您要找的是不是:
Allowing only few people to rename or move files between source-controlled folders, but still restricting everyone from deleting any file.
只允许部分人在源控制的文件夹之间重命名或者删除文件，但是仍然限制不让每个人都有删除任意文件的权限。
The source-controlled file base is built up from nothing but the steady accretion of change sets, each one building on everything that has come before it.
受源代码控制的文件库是在稳定增长的变更集基础上构建的；每个变更集都以之前的所有变更为基础。
The source-controlled file base can be partitioned into one or more separate Components, each with their own tree of folders and files, and their own history.
可以将受源代码控制的文件库划分为一个或多个单独的组件（Components），每个组件具有自己的文件夹和文件的树形结构，并具有自己的变更历史记录。
The result is a compact source of X-rays that can be controlled with great precision.
With so much at stake I was surprised that there had been no prospective, placebo-controlled trials conducted that were funded by an independent source.
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感谢您的反馈，我们会尽快进行适当修改！From Wikipedia, the free encyclopedia
Figure 1: An ideal current source, I, driving a resistor, R, and creating a voltage V
A current source is an
that delivers or absorbs an
which is independent of the voltage across it.
A current source is the
of a . 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 . 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.
Voltage source
Current source
Controlled voltage source
Controlled current source
Single cell
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 , 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.
of an ideal current source is infinite. An independent current source with zero current is identical to an ideal . The voltage across an ideal current source is completely determined by the circuit it is connected to. When connected to a , there is zero voltage and thus zero
delivered. When connected to a , 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.
The simplest non-ideal current source consists of a
in series with a resistor. The amount of current available from such a source is given by the
of the voltage across the voltage source to the resistance of the resistor (; 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.
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 ) for large laboratory versions.
In these circuits the output current is not monitored and controlled by means of .
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 example, if the load increases its resistance, the transistor decreases its present output resistance (and ) to keep up a constant total resistance in the circuit.
Active current sources have many important applications in . They are often used in place of ohmic
(e.g., a ) to generate a current that depends slightly on the voltage across the load.
configuration driven by a constant input current or voltage and
() 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
is an example of such a current source widely used in . The ,
configurations can serve as constant current sources as well.
can be made to act as a current source by tying its gate to its source. The current then flowing is the IDSS of the FET. These can be purchased with this connection already made and in this case the devices are called
or constant current diodes or current limiting diodes (CLD). An enhancement mode N channel MOSFET can be used in the circuits listed below.
An example:
current source.
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
is ideal only when the voltage across it is 0; 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 , 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, IOUT = VIN/R. The op-amp voltage-to-current converter in Figure 3, a
are typical implementations of this idea.
The floating load is a serious disadvantage of this circuit solution.
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, VIN, a positive resistor, R, a load (the capacitor, C, acting as impedance Z) and a negative impedance converter INIC (R1 = R2 = R3 = R and the op-amp). The input voltage source and the resistor R constitute an imperfect current source passing current, IR through the load (Fig. 3 in the source). The INIC acts as a second current source passing "helping" current, I-R, through the load. As a result, the total current flowing through the load is constant and the circuit impedance seen by the input source is increased. However the Howland current source isn't widely used because it requires the four resistors to be perfectly matched, and its impedance drops at high frequencies.
The grounded load is an advantage of this circuit solution.
This section does not
any . Please help
by . Unsourced material may be challenged and . (October 2014) ()
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
connected in the feedback loop. The external load of this current source is connected somewhere in the path of the current supplying the current sensing resistor but out of the feedback loop.
The voltage follower adjusts its output current IOUT flowing through the load so that to make the voltage drop VR = IOUTR across the current sensing resistor R equal to the constant input voltage VIN. Thus the voltage stabilizer keeps up a constant voltage drop across so, a constant current IOUT = VR/R = VIN/R flows through the resistor and respectively through the load.
If the input voltage varies, this arrangement will act as a
(voltage-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 ().
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).
The internal structure of a current limiting diode
The simplest constant-current source or sink is formed from one component: a
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 , as it behaves much like a dual to the constant voltage diode () used in simple voltage sources.
Due to the large variability in saturation current of JFETs, it is common to also include a source resistor (shown in the adjacent image) which allows the current to be tuned down to a desired value.
Figure 4: Typical BJT constant current source with negative feedback
(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 , when reverse biased (as shown in the circuit) has a constant
across it irrespective of the
flowing through it. Thus, as long as the Zener current (IZ) is above a certain level (called holding current), the voltage across the Zener
(VZ) will be constant. Resistor, R1, supplies the Zener current and the base current (IB) of NPN
(Q1). The constant Zener voltage is applied across the base of Q1 and emitter resistor, R2.
Voltage across R2 (VR2) is given by VZ - VBE, where VBE is the base-emitter drop of Q1. The emitter current of Q1 which is also the current through R2 is given by
{\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}}}}.}
Since VZ is constant and VBE is also (approximately) constant for a given temperature, it follows that VR2 is constant and hence IE is also constant. Due to
action, emitter current, IE, is very nearly equal to the collector current, IC, of the transistor (which in turn, is the current through the load). Thus, the load current is constant (neglecting the output resistance of the transistor due to the ) 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
{\displaystyle R_{\text{R2}}={\frac {V_{\text{Z}}-V_{\text{BE}}}{I_{\text{R2}}}}}
where VBE is typically 0.65 V for a silicon device.
