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28_Lecutre - EE 114 Lecture 28 Lecture 28 Bandgap Reference...

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EE 114 Lecture 28 R. Dutton, B. Murmann 1 EE114 (HO #32) Lecture 28 Bandgap Reference R. Dutton, Boris Murmann Stanford University R. Dutton, B. Murmann 2 EE114 (HO #32) Key Idea kT/q has a positive temperature coefficient "PTAT" proportional to absolute temperature V BE of a BJT decreases with temperature "CTAT" complementary to absolute temperature Can combine PTAT + CTAT to yield an approximately zero TC voltage reference Useful in circuits that require a stable reference voltage E.g. A/D converters
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EE 114 Lecture 28 R. Dutton, B. Murmann 3 EE114 (HO #32) Conceptual Block Diagram (figure from text, Gray & Meyer; assuming conventional BJT technology with NPN and PNP) R. Dutton, B. Murmann 4 EE114 (HO #32) Simple CMOS Realization n R 1 R 2 1 V DD Q 1 Q 2 I 1 I 2 V out (practical implementation in MOS technology; only substrate PNP available)
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EE 114 Lecture 28 R. Dutton, B. Murmann 5 EE114 (HO #32) Recall the Substrate PNP A special twist that the technology provides: a three-terminal bipolar device with the collector (“C”) always connected to substrate p+ n-well p-substrate n+ +V BE space charge + V BE - I S e qV BE kT I S " F e qV BE kT Typically " F >>1 hence the substrate component dominates R. Dutton, B. Murmann 6 +V BE I S e qV BE kT I S " F e qV BE kT diode equiv. Diode-connected BJT (special case sub-PNP) +V BE I D = I S 1 + 1 " F # $ % & ( e qV BE kT ) I S e qV BE kT Simplified Equivalent EE114 (HO #32) Solve for V BE as function of I S and I D
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EE 114 Lecture 28 R. Dutton, B. Murmann 7 EE114 (HO #32) A Closer Look at V BE Even though kT/q increases with temperature, V BE decreases because I S itself strongly depends on temperature ! ! " # $ $ % & = S C BE I I ln q kT V ! ! " # $ $ % & ( ! ! " # $ $ % & ( C 0 0 G ) q / kT /( V 0 C BE I I ln q kT V e I I ln q kT V 0 G I 0 is a device parameter, which is (unfortunately) not completely independent of temperature We'll ignore this for now V G0 is the bandgap voltage of silicon "extrapolated to 0°K" (we’ll call the diode current I C ) I S " A Emitter # n i 2 I S = f ( T ) e $ V G 0 kT / q % & ( ) * I 0 + f ( T ) (details, EE216) R. Dutton, B. Murmann 8 EE114 (HO #32) Extrapolated Bandgap 1.205eV V 205 . 1 q eV 205 . 1 V 0 G = = [Pierret, Advanced Semiconductor Fundamentals, p.85]
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EE 114 Lecture 28 R. Dutton, B. Murmann 9 EE114 (HO #32) Temperature Coefficient of V BE Assuming that both I 0
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