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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Widlar current source ： ウィキペディア英語版 Widlar current source A Widlar current source is a modification of the basic two-transistor current mirror that incorporates an emitter degeneration resistor for only the output transistor, enabling the current source to generate low currents using only moderate resistor values. The Widlar circuit may be used with bipolar transistors, MOS transistors, and even vacuum tubes. An example application is the 741 operational amplifier, and Widlar used the circuit as a part in many designs.〔See, for example, Figure 2 in (''IC voltage regulators'' ).〕 This circuit is named after its inventor, Bob Widlar, and was patented in 1967.〔RJ Widlar: US Patent Number 03320439; Filed May 26, 1965; Granted May 16, 1967: (Low-value current source for integrated circuits'' ) 〕〔See Widlar: (''Some circuit design techniques for linear integrated circuits'' ) and (''Design techniques for monolithic operational amplifiers'' )〕 ==Analysis== Figure 1 is an example Widlar current source using bipolar transistors, where the emitter resistor R2 is connected to the output transistor Q2, and has the effect of reducing the current in Q2 relative to Q1. The key to this circuit is that the voltage drop across the resistor ''R''2 subtracts from the base-emitter voltage of transistor ''Q''2, thereby turning this transistor off compared to transistor ''Q''1. This observation is expressed by equating the base voltage expressions found on either side of the circuit in Figure 1 as: :$V\_B\; =\; V\_\; =\; V\_\; +\; (\backslash beta\_2\; +\; 1)I\_R\_2\; \backslash \; ,$ where β2 is the beta-value of the output transistor, which is not the same as that of the input transistor, in part because the currents in the two transistors are very different. The variable ''I''B2 is the base current of the output transistor, ''V''BE refers to base-emitter voltage. This equation implies (using the Shockley diode law): ''Eq. 1'' :$(\backslash beta\_2\; +\; 1)I\_\; =\; \backslash left(1\; +\; \backslash frac\; \backslash right)\; I\_\; =\; \backslash frac\}\; =\; \backslash frac\; \backslash ln\; \backslash left(\backslash frac\; \}\}\backslash right)\backslash \; ,$ where ''V''T is the thermal voltage. This equation makes the approximation that the currents are both much larger than the scale currents, ''I''S1 and ''I''S2; an approximation valid except for current levels near cut off. In the following, the scale currents are assumed to be identical; in practice, this needs to be specifically arranged. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Widlar current source」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.