Lecture 6 ANNOUNCEMENTS 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 OUTLINE BJT cont d PNP transistor structure operation models BJT Amplifiers General considerations Reading Chapter 4 6 5 1 EE105 Fall 2007 Lecture 6 Slide 1 Prof Liu UC Berkeley Current 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 Jtot b E x A relatively small E field exists in the quasi neutral regions drift current EE105 Fall 2007 Jn Jp Lecture 6 Slide 2 x 0 Dn N A Ln 0 a Dp N D Lp x N D L p Dn N A Ln D p Prof Liu UC Berkeley Review of BJT Operation Active Mode The emitter junction is forward biased Carriers diffuse across the emitter junction thus minority carrier concentrations are enhanced by eVD VT 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 eV V 0 at the edges of the D T 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 collectorjunction depletion region and then drift into the quasi neutral collector The collector current is primarily due to carriers collected from the base EE105 Fall 2007 Lecture 6 Slide 3 Prof Liu UC Berkeley Common 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 IC AE qDB ni2 VBE e N BWB VT 1 The base current is equal to the current due to minority carrier injection from the base into the emitter IB AE qDE ni2 VBE e N EWE VT 1 IC The current gain can thus be expressed as a function of the BJT physical parameters DB N EWE EE105 Fall 2007 Lecture 6 Slide 4 DE N BWB Prof Liu UC Berkeley Impact 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 VCE AE qDB ni2 VBE VT 1 IC e N BWB V A The base current is not impacted AE qDE ni2 VBE VT IC IB e N EWE Thus the current gain increases with increasing VCE VCE DB N EWE VCE 0 DE N BWB V A VA EE105 Fall 2007 Lecture 6 Slide 5 0 1 VCE I E VA 0 I S e VBE Prof Liu UC Berkeley VT Small Signal Models for Independent Sources 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 EE105 Fall 2007 Lecture 6 Slide 6 Prof Liu UC Berkeley 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 Berkeley NPN 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 Berkeley PNP BJT Terminal Currents VEB VEC 1 I C I S exp VT VA IS VEB I B exp VT 0 1 VEC I E VA 0 DB N EWE 0 DE N BWB EE105 Fall 2007 Lecture 6 Slide 9 VEB I S exp VT Prof Liu UC Berkeley Large Signal Model for PNP BJT EE105 Fall 2007 Lecture 6 Slide 10 Prof Liu UC Berkeley PNP 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 Berkeley Small Signal Analysis EE105 Fall 2007 Lecture 6 Slide 12 Prof Liu UC Berkeley PNP 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 13 Prof Liu UC Berkeley Small Signal Model Example 1 EE105 Fall 2007 Lecture 6 Slide 14 Prof Liu UC Berkeley Small Signal Model Example 2 Note that the small signal model is identical to that in the previous example EE105 Fall 2007 Lecture 6 Slide 15 Prof Liu UC Berkeley Small Signal Model Example 3 Note that the small signal model is identical to that in the previous examples EE105 Fall 2007 Lecture 6 Slide 16 Prof Liu UC Berkeley Small Signal Model Example 4 EE105 Fall 2007 Lecture 6 Slide 17 Prof Liu UC Berkeley BJT Amplifiers Overview EE105 Fall 2007 Lecture 6 Slide 18 Prof Liu UC Berkeley Voltage 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 Berkeley Input Output Impedances The figures below show how input and output impedances are determined All independent sources are set to zero vx impedance ix EE105 Fall 2007 Lecture 6 Slide 20 Prof Liu UC Berkeley Input Impedance Example Note that input output impedances are usually regarded as small signal quantities The input impedance is obtained by applying a small change in the input voltage and finding the resultant change in the input current vx r ix EE105 Fall 2007 Lecture 6 Slide 21 Prof Liu UC Berkeley Impedance …
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