1、CURRENT SOURCE A 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
2、current source driving a resistor load. Figure 1 Ideal current sources 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
3、 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 controll
4、ed current source. Symbols for these sources are shown in Figure 2. Figure 2 An 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 completel
5、y 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). Thus, an ideal
6、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 source is ideal (
7、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 source (the
8、Norton equivalent circuit). Resistor current source The 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 curren
9、t 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. Nonetheless, it is o
10、ften 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 50 to 450
11、 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 the signal
12、 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 the outp
13、ut impedance of these devices is naturally high when used in the current source configuration. JFET and N-FET current source A 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 alr
14、eady 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 source Figure 3 shows a typical constant current source (CC
15、S). 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) will
16、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 base-e
17、mitter drop of Q1. The emitter current of Q1 which is also the current through R2 is given by22)( 22 R VVRVII BEERER . Figure 3 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 transistor acti
18、on, 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 Early effect) and the circuit operates as a constant current s
19、ource. As long as the 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 2R2 RBEE I VV or 265.02RZ IVR , since VBE is typically 0
20、.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 R1 at resistor R1 is calculated asBZZS IKI VVR *1 ,where, K = 1.2 to 2 (so that R1 is low enough to ensure adequate IB
21、), (min )2 )(FERECB h IIII ,and hFE(min) is the lowest acceptable current gain for the particular transistor type being used. A more common current source in integrated circuits is the current mirror. Simple transistor current source with diode compensation Temperature changes will change the output
22、 current delivered by the circuit of Figure 3 because VBE is sensitive to temperature. Temperature 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 significantly counteracts temperature dependence of the CCS. Resistance R2 is now calculated as2R2 RBEzz I VVV Since VD = VBE = 0.65 V,
