電子專業(yè)(畢業(yè)小論文外文資料及翻譯譯文-資料下載頁
2024-12-03 20:51本頁面
【導(dǎo)讀】電流源是電氣或電子裝置，可提供或吸收電流。一個電流源是一個電壓源雙。術(shù)語恒流源有時用來從一個負(fù)電壓電源饋來源。圖1顯示了一個理想的電流源驅(qū)動的電阻負(fù)載的原理圖。在電路理論，理想電流源電路元件的電流通過時與其兩端的電壓無關(guān)。這是一個數(shù)學(xué)模型。如果通過一個理想的電流源電流可以指定獨(dú)立于任何其他變量的電路，它被稱為一個獨(dú)立的電流源。這些源符號，如圖2所示?；谶@個原因，一個理想電流源內(nèi)阻是無限的。當(dāng)連接到短路，存在零電壓，從而零功率交付。對于一個幾近完美的電流源，這個電阻值應(yīng)該是非常大的，但是這意味著，在規(guī)定的電流，電壓源必須是非常大的。盡管如此，在很多情況下，這種電路將提供足夠的性能時指定的電流和負(fù)載電阻小。例如，，以在50至450歐姆負(fù)載電阻范圍。目前則是流動的FET的IDSS的。R2的電壓由下式給出VE-VBE中，在VBE中是Q1基地發(fā)射極下降。這種方法是最有效的齊納二極管，V或以上評級。
【正文】 . (For details, see the article on the ideal op amp the nullor.) The article on current mirror discusses another example of these socalled gainboosted current mirrors.Other practical sourcesIn the case of opamp circuits sometimes it is desired to 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 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 parisonMost sources of electrical energy (mains electricity, a battery, ...) are best modeled as voltage sources. Such sources provide constant voltage, which means that as long as the amount of current drawn from the source is within the source39。s capabilities, its output voltage stays constant. An ideal voltage source provides no energy when it is loaded by an open circuit (. an infinite impedance), but approaches infinite 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 realworld voltage source has a very low, but nonzero output impedance: 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 load resistance approaches infinity (an open circuit). An ideal current source has an infinite output impedance in parallel with the source. A realworld 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 because this would create the paradox of running a constant, nonzero 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 zero voltage across an element with defined zero voltage (the short circuit).Because no ideal sources of either variety exist (all realworld examples have finite and nonzero source impedance), any current source can be considered as a voltage source with the same source impedance and vice versa. These concepts are dealt with by Norton39。s and Th233。venin39。s theorems.
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