恒流源-毕业论文外文翻译.docx

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1、CURRENT SOURCEA current source is an electrical or electronic device that delivers or absorbs electric current. A current source is the dual of a voltage source. The term constant-current sink is sometimes used for sources fed from a negative voltage supply. Figure 1 shows a schematic for an ideal c

2、urrent source driving a resistor load.Figure 1Ideal current sourcesIn 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 thr

3、ough 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 c

4、urrent source. Symbols for these sources are shown in Figure 2.Figure 2An independent current source with zero current is identical to an ideal open circuit. For this reason, the internal resistance of an ideal current source is infinite. The voltage across an ideal current source is completely dete

5、rmined by the circuit it is connected to. When connected to a short circuit, there is zero voltage and thus zero power delivered. When connected to a load resistance, the voltage across the source approaches infinity as the load resistance approaches infinity (an open circuit). Thus, an ideal curren

6、t source could supply unlimited power forever and so would represent an unlimited source of energy. Connecting an ideal open circuit to an ideal non-zero current source is not valid in circuit analysis as the circuit equation would be paradoxical, e.g., 5 = 0.No real current source is ideal (no unli

7、mited energy sources exist) and all have a finite internal resistance (none can supply unlimited voltage). However, the internal resistance of a physical current source is effectively modeled in circuit analysis by combining a non-zero resistance in parallel with an ideal current source (the Norton

8、equivalent circuit).Resistor current sourceThe simplest current source consists of a voltage source in series with a resistor. The current available from such a source is given by the ratio of the voltage across the voltage source to the resistance of the resistor. For a nearly ideal current source,

9、 the value of this resistor should be very large but this implies that, for a specified current, the voltage source must be very large. 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

10、case that such a circuit will provide adequate performance when the specified current and load resistance are small. For example, a 5V voltage source in series with a 4.7k ohms resistor will provide an approximately constant current of 1mA (5%) to a load resistance in the range of 50 to 450 ohms.Act

11、ive current sources Active current sources have many important applications in electronic circuits. Current sources (current-stable resistors) are often used in place of ohmic resistors in analog integrated circuits to generate a current without causing attenuation at a point in the signal path to w

12、hich the current source is attached. The collector of a bipolar transistor, the drain of a field effect transistor, or the plate of a vacuum tube naturally behave as current sources (or sinks) when properly connected to an external source of energy (such as a power supply) because the output impedan

13、ce of these devices is naturally high when used in the current source configuration.JFET and N-FET current sourceA JFET can be made to act as a current source by tying its gate to its source. The current then flowing is the IDSS of the FET. These can be purchased with this connection already made an

14、d in this case the devices are called current regulator diodes or constant current diodes or current limiting diodes (CLD). An enhancement mode N channel MOSFET can be used in the circuits listed below.Simple transistor current sourceFigure 3 shows a typical constant current source (CCS). DZ1 is a z

15、ener diode which, when reverse biased (as shown in the circuit) has a constant voltage drop across it irrespective of the current flowing through it. Thus, as long as the zener current (IZ) is above a certain level (called holding current), the voltage across the zener diode (VZ) will be constant. R

16、esistor R1 supplies the zener current and the base current (IB) of NPN transistor (Q1). The constant zener voltage is applied across the base of Q1 and emitter resistor R2. The operation of the circuit is as follows:Voltage across R2 (VR2) is given by VZ - VBE, where VBE is the base-emitter drop of

17、Q1. The emitter current of Q1 which is also the current through R2 is given by.Figure 3Since VZ is constant and VBE is also (approximately) constant for a given temperature, it follows that VR2 is constant and hence IE is also constant. Due to transistor action, emitter current IE is very nearly equ

18、al to the collector current IC of the transistor (which in turn, is the current through the load). Thus, the load current is constant (neglecting the output resistance of the transistor due to the Early effect) and the circuit operates as a constant current source. As long as the temperature remains

