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EE 330 Lecture 31 • Current Source Biasing • Current Sources and Mirrors Basic Amplifier Gain Table Review from Last Lecture VDD VDD VDD RC RC B Vin C E Vout B VBB RE E VEE E Vin RE VEE CC/CD BJT MOS -gmRC 2IDQRC VEB Vin VSS CE/CS Rout C E Vin Rin Vout C B B RC Vout C Vout AV VDD ICQRC Vt MOS βVt ICQ BJT gm gm +gE 2IDQRE ICQRE 2IDQRE +VEB ICQRE +Vt rπ CB/CG rπ +βRE V β t RE ICQ -1 gm RC 2IDQ VEB MOS BJT CEwRE/CSwRS MOS gmRC 2IDQRC VEB - ICQRC Vt -1 gm 2IDQ VEB RC RE rπ +βRE ICQ Vt RC BJT V β t RE ICQ RC ICQ Vt Can use these equations only when circuit is EXACTLY like that shown above !! Review from Last Lecture Basic Amplifier Characteristics Summary VDD RC Vout CE/CS C B Vin E • • • • Large noninverting gain Low input impedance Moderate (or high) output impedance Used more as current amplifier or, in conjunction with CD/CS to form two-stage cascode VEE VDD • • • • C B CC/CD Vin Vout E RE VSS VDD RC Vout C CB/CG B VBB E Vin VDD RC CEwRE/ CSwRS Vout C B Vin E RE VEE Gain very close to +1 (little less) High input impedance for BJT (high for MOS) Low output impedance Widely used as a buffer • • • • Large noninverting gain Low input impedance Moderate (or high) output impedance Used more as current amplifier or, in conjunction with CD/CS to form two-stage cascode • • • • Reasonably accurate but somewhat small gain (resistor ratio) High input impedance Moderate output impedance Used when more accurate gain is required Review from Last Lecture High-gain BJT amplifier -gm AV g0 GC VDD RC Vout C B Vin E VEE -gmRC To make the gain large, it appears that all one needs to do is make RC large ! AV -ICQRC -gmRC Vt But Vt is fixed at approx 25mV and for good signal swing, ICQRC<(VDD-VEE)/2 VDD VEE AV 2Vt If VDD-VEE=5V, 5V AV 100 2 25mV Gain is practically limited with this supply voltage to around 100 Review from Last Lecture High-gain MOS amplifier -gm AV g0 GD VDD To make the gain large, it appears that all one needs to do is make RD large ! RD Vout G Vin D S VSS -gmRD AV -2IDQRD -gmRD VEB But VEB is practically limited to around 100mV and for good signal swing, IDQRD<(VDD-VSS)/2 VDD VSS AV VEB If VDD -VSS=5V and VEB=100mV, 5V AV 50 100mV Gain is practically limited with this supply voltage to around 100 Are these fundamental limits on the gain of the BJT and MOS Amplifiers? Review from Last Lecture High-gain amplifier VDD iB VOUT IB VOUT VIN VIN VIN VBE VOU gπ gmVBE Q1 Q1 -gm AV 0 VEE This gain is very large ! Too good to be true ! Need better model of MOS device! Review from Last Lecture High-gain amplifier VDD VOUT iB VOUT IB VOUT VIN VIN Q1 VIN Q1 VEE This gain is very large (but realistic) ! But how can we make a current source? VBE gπ gmVBE g0 -gm AV g0 AV -ICQ V - AF VtICQ /VAF Vt V A V - AF Vt 200V 8000 25mV High-gain amplifier VDD AV 8000 IB VOUT VIN Q1 VEE How can we build the ideal current source? What is the small-signal model of an actual current source? Current Sources/Mirrors VCC I0 Q0 AE0 VCC -0.6V R If the base currents are neglected Load R I0 I1 Q1 AE1 Current Sources/Mirrors VCC I0 Q0 AE0 VCC -0.6V R If the base currents are neglected Load R I0 I0 =JS A E0e I1 Q1 AE1 I1=JS AE1e VBE0 Vt VBE1 Vt since VBE1=VBE2 AE1 I0 AE0 Behaves as a current source ! I1 Actually termed a “sink” current since coming out of load Current Sources/Mirrors VCC R I0 Q0 AE0 I1 I1 Q1 AE1 Current Sink • • • • Multiple Outputs Possible Can be built at sourcing or sinking currents Also useful as a current amplifier MOS counterparts work very well and are not plagued by base current Current Sources/Mirrors I0 I1 Q0 AE0 Q1 I2 Q2 AE1 AE2 In Qn AEn Multiple-Output Bipolar Current Sink AEk Ik = I0 AE0 Current Sources/Mirrors VDD Q0 AE0 I0 Q1 AE1 Q2 AE2 Qn AEn I1 I2 In Multiple-Output Bipolar