Chapter 4: Difference between revisions

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*'''Saturation''': High current from the emitter to collector. Logical "on".
*'''Saturation''': High current from the emitter to collector. Logical "on".
*'''Cutoff''': Very little current flow. Logical "off".
*'''Cutoff''': Very little current flow. Logical "off".

===Large-Signal DC Analysis===
#Assume an operating region for the BJT.
#Solve the circuit to find <math>I_C, I_B, V_{CE}</math>
#Check the values to see if they match the region constraints.


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Revision as of 12:45, 3 March 2010

Bipolar Junction Transistor

Base, Emitter, Collector
  • NPN: Not Pointing iN
  • PNP: Pointing iN Please
  • The arrow is for the emitter current.
  • For an NPN the current flows into the collector. For a PNP the current flows out of the collector.

How a BJT works

Large-Signal DC models

BJT Regions
Base-Emitter Collector-Base Region
FB FB Saturation
FB RB Active
RB RB Cutoff
RB FB Reverse Active
Region Constraints
Region NPN PNP
Saturation and and
Active and and
Cutoff and and
  • Active: The normal mode of operation. This mode has the largest common-emitter current gain.
  • Reverse Active: The collector and emitter roles are reversed. Most BJTs are not symmetrical, thus and take on different values.
  • Saturation: High current from the emitter to collector. Logical "on".
  • Cutoff: Very little current flow. Logical "off".

Large-Signal DC Analysis

  1. Assume an operating region for the BJT.
  2. Solve the circuit to find
  3. Check the values to see if they match the region constraints.
IMG 0288.jpg
IMG 0287.jpg
  • What are the Saturation, Reverse Active and Cutoff regions used for?
  • Why do we always seem to use a common emitter configuration?
  • What does the Reverse Active large-signal model look like? How about graphically?
  • For the Saturation region, why is 0.2V the magic number for V_CE?