240ln11 - SUPPLY INSENSITIVE BIASING LN 11 One figure of...

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1 SUPPLY INSENSITIVE BIASING LN 11 One figure of merit for the current mirror current sources is to make them independent of the supply voltage or to make them as insensitive to power supply variations as possible Therefore the goal is to reduce cc out out cc I V V I I V S out cc = I out v o -V EE Q2 Q1 As an example BJT Current Mirror R V CC I C1 I o = I IN = [V CC –V BE(on) ]/R sup sup sup V I I V S out out Iout p V = If V cc >> V BE(on) then 1 sup Iout p V S Another example is the Widlar current mirror for which IN DD I V Iout p V S S 5 . 0 sup Clearly other designs are needed for supply insensitivity One approach is to use a base emitter reference with BJT’s Or a Vt reference with MOSFETs
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2 Q1 I o R 2 Q2 I REF I c1 This source is similar to the Wilson (see ln 9, page 6) except that R2 replaces the diode connected Q3x Base-Emitter Reference Bipolar Current Source V cc 1 ln 2 ln 2 1 1 1 IS I R V R V I I I V V IN T BE o S IN T BE = = > = Iin Vcc on BE T IIN Vcc o T Iout Vcc S V V S R I V S ) ( 2 * = = If V cc >> 2V BE(on) , I IN V cc /R 1 , so R 1 1 IN cc I V S If V BE(on) =0.7V then 037 . 0 ) ( 2
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240ln11 - SUPPLY INSENSITIVE BIASING LN 11 One figure of...

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