电信 电流 外文翻译 外文文献恒流源.docx

1、电信 电流 外文翻译 外文文献 恒流源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 schem

2、atic for an ideal current 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.

3、 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 depend

4、ent or controlled current 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

5、 is completely determined 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). Th

6、us, an ideal current 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 sourc

7、e is ideal (no unlimited 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

8、source (the Norton 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 id

9、eal current source, 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. Nonetheles

10、s, 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 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

11、 50 to 450 ohms.Active 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 t

12、he signal path to which 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

13、 the output impedance 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 connect

14、ion already made and 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 sourc

15、e (CCS). DZ1 is a zener 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)

16、will be constant. Resistor 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 ba

17、se-emitter drop of 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

18、 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 Early effect) and the circuit operates as a constant current source. As long as the

19、temperature remains 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 emitt

20、er current and is 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 par

21、ticular transistor 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. Temper

22、ature dependence can 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 significa

23、ntly counteracts temperature 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 b

24、reakdown mechanism 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

25、 voltage and also 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.

26、Figure 6 shows a 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 resi

27、stor, rather than 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

28、 inject a precisely 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

29、 of Figure 7 using 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 sourc

30、es provide constant 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 inf

31、inite power and 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

32、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 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 con