DOC PREVIEW
Berkeley ELENG 105 - Lecture Notes

This preview shows page 1-2-24-25 out of 25 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 25 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 25 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 25 pages.
Access to all documents
Download any document
Ad free experience
View full document
Premium Document
Do you want full access? Go Premium and unlock all 25 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 25 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

Lecture 6Current Flow in a “Long-Base” PN JunctionReview of BJT Operation (Active Mode)Common-Emitter Current Gain, bImpact of Early Effect on BJT CurrentsSmall-Signal Models for Independent SourcesPNP TransistorNPN vs. PNP BJTsPNP BJT Terminal CurrentsLarge-Signal Model for PNP BJTPNP BJT BiasingSmall-Signal AnalysisPNP BJT Small-Signal ModelSmall-Signal Model Example 1Small-Signal Model Example 2Small-Signal Model Example 3Small-Signal Model Example 4BJT Amplifiers: OverviewVoltage AmplifierInput/Output ImpedancesInput Impedance ExampleImpedance at a NodeImpedance seen at the CollectorImpedance seen at the EmitterSummary of BJT ImpedancesEE105 Fall 2007 Lecture 6, Slide 1 Prof. Liu, UC BerkeleyLecture 6OUTLINE•BJT (cont’d)–PNP transistor (structure, operation, models)•BJT Amplifiers–General considerationsReading: Chapter 4.6-5.1ANNOUNCEMENTS•HW#3, Prob. 2: Re-draw I-V plots for WB reduced by a factor of 2.•Discussion Section 103 (Fr 11AM-12PM) will move to 458 Evans.•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 2 Prof. Liu, UC BerkeleyCurrent 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 VD is applied across the diode, almost all of this voltage is dropped across the depletion region.x0Jtota-bpnnpADpDpnAnxpnDDLLNNLNDLNDJJ 0Ex•A relatively small E-field exists in the quasi-neutral regions  drift currentEE105 Fall 2007 Lecture 6, Slide 3 Prof. Liu, UC BerkeleyReview 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 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 WB is 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.TDVVe/0/TDVVeEE105 Fall 2007 Lecture 6, Slide 4 Prof. Liu, UC BerkeleyCommon-Emitter Current Gain, •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:–The base current is equal to the current due to minority-carrier injection from the base into the emitter:•The current gain  can thus be expressed as a function of the BJT physical parameters: 1/2TBEVVBBiBECeWNnqDAI CVVEEiEEBIeWNnqDAITBE 1/2BBEEEBWNDWNDEE105 Fall 2007 Lecture 6, Slide 5 Prof. Liu, UC BerkeleyImpact of Early Effect on BJT Currents•For a fixed value of VBE, WB decreases 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, IC increases (slightly) with increasing VCE.•The base current is not impacted:•Thus, the current gain  increases with increasing VCE.CVVEEiEEBIeWNnqDAITBE/2ACEVVBBiBECVVeWNnqDAITBE1/2TBEVVSACEEeIVVI/001ACEACEBBEEEBVVVVWNDWND0EE105 Fall 2007 Lecture 6, Slide 6 Prof. Liu, UC Berkeley•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.•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.Small-Signal Models for Independent SourcesEE105 Fall 2007 Lecture 6, Slide 7 Prof. Liu, UC BerkeleyPNP 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 8 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 9 Prof. Liu, UC BerkeleyPNP BJT Terminal CurrentsBBEEEBTEBSAECETEBSBAECTEBSCWNDWNDVVIVVIVVIIVVVVII000exp1exp1expEE105 Fall 2007 Lecture 6, Slide 10 Prof. Liu, UC BerkeleyLarge-Signal Model for PNP BJTEE105 Fall 2007 Lecture 6, Slide 11 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 12 Prof. Liu, UC BerkeleySmall-Signal AnalysisEE105 Fall 2007 Lecture 6, Slide 13 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 changes in currents and voltages.EE105 Fall 2007 Lecture 6, Slide 14 Prof. Liu, UC BerkeleySmall-Signal Model Example 1EE105 Fall 2007 Lecture 6, Slide 15 Prof. Liu, UC BerkeleySmall-Signal Model Example 2•Note that the small-signal model is identical to that in the previous example.EE105 Fall 2007 Lecture 6, Slide 16 Prof. Liu, UC BerkeleySmall-Signal


View Full Document

Berkeley ELENG 105 - Lecture Notes

Documents in this Course
Lecture 3

Lecture 3

21 pages

Lecture 9

Lecture 9

15 pages

Lecture 3

Lecture 3

19 pages

Lecture 3

Lecture 3

22 pages

Outline

Outline

16 pages

Lecture 3

Lecture 3

21 pages

Lecture 2

Lecture 2

28 pages

Lecture 3

Lecture 3

21 pages

Lecture 4

Lecture 4

22 pages

Lecture 6

Lecture 6

25 pages

Lecture 1

Lecture 1

13 pages

Lecture 5

Lecture 5

22 pages

Lecture 3

Lecture 3

21 pages

Lecture 1

Lecture 1

13 pages

Lecture 8

Lecture 8

25 pages

Lecture

Lecture

5 pages

Overview

Overview

24 pages

Lecture 5

Lecture 5

22 pages

Load more
Download Lecture Notes
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Lecture Notes and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Lecture Notes 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?