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Lecture 6
Introduction to Bipolar Transistor
Q point analysis
DC Operating point analysis and mod
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Some Resources
Semiconductor Applets http://jas.eng.buffalo.edu/
Marc E. Herniter: Schematic Capture with
Cadence PSpice (2nd Edition)
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Lecture 5 EE171
ThenpnBJT.
NPN Bipolar Junction Trans An npnBJT consists of a thin layer of p-typmaterial between two layers of n-type mateshown in the figure.
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Annpntransistor with variable biasing sources (common-emitter configurati
In most cases the base-emitter junction is forward and the base-collector junction is reverse biased (aregion).
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Lecture 5 EE171
The emitter is heavily doped compared with the baof the current is due to electrons moving from the e
Base current consists of holes crossing from the bathe emitter and of holes that recombine with electrthe base.
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Lecture 5 EE171
Common-emitter characteristics of a typical npnBJT.
Common-Emitter Characteri
Input characteristic is similar to the forward biascharacteristic of a pnjunction.
In the output characteristics, the collector current isindependent of VCE.
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Lecture 5 EE171
Secondary EffectsBase-width Modulation
Collector BreakdownLeakage Current
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Lecture 5 EE171
Base-width Modulation:
As VCE gets larger, depletion region of collector exfarther into the base. This means less recombinatiobase and hence less base current.
In case of a constant base current, the emitter curre
should increase to maintain this constant base curre That is why in reality, collector current would incr
with increasing VCE (next slide).
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Lecture 5 EE171
Early Voltage:
The straight line extension of collector currcurves all meet at a point on the negativeVThe voltage amplitude at the intersection isthe Early voltage.
Common-emitter characteristics displaying exaggerated secondary effect
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Lecture 5 EE171
Collector Breakdown: There are two main reasons for collect
breakdown:
-Avalanche Breakdown-Punch-through
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Lecture 5 EE171
Leakage Current The reason for this current is reverse
leakage current of the collector junctio
At room temperature, the leakage currextremely small, but, as the temperatuincreases so does the leakage current.
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Lecture 5 EE171
Load-line analysis of the amplifier
Load-line analysis
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Load-line analysis for Example 4.2.
Lets study Example 4.2 of the book to understandanalysis.
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Load-line analysis for Example 4.2.
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Voltage waveforms for the amplifier of Example 4.2.
Input-output waveforms
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Lecture 5 EE171
Load-line analysis for Example 4.2.
Lets study Example 4.2 of the book to understandanalysis.
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Load-line analysis for Example 4.2.
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Output of the amplifier of Example 4.2 for vin (t) =1.2 sin(2000p t) showing gross d
Distortion Distortion occurs in BJT amplifiers due to nonlinea
the input characteristic and non-uniform spacing ofoutput characteristic.
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Amplification occurs in the active region. Clipping occurs when the instantaneous operatinsaturation or cutoff. In saturation,vCE
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