Current Source AEk Ik = I0 AE0 Current Sources/Mirrors VDD Qp0 AEp0 I0 Q0 AE0 Qp1 AEp1 Qp2 AEp2 Qpn AEpn Ip1 Ip2 Ipn In0 Qn1 In1 Qn2 Inn Qnn AEn1 AEn2 AEnn Multiple-Output Bipolar Current Source and Sink Ink =? Ipk =? Current Sources/Mirrors VDD Qp0 AEp0 I0 Q0 AE0 Qp1 AEp1 Qp2 AEp2 Qpn AEpn Ip1 Ip2 Ipn In0 Qn1 In1 Qn2 Inn Qnn AEn1 AEn2 AEnn Multiple-Output Bipolar Current Source and Sink AEnk Ink = I0 AE0 AEn1 AEpk Ipk = I0 AE0 AEp0 Current Sources/Mirrors Iin Q0 AE0 Iout Q1 AE1 npn Current Mirror AE1 Iout = Iin AE0 • • • • Termed a “current mirror” Output current linearly dependent on Iin Serves as a current amplifier Widely used circuit Current Sources/Mirrors iin iout IBS Q0 AE0 MIBS Q1 AE1 M= A E1 A E0 npn current mirror amplifier i out =? Current Sources/Mirrors iin iout IBS Q0 AE0 MIBS Q1 M= AE1 A E1 A E0 npn current mirror amplifier AE1 i out = iin AE0 Amplifiers both positive and negative currents Current Sources/Mirrors Iin I0 Iout IOU T Q0 AE0 Q1 M0 W0,L0 M1 W1,L1 AE1 npn Current Mirror n-channel Current Mirror Iout =? Current Sources/Mirrors Iin Iout M0 W0,L0 M1 W1,L1 n-channel Current Mirror μC W 2 Iin = OX 1 VGS1-VT1 2L 2 Iout = μ COX W2 VGS2 -VT2 2 2L 2 If process parameters are matched, it follows that W2 L1 Iout = Iin W1 L2 Current mirror gain can be accurately controlled Layout is important to get accurate gain (for both MOS and BJT) Layout of Current Mirrors Example with M = 2 W2 L1 M= W L 1 2 Standard layout W2 2W L1 2L M= W 2 W L 2 L 1 2 2W 2W L1 2L M= 1 2 W1 2W L1 2L Gate area after fabrication depicted Layout of Current Mirrors Example with M = 2 W2 L1 M= W L 1 2 2W 2W L1 2L M= 1 2 W1 2W L1 2L Standard layout 2W 4W L1 2L M= 1 2 W1 2W L1 2L Better Layout Layout of Current Mirrors Example with M = 2 W2 L1 M= W L 1 2 Standard layout 2W 4W L1 2L M= 1 2 W 2 W L 2 L 1 1 Better Layout 2W 4W L1 2L M= 1 2 W1 2W L1 2L This is termed a common-centroid layout Even Better Layout Current Sources/Mirrors iin iout IBS MIBS M= M1 W1,L1 W2 L1 W1 L 2 M2 W2,L2 n-channel current mirror current amplifier W2 L1 i out = iin W1 L2 Amplifiers both positive and negative currents Current Sources/Mirrors I0 M0 I1 I2 In M1 M2 Mn W2,L2 Wn,Ln W1,L1 W0,L0 multiple output n-channel current sink array VDD M1 M0 W1,L1 W0,L0 I0 I1 M2 W2,L2 I2 Mn Wn,Ln In multiple output p-channel current source array Wk L0 Ik = I0 W0 Lk High-gain amplifier VDD AV 8000 IB VOUT VIN Q1 VEE How can we build the current source? What is the small-signal model of an actual current source? Basic Current Sources and Sinks Basic Bipolar Current Sinks Basic Bipolar Current Sources IX VCC IX VXX IX = JS AEe VXX Vt VCC VYY IX IX VCC VCC R VCC IX IX IX R IX IX VCC -0.6V R Very practical methods for biasing the BJTs can be used Current Mirrors often used for generating sourcing and sinking currents Basic Current Sources and Sinks Small-signal Model of BJT Current Sinks and Sources IX VXX iB VBE gπ gmVBE g0 g0 Small-signal model of all other BJT Sinks and Sources are the same Basic Current Sources and Sinks Small-signal Model of BJT Current Sinks and Sources IX VXX iB VBE gπ gmVBE g0 g0 Small-signal model of all other BJT Sinks and Sources are the same Basic Current Sources and Sinks Small-signal Model of BJT Current Sinks and Sources VGS gmVGS g0 g0 Small-signal model of all other MOS Sinks and Sources are the same High-gain amplifier VDD VOUT IB VOUT VIN VIN -gm AV g0 Q1 Q1 VEE VCC iB Q2 VYY VOUT VIN VIN Q1 VEE Q1 VIN VBE1 VOUT gπ1 gm1VB g01 E1 g02 VOUT -gm1 AV g01 g02 -gm1 2g01 High-gain amplifier VDD VCC IB VYY VOUT VOUT VIN Q1 VIN Q1 VEE VEE -gm AV g0 AV -g m1 2g 01 • Nonideal current source decreased the gain by a factor of 2 • But the voltage gain is still quite large Can the gain be made even larger?
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