ChapterBandgapReferences学习教案

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1、会计学1ChapterBandgapReferences第一页，共24页。v Simple current mirror.1211)/()/(/1LWLWgRVImDDout Iout dependent on VDD.第1页/共24页第二页，共24页。REFoutKII Iout independent on VDD but Iout is not uniquely defined.第2页/共24页第三页，共24页。22)11 (1)/(2KRLWCISNoxnout Iout independent on VDD and is uniquely defined, Fig. 11.3 (b)

2、 is for eliminating body effect.v A resistor is added.第3页/共24页第四页，共24页。 If M5 is not added, Iout = 0 satisfy the current mirror conditon, in this case, all transistor will be in off state, this problem is called “start-up” problem.When M5 is added, this will not happen again.v A diode connected tran

3、sistor is added to circuit to solve “start-up” problem.第4页/共24页第五页，共24页。v Negative-Temperature Coefficient (TC) voltage: the base-emitter voltage of bipolar transistor:TqEVmVTVgTBEBE/)4(v The TC of VBE itself also depends on the temperature;v When T=300oK, VBE750 mV, VBE/ T-1.5 mV/oK ;SCTBETBESCIIVV

4、VVIIln)/exp(qkTVTkTEbTIgmSexp4第5页/共24页第六页，共24页。v Positive-TC voltage: the difference between two base-emitter voltages:nqkTnVIIVInIVVVVTSTSTBEBEBElnlnlnln201021v VBE is proportional to absolute temperature T and its TC is independent of the temperature;v We call this PTAT reference (for example, PTA

5、T voltage or PTAT current).nqkTVBEln第6页/共24页第七页，共24页。v Add transistors to adjust coefficient:)ln(lnln201021mnVmIIVInIVVVVTSTSTBEBEBE第7页/共24页第八页，共24页。v If VO1=VO2:nVVRIVVVnVVVRIVRIVTBEBEOOTBEBEBEBEln)ln(22212121v At room temperature:KmVTVKmVTVoToBE/087. 0/5 . 1/2v If we can make lnn=17.2, then:TVKmVn

6、TVBEoT/5 . 1ln/2v And:0/21TVTVOO Zero temperature coefficient reference!第8页/共24页第九页，共24页。v Why called bandgap?:TnVTqEVmVnTVTVTVnVVVTgTBETBEREFTBEREFln/)4(ln/lnv Let VREF/T=0, we got:v Therefore:v As T0:qEVgREF/ Here comes the term “bandgap”/)4(lnqEVmVnVgTBETqEVmVgTREF/)4(第9页/共24页第十页，共24页。v How to ke

7、ep VO1=VO2 in reality? v Why not use Op Amp?332132323/ )ln(/ )(/ )(/ )(RnVRVVRVVRVVITBEBEBEXBEYRv Therefore: If keep lnn(1+R2/R3)=17.2, then Vout exhibits zero temperature coefficient!IR3v If VXVY then:)1 (ln)(ln)(32232323232RRnVVRRRnVVRRIVVTBETBERBEout第10页/共24页第十一页，共24页。v Sometimes use pnp as bannd

8、gap for compatible with CMOS technology:第11页/共24页第十二页，共24页。v Sometimes due to asymmetries, there exists input offset for op amp, it will affect the reference output:)1)(ln(322RRVnVVVOSTBEout第12页/共24页第十三页，共24页。v Use this structure can reduce the effect of offset:)1()ln(22322RRVmnVVVOSTBEoutIncrease t

9、his term to reduce the effect of VOS第13页/共24页第十四页，共24页。v More practical one: two emitter follower in series and PMOS current sources.Collectors connected to ground, compatible with CMOS process第14页/共24页第十五页，共24页。v PTAT current can be obtain by following structure:121/ )ln(RnVIIITDDoutVXVYv If VX=VY,

10、 then: Generating a PTAT current reference第15页/共24页第十六页，共24页。v A real implementation of PTAT current reference:1/ )ln(RnVITPTAT第16页/共24页第十七页，共24页。v Using PTAT current to create a bandgap voltage reference:nVRRVVIRVTBEBEPTAToutln12332第17页/共24页第十八页，共24页。v The circuit producing PTAT current reference,

11、with two series base-emitter voltages in each branch so as to reduce the effect of MOSFET mismatch:第18页/共24页第十九页，共24页。v Using low-voltage cascode to reduce the power-supply dependence;v “self-biased” is introduced to produce Vb1 and Vb2 by itself:第19页/共24页第二十页，共24页。v A zero TC foating voltage is pro

12、duced by following structure:646/ RVIBERIR6IR5113142125/ln2/)()(RnVRVVVVIIITBEBEBEBERDRv If VE=VF, then:nVRRVRRRIVRIVVVVTBERERFoutoutoutln2)()(1546455465 ,4, Proper choice of the resistor ratios and n can provides a zero TC of Vout第20页/共24页第二十一页，共24页。v To improve the supply rejection, a local supply (局部电源(dinyun)） is used and a regulation circuit (稳压电路）is added:v While VR1 can be a bandgap reference produced by Core circuit:第21页/共24页第二十二页，共24页。v The overall circuit including “start-up” circuit:第22页/共24页第二十三页，共24页。Q11.1;Q11.2;Q11.7.第23页/共24页第二十四页，共24页。