Lecture 33Equivalent Circuit for I-SourceSmall-Signal Resistance of I-SourceImproved Current SourcesCascode (or Stacked) Current SourceDrawback of Cascode I-SourceCurrent Sinks and SourcesCurrent MirrorsMultistage AmplifiersStart: Two-Stage Voltage AmplifierAdd a Third Stage: CCUsing CMOS StagesR. T. HoweEECS 105 Spring 2002 Lecture 33Dept. of EECSUniversity of California at BerkeleyLecture 33• Last time:– Frequency response of voltage and current buffers– Voltage/Current sources using MOS transistors• Today :– Improved current sources– Current mirrorsR. T. HoweEECS 105 Spring 2002 Lecture 33Dept. of EECSUniversity of California at BerkeleyEquivalent Circuit for I-SourceFind the DC current for “gray circle” equivalent circuit()222TnREFoxnOUTVVLWCI −=µSubstitute for VREFR. T. HoweEECS 105 Spring 2002 Lecture 33Dept. of EECSUniversity of California at BerkeleySmall-Signal Resistance of I-SourceR. T. HoweEECS 105 Spring 2002 Lecture 33Dept. of EECSUniversity of California at BerkeleyImproved Current SourcesGoal: increase rocApproach: look at amplifier (?) output resistance results… to see topologies that boost resistanceR. T. HoweEECS 105 Spring 2002 Lecture 33Dept. of EECSUniversity of California at BerkeleyCascode (or Stacked) Current SourceInsight: VGS2= constant ANDVDS2= constantSmall-Signal Resistance roc:R. T. HoweEECS 105 Spring 2002 Lecture 33Dept. of EECSUniversity of California at BerkeleyDrawback of Cascode I-SourceMinimum output voltage for all transistors saturated:2,44,4, GSSATDSSSATDSMINOUTVVVVV+=+= vOUT iOUTR. T. HoweEECS 105 Spring 2002 Lecture 33Dept. of EECSUniversity of California at BerkeleyCurrent Sinks and SourcesSource: output current comesfrom voltage supplySink: output current goesto groundR. T. HoweEECS 105 Spring 2002 Lecture 33Dept. of EECSUniversity of California at BerkeleyCurrent MirrorsIdea: we only need one reference current to set up all the currentsources and sinks needed for a multistage amplifier.R. T. HoweEECS 105 Spring 2002 Lecture 33Dept. of EECSUniversity of California at BerkeleyMultistage AmplifiersNecessary to meet typical specifications for any of the 4 typesWe have 2 flavors (NMOS, PMOS) of CS, CG, and CD and thenpn versions of CE, CB, and CC (for a BiCMOS process)What are the constraints?1. Input/output resistance matching2. DC coupling (no passive elements to block the signal)R. T. HoweEECS 105 Spring 2002 Lecture 33Dept. of EECSUniversity of California at BerkeleyStart: Two-Stage Voltage Amplifier• Use two-port models to explore whether the combination “works”CE2CE1Results: Rin= Rin1, Rout= Rout2, Av=R. T. HoweEECS 105 Spring 2002 Lecture 33Dept. of EECSUniversity of California at BerkeleyAdd a Third Stage: CCGoal: reduce the output resistance(important spec. for a voltage amp)CE2CC3CE1Output resistance:R. T. HoweEECS 105 Spring 2002 Lecture 33Dept. of EECSUniversity of California at BerkeleyUsing CMOS StagesCS2CD3CS1Input resistance: Voltage gain (2-port parameter): Output
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