Lecture 36 More Ampllifier Examples* Example 1: CMOS Transconductance Amplifier* Current supply must have a very high source resistance roc since otherwise it will limit the ou...Current Supply Topology* p-channel cascode current supply is an obvious solution* need to design a totem pole voltage supply to generate VG2, VG3, and VG4Totem Pole Voltage Reference* Match device sizes of M2B, M3B, and M4B to M2, M3, and M4Complete Transconductance Amplifier* VBIAS: user must supply a very precise DC voltage so that the CS/CG cascode is biased so that i...CS-CG two-port parameters: Gm = gm1Output resistance: Rout = ro2(1 + gm2 ro1) || ro3(1 + gm3 ro4)Output swing:VOUT(max) = VD4 - VSD3(sat) = VDD - VSG4B - VSG3B + VSG3 - VSD3(sat)VOUT(max) = 5 V - 1.5 V - 1.5 V + 1.5 V - 0.5 V = 3.0 VVOUT(min) = VD1 + VDS2(sat) = VG2 - VGS2 + VDS2(sat) = 2 V - 1.4 V + 0.4 V = 1 VExample 2: Multistage Voltage AmplifierExample for “dissecting” a complicated circuit* 10 MOSFETs, 3 BJTs, 1 resistor ... must identify building blocksStep-by-step approach to identifying the “important” transistors --1. replace all transistor current sources and voltage sources by their symbols -- look for diodes...2. for the (few) remaining transistors, identify the type and use two-port small- signal models t...Eliminating Current and Voltage Sources* Replace current and voltage sources with symbolsIdentifying Amplifier Stagesn-channel MOSFET M1 has its source grounded --> common sourcenpn BJT Q2 has its gate tied to a voltage source (from “totem pole” string of diode-connected tra...p-channel MOSFET M3 has its drain connected to ground --> common drainnpn BJT Q4 has its collector tied to the positive supply --> common collectorVoltage amplifier is a cascade of two-port models:1. common source/common base with cascoded current-source supply: very high output resistance Rou...2. common drain: no loading on previous stage since infinite input resistance3. common collector: low output resistanceCascode Stage Output Resistance* Cascode input stage output resistance determines gainn Output resistance: note that RS2 = ro1 >> rp2CS-CB-CD-CC Two-Port Parameters* Since CC and CD stages have unity gain (approximately), we can quickly estimate the voltage gai...n Voltage gain:n Output resistance: source resistance of CC output stage is relatively small, since it preceded ...DC Bias and Output Swing* Assuming all n-channel devices have VGS = 1.5 V and p-channel devices have VSG = 1.5 V, we can ...* Output swing: must consider the limited swing of previous stages (back to cascode) since the th...EE 105 Fall 2001 Page 1 Lecture 36Lecture 36 More Ampllifier Examples* Example 1: CMOS Transconductance Amplifier * Current supply must have a very high source resistance roc since otherwise it will limit the output resistance of the amplifierEE 105 Fall 2001 Page 2 Lecture 36Current Supply Topology* p-channel cascode current supply is an obvious solution* need to design a totem pole voltage supply to generate VG2, VG3, and VG4EE 105 Fall 2001 Page 3 Lecture 36Totem Pole Voltage Reference* Match device sizes of M2B, M3B, and M4B to M2, M3, and M4EE 105 Fall 2001 Page 4 Lecture 36Complete Transconductance Amplifier* VBIAS: user must supply a very precise DC voltage so that the CS/CG cascode is biased so that it is in the high gain regionCS-CG two-port parameters: Gm = gm1Output resistance: Rout = ro2(1 + gm2 ro1) || ro3(1 + gm3 ro4)Output swing: VOUT(max) = VD4 - VSD3(sat) = VDD - VSG4B - VSG3B + VSG3 - VSD3(sat)VOUT(max) = 5 V - 1.5 V - 1.5 V + 1.5 V - 0.5 V = 3.0 VVOUT(min) = VD1 + VDS2(sat) = VG2 - VGS2 + VDS2(sat) = 2 V - 1.4 V + 0.4 V = 1 VVBIAS1.2 V≈EE 105 Fall 2001 Page 5 Lecture 36Example 2: Multistage Voltage AmplifierExample for “dissecting” a complicated circuit* 10 MOSFETs, 3 BJTs, 1 resistor ... must identify building blocksStep-by-step approach to identifying the “important” transistors --1. replace all transistor current sources and voltage sources by their symbols -- look for diodes and current mirrors! (M5, M6/M6B, M7/M7B, and M10 and Q2B are part of current sources or a totem pole voltage reference.)2. for the (few) remaining transistors, identify the type and use two-port small-signal models to understand the circuit’s operation. (For the above amplifier, the remaining transistors are M1, Q2, M3, and Q4.)EE 105 Fall 2001 Page 6 Lecture 36Eliminating Current and Voltage Sources* Replace current and voltage sources with symbols+_+_vsVBIASRSVDD = 5 VQ4M3Q2VB2ID10-ID5 -ID6vOUTM1EE 105 Fall 2001 Page 7 Lecture 36Identifying Amplifier Stagesn-channel MOSFET M1 has its source grounded --> common sourcenpn BJT Q2 has its gate tied to a voltage source (from “totem pole” string of diode-connected transistors) --> common base p-channel MOSFET M3 has its drain connected to ground --> common drainnpn BJT Q4 has its collector tied to the positive supply --> common collectorVoltage amplifier is a cascade of two-port models:1. common source/common base with cascoded current-source supply: very high output resistance Rout(CS/CB) --> can get extremely high output resistance, with a transconductance equal to that of the CS stage2. common drain: no loading on previous stage since infinite input resistance 3. common collector: low output resistanceCSRinRout(CS/CB)RoutCB CD CCEE 105 Fall 2001 Page 8 Lecture 36Cascode Stage Output Resistance* Cascode input stage output resistance determines gainn Output resistance: note that RS2 = ro1 >> rπ2+_VBIASRSVDD = 5 VQ2VB2-ID6M1Rout,CB/CSRS2Rout,CBβo2ro2()roc6βoro2()ro61 gm6ro7+()()==EE 105 Fall 2001 Page 9 Lecture 36CS-CB-CD-CC Two-Port Parameters* Since CC and CD stages have unity gain (approximately), we can quickly estimate the voltage gain by finding vin3/vin where vin3 is the input to the CD stagen Voltage gain:n Output resistance: source resistance of CC output stage is relatively small, since it preceded by a CD stage.Avgm1–()Rout,CB≈gm1βoro2()ro61 gm6ro7+()()()–=RoutRout,CC1gm4---------RS,CC βo4-----------------+1gm4---------1gm3βo4-------------------+=≈=EE 105 Fall 2001 Page 10 Lecture 36DC Bias and Output Swing* Assuming all n-channel devices have VGS = 1.5 V and p-channel devices have VSG = 1.5 V, we can find all the node voltages ... we also assume that VBIAS has been adjusted such that the circuit is in the high-gain region* Output swing: must consider the limited swing of previous stages (back to cascode) since the the CD/CC output stages are “DC
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