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.

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Bias-Circuit Design

• Fixed-base bias circuit is a very simplistic method that does not deal well with a wide variety of $\beta$ values. The base current does not adjust for changes in $\beta$. If you want a circuit that has a particular operating point on the collector load line, the base current must change with $\beta$.
• Using two voltage sources (one for the collector and one for the base) achieves an emitter current that is relatively independent of $\beta$. 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 $\beta$.

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?