Chapter 4

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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.
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IMG 0287.jpg

Bias-Circuit Design

  • Fixed-base bias circuit is a very simplistic method that does not deal well with a wide variety of values. The base current does not adjust for changes in . If you want a circuit that has a particular operating point on the collector load line, the base current must change with .
  • Using two voltage sources (one for the collector and one for the base) achieves an emitter current that is relatively independent of . Usually only one source is available or we may wish to inject a signal into the base.
  • What's wrong with injecting the signal on top of the DC source?
  • Four-resistor bias circuit provides a constant base voltage independant of .

Questions

  • Why do we always seem to use a common emitter configuration? Common-base and common-collector have different properties.
  • Daisy chaining the voltage doubler? Go up?
  • For P3.17 make the glob of sauder assuming they're all on and check the currents running through each one. Make sure they go the right way, and add up?
  • How do we get around the problem of the fixed-base bias current? Looks like you can fix it with two voltage sources, but is there a better method?