19、 constant (or doesnt vary much), the load current will be independent of the supply voltage, R1 and the transistors gain. R2 allows the load current to be set at any desirable value and is calculated by or , since VBE is typically 0.65 V for a silicon device. (IR2 is also the emitter current and is

20、assumed to be the same as the collector or required load current, provided hFE is sufficiently large). Resistance R1 at resistor R1 is calculated as,where, K = 1.2 to 2 (so that R1 is low enough to ensure adequate IB), ,and hFE(min) is the lowest acceptable current gain for the particular transistor

21、 type being used.A more common current source in integrated circuits is the current mirror.Simple transistor current source with diode compensationTemperature changes will change the output current delivered by the circuit of Figure 3 because VBE is sensitive to temperature. Temperature dependence c

22、an be compensated using the circuit of Figure 4 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 t

23、emperature dependence of the CCS.Resistance R2 is now calculated asSince VD = VBE = 0.65 V,Therefore, .Figure 4This method is most effective for Zener diodes rated at 5.6 V or more. For breakdown diodes of less than 5.6 V, the compensating diode is usually not required because the breakdown mechanis

24、m is not as temperature dependent as it is in breakdown diodes above this voltage.Simple transistor current source with LEDAnother method is to replace the Zener diode with a light-emitting diode LED1 as shown in Figure 5. The LED voltage drop (VD) is now used to derive the constant voltage and also

25、 has the additional advantage of tracking (compensating) VBE changes due to temperature. R2 is calculated as ,and R1 as , where ID is the LED current. Figure 5 Figure 6Another common method is to use feedback to set the current and remove the dependence on the Vbe of the transistor. Figure 6 shows a

26、 very common approach using an op amp with the non-inverting input connected to a voltage source (such as the Zener in an above example) and the inverting input connected to the same node as the resistor and emitter of the transistor. This way the generated voltage is across the resistor, rather tha

27、n both the resistor and transistor. (For details, see the article on the ideal op amp - the nullor.) The article on current mirror discusses another example of these so-called gain-boosted current mirrors.Other practical sourcesIn the case of opamp circuits sometimes it is desired to inject a precis

28、ely known current to the inverting input (as an offset of signal input for instance) and a resistor connected between the source voltage and the inverting input will approximate an ideal current source with value V/R.Inductor type current sourceAmongst other applications, the circuit of Figure 7 usi

29、ng the LM317 voltage regulator is used to present a source of constant current in Class E (switching) electronic amplifiers.Figure 7Current and voltage source comparisonMost sources of electrical energy (mains electricity, a battery, .) are best modeled as voltage sources. Such sources provide const

30、ant voltage, which means that as long as the amount of current drawn from the source is within the sources capabilities, its output voltage stays constant. An ideal voltage source provides no energy when it is loaded by an open circuit (i.e. an infinite impedance), but approaches infinite power and

31、current when the load resistance approaches zero (a short circuit). Such a theoretical device would have a zero ohm output impedance in series with the source. A real-world voltage source has a very low, but non-zero output impedance: often much less than 1 ohm.Conversely, a current source provides

32、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

33、has an infinite output impedance in parallel with the source. A real-world current source has a very high, but finite output impedance. In the case of transistor current sources, impedances of a few megohms (at DC) are typical.An ideal current source cannot be connected to an ideal open circuit beca

34、use 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). Nor can an ideal voltage source be connected to an ideal short circuit (R=0), since this would result a similar paradox of finite non

35、 zero voltage across an element with defined zero voltage (the short circuit). 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.

