please help
665:
642:
633:
624:
602:
593:
590:
582:
568:
561:
554:
542:
533:bootstrapped
531:An example:
530:
502:
495:
460:
441:
434:
431:
419:
402:
393:
390:
385:voltage drop
379:
375:
371:
356:
342:
337:
333:
330:
314:open circuit
307:
302:
298:
294:
289:
287:
276:Single cell
191:
187:
180:current sink
179:
169:
152:
150:
145:
141:
137:
116:
110:January 2019
107:
97:
90:
83:
76:
64:
52:Please help
47:verification
44:
2848:data sheet"
2844:"Tung-Sol:
2705:arc welding
2605:Similarly,
2532:solar cells
1775:Resistance
894:Zener diode
852:Zener diode
543:The simple
491:common grid
487:common gate
483:common base
295:independent
178:. The term
167:across it.
2935:Categories
2795:0521370957
2717:References
1160:transistor
937:transistor
450:in analog
436:vice versa
198:Background
80:newspapers
2555:impedance
2411:converted
2327:Becquerel
2071:⋅
2040:−
2027:−
1834:−
1697:⋅
1666:−
1582:−
1366:⋅
1335:−
1221:−
1081:−
935:) of NPN
666:does not
518:N-channel
454:(e.g., a
448:resistors
410:megavolts
398:1 mA ± 5%
367:Ohm's law
299:dependent
273:of cells
2917:Archived
2905:Archived
2898:- Vishay
2733:Archived
2649:See also
2633:Because
2536:Norton's
2132:and the
1890:= 0.65 V
1722:, where
976:, where
873:In this
408:tens of
2728:Widlar
2582:megohms
2547:voltage
2524:battery
2413:to its
2409:can be
902:current
687:removed
672:sources
576:and an
271:Battery
165:voltage
94:scholar
2877:
2856:26 May
2792:
1875:Since
1495:FE,min
1394:where
1259:where
1122:Since
572:. The
521:MOSFET
481:. The
155:is an
96:
89:
82:
75:
67:
2851:(PDF)
2752:(PDF)
2611:ideal
2607:ideal
2592:ideal
2588:Ideal
2289:LM317
2237:sense
2214:sense
2207:sense
2173:0.4/R
915:diode
363:ratio
322:power
174:of a
101:JSTOR
87:books
2875:ISBN
2858:2013
2790:ISBN
2538:and
2522:, a
2300:The
1620:and
1489:and
843:JFET
670:any
668:cite
498:JFET
489:and
461:The
308:The
172:dual
73:news
2786:182
2567:ohm
1629:as
1469:min
1418:),
814:).
807:).
771:OUT
750:OUT
734:OUT
681:by
620:= R
616:= R
612:= R
558:OUT
506:DSS
301:or
288:An
56:by
2937::
2826:^
2813:BE
2788:.
2542:.
2331:Ra
2268:BE
2244:be
2229:be
2221:be
2184:be
2168:be
2155:be
2148:be
2136:.
1976:BE
1967:BE
1933:R2
1892:,
1888:BE
1883:=
1856:R2
1769:BE
1750:BE
1601:R2
1590:BE
1533:BE
1406:R1
1286:FE
1277:R2
1265:BE
1240:R2
1229:BE
1146:R2
1137:BE
1089:BE
1048:R2
1001:R2
982:BE
973:BE
968:−
957:R2
892:A
789:IN
784:=
773:=
762:IN
745:=
637:−R
597:IN
565:IN
560:=
496:A
485:,
424:.
403:A
374:=
369:;
151:A
2860:.
2821:.
2811:V
2798:.
2499:h
2496:t
2491:R
2467:o
2464:n
2459:R
2452:o
2449:n
2444:I
2440:=
2434:h
2431:t
2426:V
2394:o
2391:n
2386:R
2362:o
2359:n
2354:I
2266:V
2242:V
2235:R
2227:V
2219:V
2212:R
2205:R
2181:V
2175:e
2165:V
2153:V
2146:V
2105:D
2102:V
2079:B
2075:I
2068:K
2065:+
2060:Z
2056:I
2048:D
2044:V
2035:Z
2031:V
2022:S
2018:V
2011:=
2006:1
2002:R
1987:1
1984:R
1973:V
1964:V
1958:D
1955:V
1929:I
1923:Z
1919:V
1913:=
1908:2
1904:R
1885:V
1881:D
1878:V
1852:I
1845:E
1842:B
1838:V
1829:D
1825:V
1821:+
1816:Z
1812:V
1805:=
1800:2
1796:R
1781:2
1778:R
1766:V
1760:D
1757:V
1754:(
1748:V
1727:D
1724:I
1705:B
1701:I
1694:K
1691:+
1686:D
1682:I
1674:D
1670:V
1661:S
1657:V
1650:=
1645:1
1641:R
1626:1
1623:R
1597:I
1586:V
1577:D
1573:V
1566:=
1561:2
1557:R
1542:2
1539:R
1530:V
1524:D
1521:V
1492:h
1465:,
1462:E
1459:F
1455:h
1449:C
1445:I
1439:=
1434:B
1430:I
1415:B
1412:I
1403:R
1397:K
1374:B
1370:I
1363:K
1360:+
1355:Z
1351:I
1343:Z
1339:V
1330:S
1326:V
1319:=
1314:1
1310:R
1295:1
1292:R
1283:h
1274:I
1271:(
1262:V
1236:I
1225:V
1216:Z
1212:V
1205:=
1200:2
1196:R
1177:C
1174:I
1168:E
1165:I
1155:E
1152:I
1143:V
1134:V
1128:Z
1125:V
1107:.
1100:2
1096:R
1085:V
1076:Z
1072:V
1065:=
1058:2
1054:R
1044:V
1038:=
1035:)
1030:C
1026:I
1022:=
1017:E
1013:I
1009:=
1006:(
997:I
979:V
970:V
966:Z
963:V
954:V
951:(
948:2
945:R
932:B
929:I
923:Z
920:V
917:(
910:Z
907:I
793:R
791:/
786:V
782:R
780:/
778:R
775:V
768:I
759:V
753:R
747:I
743:R
740:V
731:I
708:)
702:(
697:)
693:(
689:.
675:.
634:I
628:R
625:I
618:3
614:2
610:1
608:R
603:Z
594:V
569:R
567:/
562:V
555:I
503:I
469:(
380:R
378:/
376:V
372:I
146:V
142:R
138:I
123:)
117:(
112:)
108:(
98:·
91:·
84:·
77:·
50:.
20:)
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