模拟集成电路的分析与设计：Chapter 11-Bandgap References

1、General Consideration v The best references should be: independent on supply; independent on temperature v An accurate circuit design needs an accurate voltage or current reference. v In practice, there are several kinds of references exist: 1. Supply-independent biasing; 2. Proportional to absolute

2、 temperature (PTAT) reference; 3. Constant-Gm behavior reference; 4. Temperature independent reference. v Other parameters affecting references may be: output impedance, output noise and power dissipation. Supply independent biasing (1) v Simple current mirror. 1 2 11 )/( )/( /1LW LW gR V I m DD out

3、 Iout dependent on VDD. Supply independent biasing (2) REFout KII Iout independent on VDD but Iout is not uniquely defined. Supply independent biasing (3) 2 2 ) 1 1 ( 1 )/( 2 KRLWC I S Noxn out Iout independent on VDD and is uniquely defined. v A resistor is added at source of NMOS. REFout II PMOS m

4、irror SDGSGS RIVV 221 SoutTH Noxn out TH Noxn out RIV LWKC I V LWC I 21 )/( 2 )/( 2 Supply independent biasing (4) 2 2 ) 1 1 ( 1 )/( 2 KRLWC I S Poxn out For eliminating body effect ! v A resistor is added at source of PMOS. Supply independent biasing (5) If M5 is not added, Iout = 0 satisfy the cur

5、rent 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 transistor is added to the circuit to solve “start-up” problem. PTAT References (1) v Negative-Temperature Coefficient

6、 (TC) voltage: the base- emitter voltage of bipolar transistor: T qEVmV T V gTBE BE /)4( v The TC of VBE itself also depends on the temperature; v When T=300oK, VBE750 mV, VBE/ T-1.5 mV/oK ; S C TBETBESC I I VVVVIIln)/exp( q kT VT kT E bTI gm S exp 4 PTAT References (2) v Positive-TC voltage: the di

7、fference between two base-emitter voltages: n q kT nV I I V I nI V VVV T S T S T BEBEBE ln ln lnln 2 0 1 0 21 v VBE is proportional to absolute temperature T and its TC is independent of the temperature; v We call this PTAT reference (for example, PTAT voltage or PTAT current). n q k T VBE ln PTAT R

8、eferences (3) v Add transistors to adjust coefficient: )ln( lnln 2 0 1 0 21 mnV mI I V I nI V VVV T S T S T BEBEBE Bandgap References (1) If VO1=VO2: nVVRIVVV nVVVRI VRIV TBEBEOO TBEBE BEBE ln )ln( 2221 21 21 At room temperature: KmVTV KmVTV o T o BE /087. 0/ /5 . 1/ 2 If we can make lnn=17.2, then:

9、 TVKmVnTV BE o T /5 . 1ln/ 2 And: 0/ 21 TVTV OO Zero temperature coefficient reference! Bandgap References (2) v Why called bandgap?: T nV T qEVmV nTVTVTV nVVVV T gTBE TBEREF TBEOREF ln /)4( ln/ ln 2 2 22 Let VREF/T=0, we got: Therefore: As T0: qEV gREF / Here comes the term “bandgap” /)4(ln 2 qEVmV

10、nV gTBET qEVmV gTREF /)4( Bandgap References (3) v How to keep VO1=VO2 in reality? Why not use Op Amp? 3 321 32 323 / )ln( / )( / )( / )( RnV RVV RVV RVVI T BEBE BEX BEYR Therefore: If keep lnn(1+R2/R3)=17.2, then Vout exhibits zero temperature coefficient! IR3 If VXVY then: )1 (ln )( ln )( 3 2 2 32

11、 3 2 3232 R R nVV RR R nV V RRIVV TBE T BE RBEout Bandgap References (4) v Sometimes use pnp as banndgap for compatible with CMOS technology: Bandgap References (5) v Sometimes due to asymmetries, there exists input offset for op amp, it will affect the reference output: )1)(ln( 3 2 2 R R VnVVV OSTB

12、Eout Bandgap References (6) v Use this structure can reduce the effect of offset: )1()ln(22 3 2 2 R R VmnVVV OSTBEout Increase this term to reduce the effect of VOS Bandgap References (7) v More practical one: two emitter follower in series and PMOS current sources. Collectors connected to ground, c

13、ompatible with CMOS process PTAT current (1) v PTAT current can be obtained by following structure: 121 / )ln(RnVIII TDDout VXVY v If VX=VY, then: Generating a PTAT current reference PTAT current (2) v A real implementation of PTAT current reference: 1 / )ln(RnVI TPTAT PTAT current (3) v Using PTAT

14、current to create a bandgap voltage reference: nV R R V VIRV TBE BEPTATout ln 1 2 3 32 Case study (1) v The circuit producing PTAT current reference, with two series base-emitter voltages in each branch so as to reduce the effect of MOSFET mismatch: Case study (2) v Using low-voltage cascode to redu

15、ce the power-supply dependence; v “self-biased” is introduced to produce Vb1 and Vb2 by itself: Case study (3) v A zero TC floating voltage is produced by following structure: 646 / RVI BER IR6 IR5 1 13142 125 /ln2 /)()( RnV RVVVV III T BEBEBEBE RDR v If VE=VF, then: nV R R V R R RIVRIV VVV TBE RERF

16、 outoutout ln2 )()( 1 5 4 6 4 5546 5 ,4, Proper choice of the resistor ratios and n can provides a zero TC of Vout Case study (4) v To improve the supply rejection, a local supply (局部电源） is used and a regulation circuit (稳压电路）is added: v While VR1 can be a bandgap reference produced by Core circuit: Case study (5) v The overall circuit including “start-up” circuit: Questions for Chap.11 (1) (1) How do the best references should be? (2) Tell us at least three kinds of reference in practice. (3) Calculate Iout of the circuit in Figure 11.3 and explain why it is a suppl