36、 These concepts are dealt with by Nortons and Thvenins theorems.恒流源电流源是电气或电子装置，可提供或吸收电流。一个电流源是一个电压源双。术语恒流源有时用来从一个负电压电源馈来源。图1显示了一个理想的电流源驱动的电阻负载的原理图。图1理想电流源1、理想电流源在电路理论，理想电流源电路元件的电流通过时与其两端的电压无关。这是一个数学模型。如果通过一个理想的电流源电流可以指定独立于任何其他变量的电路，它被称为一个独立的电流源。相反，如果其他一些电压或电路中的电流通过一个理想电流源的电流决定，它被称为从属或控制的电流源。这些源符号，如图

37、2所示。图2各种电流源符号一个独立的电流源与零电流是相同的理想开路。基于这个原因，一个理想电流源内阻是无限的。在一个理想的电流源的电压是完全取决于它的连接电路。当连接到短路，存在零电压，从而零功率交付。当连接到负载电阻两端的电压接近源的负载电阻接近无穷大（开路）。因此，一个理想的电流源可提供无限的能量将代表无限的能源来源。连接的理想开路理想非零电流源是无效的，在电路的电路方程分析将是自相矛盾的。没有真正的电流源是理想的（不存在无限的能源），并且所有的有限的内部电阻（没有人能提供无限的电压）。然而，内部电阻电流源建模的有效结合电路分析与理想电流源非零并联电阻（诺顿等效电路）。2、电阻电流源最简单

38、的电流源包括一个与一个电阻器系列电压源。目前从这样的来源可以是由两端的电压源电压比电阻器的电阻提供。对于一个几近完美的电流源，这个电阻值应该是非常大的，但是这意味着，在规定的电流，电压源必须是非常大的。因此，效率低（由于功率的电阻损耗），它通常是不切实际的建好这样的电流源。尽管如此，在很多情况下，这种电路将提供足够的性能时指定的电流和负载电阻小。例如，与一个4.7K的欧姆电阻器系列5V的电压源将提供一个大约1mA的恒定电流（ 5），以在50至450欧姆负载电阻范围。 3、主动电流源主动电流源在电子电路中的许多重要的应用。 （电流）稳定电阻电流源通常用于在模拟集成电路的欧姆电阻的地方产生的电流而

39、不会导致一个在信号路径的电流源连接点的衰减。一个双极晶体管的集电极，一个场效应晶体管，或一个真空管自然表现为（或汇漏电流源盘）当正确连接到外部的能源来源（如电力供应），因为输出这些设备的高阻抗，自然是当电流源配置中使用。 4、结型场效应管和N - FET电流源一款JFET可作为一所捆绑的大门，它的源电流源。目前则是流动的FET的IDSS的。这些就可以买到这个已经在此设备被称为电流稳压二极管或恒定电流二极管或限流二极管（CLD）的案件有关。一个增强型N沟道MOSFET，可用于下列电路5、简单晶体管电流源图3显示了一个典型的恒定电流源（CCS）的。 DZ1是一个齐纳二极管，当这种反向偏置（所示电路

40、），它有一个恒定的电压上，不论是流经它的电流下降。因此，只要齐纳电流（输出型）超过一定水平（称为维持电流），对面的齐纳二极管（VZ）的电压将保持不变。电阻R1用品齐纳电流和基极电流（IB）的的NPN晶体管（Q1）。恒定纳电压是适用于整个Q1和发射极电阻R2基地。电路的操作如下：R2的（VR2）电压由下式给出VE- VBE中，在VBE中是Q1基地发射极下降。Q1的发射极电流，也是经过R2的电流由下式给出图3典型恒流源由于VE不变，VBE中也（大约）某一温度恒定，可以得出VR2是恒定的，所以IE也不变。由于晶体管的作用，发射极电流IE是非常接近等于集电极电流的晶体管集成电路（反过来，是当前通过负载

