R. T. HoweEECS 105 Spring 2002 Lecture 24Dept. of EECSUniversity of California, BerkeleyLecture 24• Last time:– Multi-stage amplifiers: voltage, transconductance• Today :– Cascode: merged CS/CG cascadeR. T. HoweEECS 105 Spring 2002 Lecture 24Dept. of EECSUniversity of California, BerkeleyCG Cascade: Sharing a SupplyFirst stage has no currentsupply of its own Æ its outputresistance is modifiedR. T. HoweEECS 105 Spring 2002 Lecture 24Dept. of EECSUniversity of California, BerkeleyMultistage Amplifier Design ExamplesStart with basic two-stage transconductance amplifier:Why do this combination?R. T. HoweEECS 105 Spring 2002 Lecture 24Dept. of EECSUniversity of California, BerkeleyTwo-Stage Amplifier TopologyDirect DC connection: use NMOS then PMOSR. T. HoweEECS 105 Spring 2002 Lecture 24Dept. of EECSUniversity of California, BerkeleyCurrent Supply DesignAssume that the reference is a “sink” set by a resistorMust mirror the reference current and generate a sink for iSUP 2R. T. HoweEECS 105 Spring 2002 Lecture 24Dept. of EECSUniversity of California, BerkeleyUse Basic Current SuppliesR. T. HoweEECS 105 Spring 2002 Lecture 24Dept. of EECSUniversity of California, BerkeleyComplete Amplifier TopologyWhat’s missing? The device dimensions and the biasvoltage and reference resistorR. T. HoweEECS 105 Spring 2002 Lecture 24Dept. of EECSUniversity of California, BerkeleyThe Cascode ConfigurationDC bias: Two-port model: first stage has no current supply of its ownCommon source / common gatecascade is one version of a cascode(all have shared supplies)R. T. HoweEECS 105 Spring 2002 Lecture 24Dept. of EECSUniversity of California, BerkeleyCascode Two-Port Model CS1* CG2Output resistance of first stage = 1,,*oCSdownCSoutrRR==Why is the cascode such an important configuration?R. T. HoweEECS 105 Spring 2002 Lecture 24Dept. of EECSUniversity of California, BerkeleyMiller Capacitance of Input StageFind the Miller capacitance for Cgd1Input resistance to common-gatesecond stage is low Æ gain acrossCgd1 is small.R. T. HoweEECS 105 Spring 2002 Lecture 24Dept. of EECSUniversity of California, BerkeleyTwo-Port Model with CapacitorsMiller capacitance:1)1(1gdvCMCACgd−=R. T. HoweEECS 105 Spring 2002 Lecture 24Dept. of EECSUniversity of California, BerkeleyOther Cascode ConfigurationsCSn-CBCEn-CGnBasic configuration: transconductance stage followed by current
View Full Document