Finding a replacement for your Bipolar Transistor

[Barley_Li]

When designing or repairing electronic equipment, it sometimes becomes necessary to find replacements for Bipolar Transisitors (BJTs). It can feel frustrated to choose the right replacement when reading over the many parameters in these parts’ specifications.

When looking for the correct BJT replacement, it is necessary to understand and consider a few related parameters in the main specification. Beyond operation temperatures and typical applications, the below basic transistor parameters need to be considered.

1. Polarity
   It is necessary to find out whether the transistor is either NPN or PNP. Incorrect type will reverse the voltages and may result in the destruction of the transistor or damage to your equipment.
   

2. Package and Pin-Out configuration
   BJTs are available in many packages. It is often necessary to match the replacement package size closely, so it can physically fit on the pads/ holes of printed circuit boards. Most importantly, you need to be aware of the pin-out configuration because different suppliers may have different pin-out design configuration.
   

3. Voltage Breakdown
   Choosing this parameter such as Vceo to match the specification is necessary. It represents the BJT’s ability to withstand the voltage. Exceeding voltage breakdown is fine, but you should never go below the rated breakdown voltage of the transistor you’re replacing.

4. Current gain
   Current gain is usually denoted as B or hfe. Choosing a replacement BJT with approximately the same current gain is necessary.

5. Frequency
   It is usually important to ensure that the replacement BJT can meet any frequency limits or requirements in your device, so it will not affect the function of original application.
   

6. Power dissipation
   It is necessary to ensure that the replacement transistor can dissipate sufficient power. The package type is one factor that affects power dissipation.
   

For more transistor technical information, please check out these related threads

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[Gorilla]

Great article, formatted perfectly. I’d like to see more like this with different components.