41、）。因此，负载电流为常数（忽略了晶体管，由于早期的效果输出电阻）和电路作为一个恒定电流源的运作。只要温度保持不变（或变化不大），负载电流将是电源电压，R1和晶体管的增益无关。 R2的允许负载电流在任何可取的值集，并计算或,由于VBE中通常是0.65 V的硅器件（IR2也是发射极电流，并假设作为收藏家或负载所需的电流，同时提供HFE的足够大）。阻力在电阻，其中，K= 1.2到2（使R1是足够低，以确保有足够的IB）， 是最低的，特别是可以接受的类型正在使用的晶体管的电流增益。6、简单晶体管电流源与二极管补偿温度的变化会改变输出电流由图3电路交付因为VBE对温度很敏感。温度补偿的依赖可以用图4电路

42、，包括一个标准（作为晶体管的半导体材料相同）与齐纳二极管系列二极管D为在图像显示在左侧。该二极管压降（VD）的追踪VBE中由于温度变化和温度，从而大大抵消了对CCS的依赖。电阻,由于VD= VBE中= 0.65V，因此， ,R1的计算方法 图4补偿电路这种方法是最有效的齐纳二极管，在5.6 V或以上评级。对于小于5.6 V时，补偿二极管故障二极管通常不是必需的，因为击穿机理是温度依赖并不像它在上述这个电压击穿二极管的。7、简单晶体管电流源与LED另一种方法是取代轻齐纳二极管的发光二极管LED1，如图5。 LED的电压降（VD）现在用于推导恒压，也有额外的跟踪优势（补偿）VBE中温度引起的变化。

43、 R2的计算公式为, R1的计算公式为,其中ID是LED电流。 图5 LED1代替DZ1 图6反馈电路另一种常见的方法是使用反馈来设置当前和消除对晶体管的VBE中的依赖。图6显示了一个非常普遍的使用方法与非反相在上面的例子中输入连接到一个电压源和反相输入端连接到的电阻和晶体管的发射极相同的节点。这种方式产生的电压是电阻两端，而不是两个电阻器和晶体管。 （详见上的理想运算放大器的文章 - 在零器。）电流镜上的文章讨论的另一个这些所谓的收益，例如，提高了电流镜。反馈也被用于两个晶体管发射极电流镜变性。反馈是在某些电流镜使用诸如维德拉电流源和威尔逊电流源，多晶体管的基本特征。8、其他实际来源在运放电

44、路的情况下，有时候是理想的注入电流精确已知的反相输入（作为一个信号，例如输入偏移量）和源之间的电压和反相输入端连接一个电阻将接近理想值为电流源V/R。9、电感式电流源除其他应用中，图7电路采用LM317稳压器是用来向一个不断在E级电流（开关源）的电子放大器。 图7电感式电流源10、当前及电压源比较 大部分的电能的来源（主要电力，电池，.）是最好的建模为电压源。这种来源提供恒定的电压，这意味着只要当前从源头上得出的数额在源的能力，但是它的输出电压保持不变。一个理想的电压源规定，如果是由开路加载没有能源（即一个无限阻抗），但方法无限的功率和电流时，负载电阻趋近于零（短路）。这种理论的设备将有一个与

45、源系列零欧姆的输出阻抗。一个真正的世界电压源具有非常低的，但不为零输出阻抗：通常远小于1欧姆。 相反，电流源提供恒定电流，只要负载连接到源码头已经足够低的阻抗。一个理想的电流源将提供没有力气短路和方法无限的能源和负载电阻接近无穷大电压（开路）。一个理想的电流源具有同时与源无限输出阻抗。一个真正的世界电流源具有非常高，但有限的输出阻抗。在晶体管电流源情况下，一（在直流）数兆欧阻抗典型。一个理想电流源不能被连接到一个理想的开路，因为这会造成正在运行的一个常量，非零电流（从电流源）通过与定义零电流（开放的电路元件）的矛盾。也不能理想电压源连接到一个理想的短路相关（r = 0），因为这将导致对有限非零元素上定义的电压零电压类似的悖论（短路）。由于没有理想的品种或来源存在（所有现实世界的例子有限和非零源阻抗），任何电流源，可作为具有相同的源阻抗，反之亦然电压源考虑。这些概念，均由诺顿和戴维南定理。