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Understanding Transistor Biasing: NPN and PNP Configurations Explained

This document provides a concise overview of transistor biasing, focusing on NPN and PNP configurations. It explains how the collector junction operates in a reverse-biased manner and the necessity of forward biasing the emitter junction for proper function. Key concepts include the flow of current in both configurations and the importance of voltage arrangements. Additionally, it highlights the relationships between collector current (Ic), base current (IB), and emitter current (IE), and illustrates the differences between NPN and PNP transistors side by side.

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Understanding Transistor Biasing: NPN and PNP Configurations Explained

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  1. SEMICONDUCTORS Transistor Biasing

  2. SEMICONDUCTORS • Here we have a collector junction that forms a PN junction diode configuration that is reverse biased that functions like a reverse biased diode and conducts a very small leakage current.

  3. SEMICONDUCTORS • We should take a moment here and review a basic circuit symbol, the battery. • The small cell is negative and the large cell is positive, current flows from negative to positive. • We can put multiple cells together to increase voltage.

  4. SEMICONDUCTORS • If one cell equals 1.5 volts, then how many volts does 4 cells equal? (to increase voltage batteries are connected in series) SERIES CONNECTED BATTERIES

  5. SEMICONDUCTORS • The collector junction in an NPN transistor is typically connected as a reverse biased arrangement with the positive lead connected to the collector lead.

  6. SEMICONDUCTORS • Here we have an emitter junction that forms a PN junction diode configuration that is forward biased that functions like a forward biased diode and conducts a small amount of current.

  7. SEMICONDUCTORS • Normally the emitter junction must be forward biased to function properly

  8. SEMICONDUCTORS • You have just seen the action which takes place in an NPN transistor when the collector junction and the emitter junction are properly biased. • A transistor must be simultaneously biased with the emitter junction forward biased and the collector junction reverse biased in an NPN transistor.

  9. SEMICONDUCTORS • Here is the PNP transistor showing the proper biasing. • The external voltages have been reversed from that of the NPN transistor. The battery (voltage) has been reversed

  10. SEMICONDUCTORS • Compare the two transistor types and their configurations in this side by side diagram. • Remember that both of these are properly biased.

  11. SEMICONDUCTORS • In an NPN transistor the majority (95 to 99%) of the current supplied by the emitter flows through the collector. • This is referred to as collector current and designated as Ic, a small percentage (1 to 5%) of the current flows through the base and is designated as IB. • The current the flows into the emitter is designated as IE.

  12. SEMICONDUCTORS • IC = Collector current • IB = Base current • IE= Emitter current IE = IB + IC

  13. SEMICONDUCTORS • In a PNP configuration the majority of the current supplied by the collector flows through the emitter and only a small percentage of the current flows through the base. • This is the same action that occurs in an NPN transistor but in the opposite direction with collector current flowing through the emitter.

  14. SEMICONDUCTORS

  15. SEMICONDUCTORS • In this diagram we see the transistor is labeled with the letter designation Q which is typical. • Notice the battery on the emitter side of the transistor, what does the arrow mean?

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