(IR2 is also the emitter current and is assumed to be the same as the collector or required load current, provided hFE is sufficiently large). Resistance, RR1, at resistor, R1, is calculated as
{\displaystyle R_{\text{R1}}={\frac {V_{\text{S}}-V_{\text{Z}}}{I_{\text{Z}}+K\cdot I_{\text{B}}}}}
where K = 1.2 to 2 (so that RR1 is low enough to ensure adequate IB),
{\displaystyle I_{\text{B}}={\frac {I_{\text{C}}}{h_{FE,{\text{min}}}}}}
and hFE,min is the lowest acceptable current gain for the particular transistor type being used.
Figure 5: Typical constant current source (CCS) using LED instead of Zener diode
The Zener diode can be replace e.g., a
LED1 as shown in Figure 5. The LED voltage drop (VD) is now used to derive the constant voltage and also has the additional advantage of tracking (compensating) VBE changes due to temperature. RR2 is calculated as
{\displaystyle R_{\text{R2}}={\frac {V_{\text{D}}-V_{\text{BE}}}{I_{\text{R2}}}}}
{\displaystyle R_{\text{R1}}={\frac {V_{\text{S}}-V_{\text{D}}}{I_{\text{D}}+K\cdot I_{\text{B}}}}}
, where ID is the LED current.
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 VBE is sensitive to temperature. Temperature dependence can be compensated using the circuit of Figure 6 that includes a standard diode, D, (of the same semiconductor material as the transistor) in series with the Zener diode as shown in the image on the left. The diode drop (VD) tracks the VBE changes due to temperature and thus significantly counteracts temperature dependence of the CCS.
Resistance R2 is now calculated as
{\displaystyle R_{2}={\frac {V_{\text{Z}}+V_{\text{D}}-V_{BE}}{I_{\text{R2}}}}}
Since VD = VBE = 0.65 V,
{\displaystyle R_{2}={\frac {V_{\text{Z}}}{I_{\text{R2}}}}}
(In practice, VD is never exactly equal to VBE and hence it only suppresses the change in VBE rather than nulling it out.)
R1 is calculated as
{\displaystyle R_{1}={\frac {V_{\text{S}}-V_{\text{Z}}-V_{\text{D}}}{I_{\text{Z}}+K\cdot I_{\text{B}}}}}
(the compensating diode's forward voltage drop, VD, appears in the equation and is typically 0.65 V for silicon devices.)
This method is most effective for
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
mechanism is not as temperature dependent as it is in breakdown diodes above this voltage.
Series negative feedback is also used in the . Negative feedback is a basic feature in some
using multiple transistors, such as the
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 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 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 Vbe junction and hence 0.4 V 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 0.6 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 illustrate the issue.
Current limiter with NPN transistors
The circuit to the left overcomes the thermal problem (see also, ). 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 Rsense to ground. When this current through Rsense 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, 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 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 Rsense will remain steady at ~0.6 V, 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.
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 to compensate for the VBE 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 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
discusses another example of these so-called gain-boosted current mirrors.
Figure 8: Constant current source using the
voltage regulator
can be implemented by an IC voltage regulator ( on Figure 8). As with the bare
and the precise
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.
Nitrogen-filled glass tubes with two electrodes and a calibrated
(fissions per second) amount of
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.
Most sources of electrical energy (, a , etc.) are best modeled as . Such sources provide constant voltage, which means that as long as the current drawn from the source is within the source's capabilities, its output
stays constant. An ideal voltage source provides no energy when it is loaded by an
(i.e., an infinite ), but approaches infinite power and current when the
approaches zero (a ). Such a theoretical device would have a zero
in series with the source. A real-world voltage source has a very low, but non-zero : 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
approaches infinity (an open circuit). An ideal current source has an
in parallel with the source. A real-world current source has a very high, but finite . In the case of transistor current sources, impedances of a few
(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,
circuits, etc.)
Similarly, an ideal
cannot be connected to an ideal
(R = 0), since this would result a similar paradox of finite non-zero voltage across an element with defined zero voltage (the short circuit). Also, a voltage source should not be connected to another voltage source if their voltages differ but again this arrangement is frequently used (e.g., in
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 ,
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
and vice versa. These concepts are dealt with by
, a compensated current source
, a device used for , many of which are designed as constant current devices.
(PDF) (PDF). Texas Instruments, Inc. 2013.
Horowitz, P Winfield Hill (1989). The Art of Electronics, 2nd Ed. UK: Cambridge University Press. p. 182.  .
The value for VBE varies logarithmically with current level: for more detail see .
See above note on logarithmic current dependence.
(PDF) 2013.
"Current Sources & Voltage References" Linden T. H Publ. Elsevier-Newnes -
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