Chapter 4

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.

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 $I_{B}>0$ and $\beta I_{B}>I_{C}>0$ $I_{B}>0$ and $\beta I_{B}>I_{C}>0$ Active $I_{B}>0$ and $V_{CE}>0.2$ $I_{B}>0$ and $V_{CE}<-0.2$ Cutoff $V_{BE}<0.5$ and $V_{BC}<0.5$ $V_{BE}>-0.5$ and $V_{BC}>-0.5$

• 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 $\alpha$ and $\beta$ 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 $I_{C},I_{B},V_{CE}$
3. Check the values to see if they match the region constraints.

Bias-Circuit Design

• Fixed-base bias circuit is a very simplistic method that does not deal well with a wide variety of $/beta$ values.

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?