The bipolar junction transistor (BJT, BJTs) can either be used as a on/off switch for a circuit or to amplify a signal.
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– There are 2 types of BJT, the NPN and the PNP. They both have the same terminals names of Collector (C) – Base (B) – Emitter (E). Make sure to check the pin layout of a particular transistor by looking at it’s datasheet. To find a datasheet for a particular component, do a google search of any component’s part number and the word “datasheet”.
- NPN: Collector voltage (Vc) is higher than emitter (Ve) – (Vce). Base V (Vb) should not drop below Collector V (Vc) – (Vbc)
- PNP: Collector voltage (Vc) is lower than emitter (Ve) – (Vce). Base V (Vb) should not rise above Collector V (Vc) – (Vbc)
– Gain: A small amount of base to emitter current (Ibe) controls a larger amount of collector to emitter current (Ice). If there is 100mA of collector current while there is 1mA of base current, then that transistor gas a gain of 100. 200mA of collector current per 1mA of base current would be a gain of 200. And so on. The actual gain of a BJT transistor changes a lot during use, and therefore real life circuits aren’t made based on a specific transistor’s gain.
– 3 modes of Operation: A low voltage across and current through the base to emitter allows a larger voltage across and current through the collector to emitter (amplification).
- Cutoff – The BJT collector to emitter is not conducting. Off.
- Active region – The BJT collector to emitter is conducting, but no fully.
- Saturated – The BJT is fully conducting as well as it can. On.
– Common biasing (how voltages are applied to the 3 terminals)
- Base biased
- Emitter biased
- Voltage divider biasing
Some circuits in no particular order
- Saturated switch
- Schmitt trigger
- Current Source/sink – Driving an LED – Mirror
- Cascading NPN and PNP
- Parallel BJTs
- Transistor as a diode
- Emitter follower/common collector – Voltage regulator
- Differential amplifier
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