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Berkeley ELENG 105 - Lecture 8

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Lecture 8Emitter DegenerationSmall-Signal AnalysisEmitter Degeneration Example 1Emitter Degeneration Example 2Input Impedance of Degenerated CE StageOutput Impedance of Degenerated CE StageDegenerated CE Stage as a “Black Box”Degenerated CE Stage with Base ResistanceDegenerated CE Stage: Input/Output ImpedancesEmitter Degeneration Example 3Output Impedance of Degenerated CE Stage with VA<∞Two Special CasesAnalysis by InspectionExample: Degeneration by Another BJTBad Input ConnectionUse of Coupling CapacitorDC and AC AnalysisBad Output ConnectionUse of Coupling Capacitor at OutputCE Stage with Voltage-Divider BiasingCE Stage with Robust BiasingElimination of Emitter Degeneration for AC SignalsComplete CE StageSummary of CE ConceptsEE105 Fall 2007 Lecture 8, Slide 1 Prof. Liu, UC BerkeleyLecture 8OUTLINE•BJT Amplifiers (cont’d)–Common-emitter topology–CE stage with emitter degeneration–Impact of Early effect (ro)Reading: Finish Chapter 5.3.1ANNOUNCEMENTS•A summary of frequently misunderstood/missed concepts is now posted on the class website, and will be updated regularly.•Graded HW assignments can be picked up in lab (353 Cory).  Please indicate your lab section on your HW assignments!•Midterm #1 Review: 3-5PM in 306 Soda – will be archived onlineEE105 Fall 2007 Lecture 8, Slide 2 Prof. Liu, UC BerkeleyEmitter Degeneration•By inserting a resistor in series with the emitter, we “degenerate” the CE stage. •This topology will decrease the gain of the amplifier but improve other aspects, such as linearity, and input impedance.EE105 Fall 2007 Lecture 8, Slide 3 Prof. Liu, UC BerkeleySmall-Signal Analysis•The gain of a degenerated CE stage = the total load resistance seen at the collector divided by 1/gm plus the total resistance placed in series with the emitter. EmCEmCmvRgRRgRgA11EE105 Fall 2007 Lecture 8, Slide 4 Prof. Liu, UC BerkeleyEmitter Degeneration Example 1Note that the input impedance of Q2 is in parallel with RE. 21||1rRgRAEmCvEE105 Fall 2007 Lecture 8, Slide 5 Prof. Liu, UC BerkeleyEmitter Degeneration Example 2Note that the input impedance of Q2 is in parallel with RC.EmCvRgrRA121||EE105 Fall 2007 Lecture 8, Slide 6 Prof. Liu, UC BerkeleyInput Impedance of Degenerated CE Stage•With emitter degeneration, the input impedance is increased from r to r + (+1)RE ― a desirable effect.ExxinxExxARrivRiRirvV)1()1()(EE105 Fall 2007 Lecture 8, Slide 7 Prof. Liu, UC BerkeleyOutput Impedance of Degenerated CE Stage•Emitter degeneration does not alter the output impedance, if the Early effect is negligible.CxxoutEminARivRvRvgrvvvV00)(EE105 Fall 2007 Lecture 8, Slide 8 Prof. Liu, UC BerkeleyDegenerated CE Stage as a “Black Box”•If gmRE >> 1, Gm is more linear.EmminoutmEminmoutARggviGRgrvgiV1)(1)(1EE105 Fall 2007 Lecture 8, Slide 9 Prof. Liu, UC BerkeleyDegenerated CE Stage with Base Resistance11)1(.)(BEmCvBECinoutAoutinAinoutARRgRARRrRvvvvvvvvVEE105 Fall 2007 Lecture 8, Slide 10 Prof. Liu, UC BerkeleyDegenerated CE Stage: Input/Output Impedances •Rin1 is more important in practice, because RB is often the output impedance of the previous stage.CoutEBinEinARRRrRRRrRV)1()1()(21EE105 Fall 2007 Lecture 8, Slide 11 Prof. Liu, UC BerkeleyEmitter Degeneration Example 31221||)1(11)||(RRRRrRRRgRRACoutinBmCvEE105 Fall 2007 Lecture 8, Slide 12 Prof. Liu, UC BerkeleyOutput Impedance of Degenerated CE Stage with VA<∞•Emitter degeneration boosts the output impedance.–This improves the gain of the amplifier and makes the circuit a better current source.   )||(1)||)(1(||)||(1rRgrRrRrgrRrRrrRgREmOoutEOmOoutEOEmoutEE105 Fall 2007 Lecture 8, Slide 13 Prof. Liu, UC BerkeleyTwo Special CasesOEmou tEOmOoutErRgRrRrrgrRrR)1( : )2)1( : )1Stage with explicit depiction of ro:EE105 Fall 2007 Lecture 8, Slide 14 Prof. Liu, UC BerkeleyAnalysis by Inspection•This seemingly complicated circuit can be greatly simplified by first recognizing that the capacitor creates an AC short to ground, and gradually transforming the circuit to a known topology.  12||)||(1 RrrRgROmout OmoutrrRgR )||(12111||outoutRRR EE105 Fall 2007 Lecture 8, Slide 15 Prof. Liu, UC BerkeleyExample: Degeneration by Another BJT•Called a “cascode”, this circuit offers many advantages that we will study later... 1121)||(1OOmoutrrrgREE105 Fall 2007 Lecture 8, Slide 16 Prof. Liu, UC BerkeleyBad Input Connection•Since the microphone has a very low resistance (connecting the base of Q1 to ground), it attenuates the base voltage and renders Q1 with a very small bias current.EE105 Fall 2007 Lecture 8, Slide 17 Prof. Liu, UC BerkeleyUse of Coupling Capacitor•A capacitor is used to isolate the DC bias network from the microphone , and to short (or “couple”) the microphone to the amplifier at higher frequencies.EE105 Fall 2007 Lecture 8, Slide 18 Prof. Liu, UC BerkeleyDC and AC Analysis•The coupling capacitor is replaced with an open circuit for DC analysis, and then replaced with a short circuit for AC analysis.OCoutBinOCmvrRRRrRrRgA||||)||(EE105 Fall 2007 Lecture 8, Slide 19 Prof. Liu, UC BerkeleyBad Output Connection•Since the speaker has an inductor with very low DC resistance, connecting it directly to the amplifier would ~short the collector to ground, causing the BJT to go into deep saturation mode.EE105 Fall 2007 Lecture 8, Slide 20 Prof. Liu, UC BerkeleyUse of Coupling Capacitor at Output•The AC coupling indeed allows for correct biasing. However, due to the speaker’s small input impedance, the overall gain drops considerably.EE105 Fall 2007 Lecture 8, Slide 21 Prof. Liu, UC BerkeleyCE Stage with Voltage-Divider BiasingOCoutinOCmvrRRRRrRrRgA||||||)||(21EE105 Fall 2007 Lecture 8, Slide 22 Prof. Liu, UC BerkeleyCE Stage with Robust Biasing  CoutEinEmCvRRRRRrRRgRA21||||)1(1AVEE105 Fall 2007 Lecture 8, Slide 23 Prof. Liu, UC BerkeleyElimination of Emitter Degeneration for AC Signals•The capacitor C2 shorts out RE at higher frequencies to eliminate the


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Berkeley ELENG 105 - Lecture 8

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