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 output resistance of the amplifier EE 105 Fall 2001 Page 1 Lecture 36 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 VG4 EE 105 Fall 2001 Page 2 Lecture 36 Totem Pole Voltage Reference Match device sizes of M2B M3B and M4B to M2 M3 and M4 EE 105 Fall 2001 Page 3 Lecture 36 Complete Transconductance Amplifier VBIAS user must supply a very precise DC voltage V BIAS 1 2 V so that the CS CG cascode is biased so that it is in the high gain region CS CG two port parameters Gm gm1 Output 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 V VOUT min VD1 VDS2 sat VG2 VGS2 VDS2 sat 2 V 1 4 V 0 4 V 1 V EE 105 Fall 2001 Page 4 Lecture 36 Example 2 Multistage Voltage Amplifier Example for dissecting a complicated circuit 10 MOSFETs 3 BJTs 1 resistor must identify building blocks Step 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 smallsignal 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 5 Lecture 36 Eliminating Current and Voltage Sources Replace current and voltage sources with symbols VDD 5 V ID5 ID6 Q4 M3 Q2 VB2 vOUT RS vs M1 VBIAS EE 105 Fall 2001 Page 6 ID10 Lecture 36 Identifying Amplifier Stages n channel MOSFET M1 has its source grounded common source npn 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 drain npn BJT Q4 has its collector tied to the positive supply common collector Voltage amplifier is a cascade of two port models CS Rin CB CD Rout CS CB CC Rout 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 stage 2 common drain no loading on previous stage since infinite input resistance 3 common collector low output resistance EE 105 Fall 2001 Page 7 Lecture 36 Cascode Stage Output Resistance Cascode input stage output resistance determines gain VDD 5 V ID6 Rout CB CS Q2 VB2 RS2 RS M1 VBIAS n Output resistance note that RS2 ro1 r 2 Rout CB o2 ro2 r oc6 o r o2 ro6 1 g m6 r o7 EE 105 Fall 2001 Page 8 Lecture 36 CS 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 stage n Voltage gain A v g m1 R out CB g m1 o r o2 r o6 1 g m6 r o7 n Output resistance source resistance of CC output stage is relatively small since it preceded by a CD stage R S CC 1 1 1 R out R out CC o4 g m4 g m4 g m3 o4 EE 105 Fall 2001 Page 9 Lecture 36 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 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 level shifters EE 105 Fall 2001 Page 10 Lecture 36
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