【正文】 urrent 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.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 pletely 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 current source could supply unlimited power forever and so would represent an unlimited source of energy. Connecting an ideal open circuit to an ideal nonzero current source is not valid in circuit analysis as the circuit equation would be paradoxical, ., 5 = 0.No real current source 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 bining a nonzero resistance in parallel with an ideal current 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 ideal 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 39。由于沒有理想的品種或來源存在（所有現(xiàn)實世界的例子有限和非零源阻抗），任何電流源，可作為具有相同的源阻抗，反之亦然電壓源考慮。一個理想電流源不能被連接到一個理想的開路，因為這會造成正在運行的一個常量，非零電流（從電流源）通過與定義零電流（開放的電路元件）的矛盾。一個真正的世界電流源具有非常高，但有限的輸出阻抗。一個理想的電流源將提供沒有力氣短路和方法無限的能源和負載電阻接近無窮大電壓（開路）。一個真正的世界電壓源具有非常低的，但不為零輸出阻抗：通常遠小于1歐姆。一個理想的電壓源規(guī)定，如果是由開路加載沒有能源（即一個無限阻抗），但方法無限的功率和電流時，負載電阻趨近于零（短路）。 圖7電感式電流源當前及電壓源比較 大部分的電能的來源（主要電力，電池，...）是最好的建模為電壓源。其他實際來源在運放電路的情況下，有時候是理想的注入電流精確已知的反相輸入（作為一個信號，例如輸入偏移量）和源之間的電壓和反相輸入端連接一個電阻將接近理想值為電流源V/R。反饋也被用于兩個晶體管發(fā)射極電流鏡變性。 （詳見上的理想運算放大器的文章 在零器。圖6顯示了一個非常普遍的使用方法與非反相在上面的例子中輸入連接到一個電壓源和反相輸入端連接到的電阻和晶體管的發(fā)射極相同的節(jié)點。 R2的計算公式為, R1的計算公式為,其中ID是LED電流。簡單晶體管電流源與LED另一種方法是取代輕齊納二極管的發(fā)光二極管LED1，如圖5。電阻,由于VD= VBE中= ，因此， ,R1的計算方法 圖4補償電路這種方法是最有效的齊納二極管， V或以上評級。溫