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5.6: Summary

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    DC biasing is required in order to maintain the proper junction potentials and operation of the BJT. Several different circuit configurations are available to establish a DC bias on both NPN and PNP transistors. These circuits vary in complexity and their ability to maintain a constant operating point, or Q point, in the face of variations of \(\beta\).

    The two-supply emitter bias topology offers very high Q point stability. It achieves this through the use of two powers supplies; one connected through a resistor to the emitter and a second unit connected through a resistor to the collector. It is unique in that the supplies are bipolar; one being positive and the other being negative.

    The voltage divider bias circuit offers similar stability performance to the two-supply emitter bias circuit. It uses a single supply and a resistive voltage divider to establish a second, lower potential at the base terminal.

    The three feedback bias configurations offer only modest enhancements in stability but use the least amount of parts. They all rely on a single DC power source.

    A DC load line is a plot of all possible collector current and corresponding collector-emitter voltage operating points. No matter what the \(\beta\) for a circuit happens to be, the transistor's operating point must lie on this line. It is a valuable DC analysis tool.

    5.6.1: Review Questions

    1. Explain the need for DC biasing. Why can't we just apply an AC signal to the base of a BJT and expect proper amplification of the signal?

    2. What is a Q point?

    3. What are the four values found on a DC load line?

    4. Rank the bias configurations presented in this chapter in terms of their Q point stability relative to \(\beta\).

    5. Rank the bias configurations presented in this chapter in terms of their circuit complexity.

    6. Describe the process of making a PNP version of an NPN bias circuit.

    This page titled 5.6: Summary is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by James M. Fiore via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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