Bipolar Junction Transistors - How does one read the datasheet?

In summary, the datasheet for the PNP BJT shows a wide spread of hFE values, indicating that the circuit should be designed to be tolerant of this variation. The variation of hFE with VCE is likely overshadowed by the variation with IC, as shown by a plot on another manufacturer's datasheet.
  • #1
d.arbitman
101
4
In particular, I am looking at a datasheet of a PNP BJT:
http://www.fairchildsemi.com/ds/2N/2N3906.pdf

I noticed that in the ON CHARACTERISTICS sub-heading, there were multiple [itex]h_{FE}[/itex] values. I'm wondering how I could extract any meaningful information from that since the most common [itex]h_{FE}[/itex] value is ~100. I also don't understand why the test [itex]V_{CE}[/itex] value is only -1.0V; shouldn't there be a wider range of applicable voltage drops from collector to emitter. How would I interpret the information provided to me in that section?
 
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  • #2
There is a wide spread of hFE for this device. Furthermore, hFE also varies with collector current (there's a graph showing this). This tells you that whatever circuit you use, that circuit should be designed to be tolerant of a wide variation of hFE. Either that or you must individually test a handful of devices and pick the one with the highest gain. (Not advisable, as repair and replacement may well see the circuit not function.)

I think you'll find that the variation of hFE with VCE is of the same order, or overshadowed by, the variation of hFE with IC. You can judge for yourself if you unearth a plot of IC versus VCE on another manufacturer's datasheet: the more horizontal the lines, the less the variation.
 

1. What is a bipolar junction transistor (BJT)?

A bipolar junction transistor is a type of semiconductor device that is commonly used as an electronic switch or amplifier. It is made up of three layers of doped semiconductor material and has three terminals: a base, collector, and emitter.

2. How do I read the datasheet for a BJT?

The datasheet for a BJT contains important information about its electrical and physical characteristics. It typically includes specifications such as maximum voltage and current ratings, gain, and frequency response. It is important to carefully review the datasheet to ensure the BJT is suitable for your intended application.

3. What is the difference between an NPN and PNP BJT?

The main difference between NPN and PNP BJTs is the direction of current flow. In an NPN BJT, current flows from the collector to the emitter, while in a PNP BJT, current flows from the emitter to the collector. Additionally, the direction of the majority carriers (electrons or holes) is opposite in the two types of BJTs.

4. What is the maximum collector current for a BJT?

The maximum collector current, also known as ICmax, is the maximum amount of current that can flow through the collector terminal without damaging the BJT. This value is typically listed on the datasheet and is an important consideration when designing a circuit using a BJT.

5. How do I choose the appropriate BJT for my circuit?

There are several factors to consider when choosing a BJT for your circuit, including maximum voltage and current ratings, gain, and frequency response. It is important to carefully review the datasheet and select a BJT that meets the requirements of your circuit and can handle the expected operating conditions.

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