1 1 Lecture20-Current Mirrors EE105 Fall 2014Microelectronic Devices and CircuitsProf. Ming C. Wu wu@eecs.berkeley.edu511 Sutardja Dai Hall (SDH)2 Lecture20-Current Mirrors Common-Emitter Amplifierswith Ideal Current Source LoadVoltage gain:AV = !gm(RC|| ro) = !gmRCAV = ! ICVTRC = !VRcVTTypical values:IC~1mARC~ 5k"AV~ !200Voltage gain:AV = !gm("|| ro) = !gmroAV = ! ICVTVAIC= !VAVTTypical values:VA =100VAV~ !4000DC voltage set by the next stage Q1Rcv_isineQ2v_isineIcv_o v_o2 3 Lecture20-Current Mirrors Simple Current Source Current MirrorQ1v_isinev_oQ2 Q3VccR_refQ2 Q3VccR_ref1 IREFIONote VBE3 =VBE2For simplicity, assume VA = !IC3 = ISexp VBE3VT"#$%&' IC2 = ISexp VBE2VT"#$%&' = IC3IREF = IC3+ IB3+ IB2 = IC2 + IC2! + IC2!IO =IREF1+2 ! ( IREFQ2 and Q3 are matched (identical): Same IS

Same ! In this example, IREF =VCC !VBE3RREFSince IO " IREF this is called current mirror 4 Lecture20-Current Mirrors CE Amplifier with Current Mirror BiasQ1v_isinev_oQ2 Q3VccR_refQ2 Q3VccR_ref1 IREFIOIO =IREF1+2 ! ! IREFQ2 and Q3 form a current mirror pair AV = !gm1(ro1|| ro2)Example: IREF =1mA,!1 = !2 =100,VA1 =VA2 =100Vgm1 = 40mSro1 = ro2 =100V /1mA =100k"AV = !40mS # 50k"= !20003 5 Lecture20-Current Mirrors Mirror with Multiple Current SourcesIREFQ1VccR_refQ2Q3Q4Qn I2I3I4InFor simplicity, assume VA = !All VBE are equalIC2 = IC3 = IC4 =.... = ICnIREF = IC1+ IB1+ IB2 +... + IBnIi = IREF1+ n!When n is large, n! is not negligible One reference current can support multiple amplifiers: Circuit block e.g., amplifier 6 Lecture20-Current Mirrors Mirror with Multiple Current SourcesQ1VccR_refQ1AIREFQ2Q3Q4Qn I2I3I4InVA = !IREF = IC1+ IB1A = IC1+ IE1A1+!= IC1+ IB2 + IB3+... + IBn1+!= IC1+ nIB1+! = IC1+ IC1 / !1+!Ii = IC1 =IREF1+n!(! +1)" IREFUse a BJT to boost up supplies to multiple base currents: 4 7 Lecture20-Current Mirrors Current Mirror with Different CurrentsQ1v_isinev_oQ2 Q3VccR_refQ2 Q3VccR_ref1 IREFI2VBE3 =VBE2For simplicity, assume VA = !IC3 = IS3exp VBE3VT"#$%&' Recall IS = qAEDnWBni2NB(AE

IC2 = IS2exp VBE2VT"#$%&' =AE2AE3"#$%&'IC3IREF = IC3+ IB3+ IB2 = IC3+ IC3! + IC2!I2 =AE2AE3"#$%&'IC3 =AE2AE3"#$%&'IREF1+1 ! +" ! )AE2AE3"#$%&'IREF" = AE2AE3Q2 and Q3 have same current gain, but different area.IREF =VCC !VBE3RREF8 Lecture20-Current Mirrors VccR_refQ3 Q2Q1Current Mirrors for PNP CE-AmplifiersIREFIREF =VCC !VBE3RREFAV = !gm1(ro1|| ro2)Example: IREF =1mA,!1 = !2 =100,VA1 =VA2 =100Vgm1 = 40mSro1 = ro2 =100V /1mA =100k"AV = !40mS # 50k"= !2000vivo5 9 Lecture20-Current Mirrors MOS Current Mirrorsdc AnalysisMOSFETs M1 and M2 are assumed to have identical VTN, Kn, l, and W/L ratios. IREF provides operating bias to mirror. VDS1 = VGS1 = VGS2 =VGS VGS =VTN +2IREFKn 1+!VDS1( )IO = ID2 = Kn2VGS !VTN( )2IO = IREF10 Lecture20-Current Mirrors MOS Current MirrorsChanging the Mirror RatioMirror ratio can be changed bymodifying W/L ratios of the two transistors forming the mirror. In the given current mirror, Io = 5IREF . For non-zero #, the current ratio will vary slightly due to mismatch in VDS

Kn1 = Kn'WL!"#$%&1 Kn2 = Kn'WL!"#$%&2IO = IREFKn21+!VDS2( )Kn11+!VDS1( )For simplicity, ! ' 0IO = IREFWL!"#$%&2WL!"#$%&16 11 Lecture20-Current Mirrors Current Mirrors with Multiple Current SourcesMultiple sources can be operated from one reference transistor Reference current enters diode-connected transistor M1 establishing gate-source voltage to bias M2 through M5, each with a different W/L ratio. Absence of base current permits large numbers of MOSFETs to be driven by one reference transistor. 12 Lecture20-Current Mirrors Current MirrorsInput and Output ResistancesThe input resistance of the current mirror is set by the diode connected transistors: The output resistance is equal to the output resistance of the Q2 or M2: !Rin "1gm1!Rout = ro2