EECS 105 Spring 2002 Lecture 23 R T Howe EECS 105 Spring 2002 Lecture 23 Lecture 23 R T Howe Multistage Amplifiers Necessary to meet typical specifications for any of the 4 types Last time Frequency response of voltage and current buffers We have 2 flavors NMOS PMOS of CS CG and CD What are the constraints Today 1 Input output resistance matching Start multi stage amplifiers 2 DC coupling no passive elements to block the signal why not University of California Berkeley Dept of EECS EECS 105 Spring 2002 Lecture 23 R T Howe University of California Berkeley Dept of EECS EECS 105 Spring 2002 Lecture 23 R T Howe Using CMOS Stages Start Two Stage Voltage Amplifier Use two port models to explore whether the combination works CS1 CS1 CS2 CD3 CS2 Input resistance Voltage gain 2 port parameter Output resistance Results Rin Rin1 Rout Rout2 Av Dept of EECS University of California Berkeley Dept of EECS University of California Berkeley 1 EECS 105 Spring 2002 Lecture 23 R T Howe EECS 105 Spring 2002 Lecture 23 Multistage Current Buffers R T Howe Common Gate 2nd Stage Are two cascaded common base stages better than one CG2 CG1 Rout Rout 2 r02 1 g m 2 RS 2 roc 2 Input resistance Rin Rin1 University of California Berkeley Dept of EECS EECS 105 Spring 2002 Lecture 23 R T Howe Summary of Cascaded Amplifiers 1 Boost the gain parameter except for buffers 2 Optimize the input and output resistances Dept of EECS University of California Berkeley EECS 105 Spring 2002 Lecture 23 R T Howe Second Design Issue DC Coupling Constraint large inductors and capacitors are not available General goals Voltage Current Transconductance Transresistance Dept of EECS Rin Output of one stage is directly connected to the input of the next stage must consider DC levels why Rout University of California Berkeley Dept of EECS University of California Berkeley 2 EECS 105 Spring 2002 Lecture 23 R T Howe EECS 105 Spring 2002 Lecture 23 Alternative CG CC Cascade R T Howe CG Cascade DC Biasing Use a PMOS CD Stage DC level shifts upward Two stages can have different supply currents Extreme case IBIAS2 0 A Dept of EECS EECS 105 Spring 2002 Lecture 23 University of California Berkeley Dept of EECS University of California Berkeley R T Howe CG Cascade Sharing a Supply First stage has no current supply of its own its output resistance is modified Dept of EECS University of California Berkeley 3
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