Page 4 – Bipolar Junction Transistors BJTs basics

The transistor is a 3 terminal component where 1 terminal controls how well the other 2 conduct. Most transistors are actually built as part of an integrated circuit but individual transistors are still often used. The Bipolar Junction Transistor (BJT) is the kind that most people learn first.

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Basic properties of Bipolar Junction transistors and circuits

2N3904 NPN BJT pin layout schematic symbol and current paths diagram by electronzap electronzapdotcom
2N3904 NPN BJT pin layout schematic symbol and current paths diagram by electronzap electronzapdotcom
2N3906 PNP bipolar junction transistor BJT schematic symbol and pin layout diagram by electronzap electronzapdotcom
2N3906 PNP bipolar junction transistor BJT schematic symbol and pin layout diagram by electronzap electronzapdotcom

There are 2 types of BJT:

  • NPN
  • PNP

N stands for N type material and P stands for P type material. NPN has P type material sandwiched between N type material, while PNP has N type material sandwiched between P type material.

  • Gain

BJTs have gain. The amount of current that flows from base to emitter will allow many times that amount of current to flow from collector to emitter. The amount of gain aka. beta (ß) or hFE will very a lot based the particular BJT being used and how it is being used. Ultimately you select a BJT that is guaranteed to have more gain than you need.

Circuits

There are a few basic BJT circuits that are easy to observe what is happening. Understanding them will help a lot in understanding the more complex circuits that are covered elsewhere.

  • Switch.
  • Current source.
  • Emitter follower.

Switch circuit

NPN Bipolar Junction Transistor switch basic circuit schematic electronics diagram by electronzap electronzapdotcom
NPN Bipolar Junction Transistor switch basic circuit schematic electronics diagram by electronzap electronzapdotcom

The NPN BJT collector must be able to handle the amount of current needed by the load. This maximum collector current (Ic) varies widely between different transistors. Always check the datasheet for the particular part number you are using to learn all it’s limitations.

Quick NPN BJT switch circuit build using 2N3904 Bipolar Junction Transistor schematic to breadboard
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PNP
Simple PNP bipolar junction transistor BJT switch circuit schematic diagram by electronzap electronzapdotcom
Simple PNP bipolar junction transistor BJT switch circuit schematic diagram by electronzap electronzapdotcom

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Base resistor must provide enough signal current to saturate (fully turn on) the PNP transistor when the switch is closed. A 10KΩ resistor typically works well. The transistor must be able to handle the current demands of the load.

The 2N3906 has a maximum of 200mA of collector current. As always, consult the component datasheet of the particular component you are using for the most accurate specifications.

Quick PNP BJT switch circuit using 2N3908 bipolar junction transistor schematic to breadboard build

Timed off switch

Simple addition of a capacitor makes the switch circuit more interesting by taking time to turn off, and fading away as it does so.

Simple NPN BJT RC time constant fade off switch circuit schematic diagram by electronzap electronzapdotcom
Simple NPN BJT RC time constant fade off switch circuit schematic diagram by electronzap electronzapdotcom

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Quick timed fade off NPN BJT switch circuit using 2N3904 bipolar junction transistor by electronzap

PNP
Simple NPN BJT RC time constant fade off switch circuit schematic diagram by electronzap electronzapdotcom
Simple NPN BJT RC time constant fade off switch circuit schematic diagram by electronzap electronzapdotcom

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Quick PNP BJT fade off switch circuit using 2N3906 bipolar junction transistor

Current source

Simple NPN BJT trimpot adjustable current source bipolar junction transistor circuit by electronzap electronzapdotcom
Simple NPN BJT trimpot adjustable current source bipolar junction transistor circuit by electronzap electronzapdotcom

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1KΩ emitter resistor makes the math easy. For each volt across it, it will pass 1mA of current.  Connect a wire or load to Vcc (positive supply power of the voltage you are/will be using) and measure the voltage across the 1KΩ resistor. 3V across it, for example, will mean that it passes 3mA of current, as will pass through the transistor and load. Remember that the NPN BJT base to emitter drops about 0.6V of the trimpot output voltage from reaching the emitter resistor in this circuits.

This process is demonstrated in the video below.

Quick NPN BJT current source using trimpot voltage divider circuit schematic to breadboard build

PNP
Simple pnp bipolar junction transistor BJT current source set by trimpot voltage divider circuit by electronzap electronzapdotcom
Simple pnp bipolar junction transistor BJT current source set by trimpot voltage divider circuit by electronzap electronzapdotcom

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2N3906 PNP BJT current source set with trimpot voltage divider circuit build by electronzap

Emitter follower

Simple NPN bipolar junction transistor BJT emitter follower set by trimpot divider schematic diagram by electronzap electronzapdotcom
Simple NPN bipolar junction transistor BJT emitter follower set by trimpot divider schematic diagram by electronzap electronzapdotcom

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Quick NPN BJT emitter follower set with trimpot voltage divider using 2N3904 transistor

PNP
Simple pnp bipolar junction transistor BJT current source set by trimpot voltage divider circuit by electronzap electronzapdotcom
Simple pnp bipolar junction transistor BJT current source set by trimpot voltage divider circuit by electronzap electronzapdotcom

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2N3906 PNP BJT current source set with trimpot voltage divider circuit build by electronzap



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  • Information on this site is not guaranteed to be accurate. Always consult the manufacturer info/datasheet of parts you use. Research the proper safety precautions for everything you do.