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Berkeley ELENG 105 - Lecture 6

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Lecture 6OUTLINEANNOUNCEMENTS• HW#3, Prob. 2: Re-draw I-V plots for WBreduced by a factor of 2.• In case of a major earthquake: – Try to duck/crouch on the floor in front of the seats for cover.– Once the earthquake stops, evacuate the building in an orderly manner.EE105 Fall 2007 Lecture 6, Slide 1 Prof. Liu, UC BerkeleyOUTLINE• BJT (cont’d)– PNP transistor (structure, operation, models)• BJT Amplifiers– General considerationsReading: Chapter 4.6-5.1Current Flow in a “Long-Base” PN Junction• The quasi-neutral N-type and P-type regions have low resistivity, whereas the depletion region has high resistivity.– When an external voltage VDis applied across the diode, almost all of this voltage is dropped across the depletion region.JtotEE105 Fall 2007 Lecture 6, Slide 2 Prof. Liu, UC Berkeleyx0 a-bpnnpADpDpnAnxpnDDLLNNLNDLNDJJ⋅⋅=== 0Ex• A relatively small E-field exists in the quasi-neutral regions  drift currentReview of BJT Operation (Active Mode)• The emitter junction is forward biased. Carriers diffuse across the emitter junction; thus, minority-carrier concentrations are enhanced (by ) at the edges of the emitter-junction depletion region.More minority carriers are “injected” into the base vs. emitter, because the emitter is more heavily doped than the base.• The collector junction is reverse biased (or not strongly forward biased).Minority-carrier concentrations are ~0 (since ) at the edges of the TDVVe/0/≅VVeEE105 Fall 2007 Lecture 6, Slide 3 Prof. Liu, UC BerkeleyMinority-carrier concentrations are ~0 (since ) at the edges of the emitter-junction depletion region.• The minority-carrier concentration gradient in the quasi-neutral base region (of width WB) results in minority-carrier diffusion toward the collector junction.• If WBis much shorter than the minority-carrier diffusion length, then most of the minority carriers injected from the emitter will reach the collector-junction depletion region, and then drift into the quasi-neutral collector. • The collector current is primarily due to carriers “collected” from the base.0/≅TDVVeCommon-Emitter Current Gain, ββββ• Assuming that no minority-carrier recombination occurs within the quasi-neutral base region:– The collector current is equal to the current due to minority-carrier injection from the emitter into the base:()1/2−=TBEVVBBiBECeWNnqDAIEE105 Fall 2007 Lecture 6, Slide 4 Prof. Liu, UC Berkeley– The base current is equal to the current due to minority-carrier injection from the base into the emitter:• The current gain βcan thus be expressed as a function of the BJT physical parameters:BBWN()βCVVEEiEEBIeWNnqDAITBE≡−= 1/2BBEEEBWNDWND=βImpact of Early Effect on BJT Currents• For a fixed value of VBE, WBdecreases with increasing VCE(because the width of the collector-junction depletion region increases with increasing reverse bias), so that the minority-carrier concentration gradient in the quasi-neutral base region increases. Thus, ICincreases (slightly) with increasing VCE.+≅CEVViBEVenqDAITBE1/2EE105 Fall 2007 Lecture 6, Slide 5 Prof. Liu, UC Berkeley• The base current is not impacted:• Thus, the current gain βincreases with increasing VCE.βCVVEEiEEBIeWNnqDAITBE≡=/2+≅ACEBBiBECVeWNITBE1TBEVVSACEEeIVVI/001++=ββACEACEBBEEEBVVVVWNDWND+≡+=0ββ• The voltage across an independent voltage source does not vary with time. Its small-signal voltage is always zero. Thus, it is regarded as a short circuit for the purpose of small-signal analysis.Small-Signal Models for Independent SourcesEE105 Fall 2007 Lecture 6, Slide 6 Prof. Liu, UC Berkeley• The current through an independent current source does not vary with time Its small-signal current is always zero.Thus, it is regarded as an open circuit for the purpose of small-signal analysis.PNP Transistor• The operating principle of a PNP BJT is the same as that of an NPN BJT. Note that the bias-voltage polarities are reversed for the PNP device, compared to an NPN device.– The emitter is biased at a higher potential than the base.– The collector is biased at a lower potential than the base. EE105 Fall 2007 Lecture 6, Slide 7 Prof. Liu, UC BerkeleyNPN vs. PNP BJTs• The directions of current flow and operation modes for NPN and PNP BJTs are shown below:EE105 Fall 2007 Lecture 6, Slide 8 Prof. Liu, UC BerkeleyPNP BJT Terminal CurrentsTEBSBAECTEBSCVVIIVVVVII=+=exp1expβEE105 Fall 2007 Lecture 6, Slide 9 Prof. Liu, UC BerkeleyBBEEEBTEBSAECETWNDWNDVVIVVIV=++=000exp1ββββLarge-Signal Model for PNP BJTEE105 Fall 2007 Lecture 6, Slide 10 Prof. Liu, UC BerkeleyPNP BJT Biasing• Note that the emitter is biased at a higher potential than the base and the collector.EE105 Fall 2007 Lecture 6, Slide 11 Prof. Liu, UC BerkeleySmall-Signal AnalysisEE105 Fall 2007 Lecture 6, Slide 12 Prof. Liu, UC BerkeleyPNP BJT Small-Signal Model• The small-signal model for a PNP transistor is identical to that of an NPN transistor.– Note that the polarity of the small-signal currents and voltages are defined to be in the opposite direction with respect to the large-signal model. This is OK, because the small-signal model is used only to determine changesin EE105 Fall 2007 Lecture 6, Slide 13 Prof. Liu, UC Berkeleysmall-signal model is used only to determine changesin currents and voltages.Small-Signal Model Example 1EE105 Fall 2007 Lecture 6, Slide 14 Prof. Liu, UC BerkeleySmall-Signal Model Example 2EE105 Fall 2007 Lecture 6, Slide 15 Prof. Liu, UC Berkeley• Note that the small-signal model is identical to that in the previous example.Small-Signal Model Example 3EE105 Fall 2007 Lecture 6, Slide 16 Prof. Liu, UC Berkeley• Note that the small-signal model is identical to that in the previous examples.Small-Signal Model Example 4 EE105 Fall 2007 Lecture 6, Slide 17 Prof. Liu, UC BerkeleyBJT Amplifiers: OverviewEE105 Fall 2007 Lecture 6, Slide 18 Prof. Liu, UC BerkeleyVoltage Amplifier• In an ideal voltage amplifier, the input impedance is infinite and the output impedance is zero.• In reality, the input and output impedances depart from their ideal values.EE105 Fall 2007 Lecture 6, Slide 19 Prof. Liu, UC Berkeleyfrom their ideal


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Berkeley ELENG 105 - Lecture 6

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