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Understanding NPN and PNP Bipolar Transistor Operations in Embedded Systems

This guide explores the principles of NPN and PNP bipolar transistors, focusing on their key operational rules and characteristics in embedded systems. It covers essential concepts such as the proper biasing of the base-emitter and base-collector junctions, current relationships, and the implications of transistor saturation. The guide also delves into practical applications, including emitter followers, Darlington pairs, and push-pull amplifier stages, helping users grasp how to utilize these components effectively for reliable circuit design.

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Understanding NPN and PNP Bipolar Transistor Operations in Embedded Systems

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  1. Embedded SystemsTransistors

  2. E B C N P N B NPN E Bipolar Transistor C C B PNP E • NPN rules: • The collector must be more positive than the emitter • B-E and B-C behave like diodes • B-E forward biased • B-C reverse biased • Limitations on IC, IB, and VCE • When rules (1)-(3) are obeyed, IC=hFEIB=IB hFE (current gain) ~ 100 iC governed by transistor action

  3. E B N P N small IC large ICE + + Bipolar Transistor C

  4. E B N P N Transistor Action C B C reverse biased forward biased IC=IB E

  5. 10 V lamp 0.1A 10 V C B E 1K Transistor Saturation switch closes: VBE=0.6V IB=9.4V/1K=9.4mA ICE=(IB)=100(9.4) =940mA to get that much current through the lamp VC would have to go significantly below ground, which is illegal by rule #1 “The collector must be more positive than the emitter” the transistor is saturated---only enough IC currentflows through the transistor to keep the collector 0.05 to 0.2 V more positive than the emitter.

  6. Emitter-Follower • VE = VB-0.6V • when Vin > 0.6V • big current/power gain at Vout=Vin-0.6V • impedance buffer Vin Vout

  7. Power Transistors • Darlington pairs • hFE=(hFE)2 • TIP102/106

  8. Push-Pull Amplifier Stages NPN can only “source” current “crossover” distortion PNP can only “sink” current

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