Every reference I can find on transistors immediately launches into theory-heavy alphabet soup. The above seems also to be assumed knowledge for reading a datasheet. I don't care; I just want to get one to work.

I understand there's some relationship between the current/voltage applied to the base to get a particular current to flow from the collector to the emitter. Which numbers on the datasheet relate to that? If I'm only trying to operate the transistor in "switch" mode, do I really need to care what current I apply to the base or will I be fine just whacking a 1k resistor in between my logic level output and the transistor base?

Is the only difference between an NPN and a PNP transistor which way the current flows when a current is applied to the base?

  • 21
    \$\begingroup\$ You say you do not care how they work, but you want to use one. You can learn transistors by rote, but with a basic understanding they are rather easy to use(barring special cases like >100 Mhz operation) \$\endgroup\$
    – Kortuk
    Oct 18 '10 at 17:00
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    \$\begingroup\$ you need to at least figure out the differences between BJTs and FETs. \$\endgroup\$
    – Mark
    Oct 18 '10 at 20:12
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    \$\begingroup\$ @Rick_2047: that was smart posting it as a comment. You can't downvote comments ;-) \$\endgroup\$ Oct 20 '10 at 4:40
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    \$\begingroup\$ Learn about transistors from a book like The Art of Electronics. That starts with practical circuits and simple models, and then gets into more detail when you need it. The textbook way of teaching transistors is terrible, and makes simple things seem amazingly difficult. \$\endgroup\$
    – endolith
    Mar 31 '11 at 14:53
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    \$\begingroup\$ suggest re-title from "I don't care how a transistor works, how do I get one to work?" to "I don't care about transistor theory, how do I get one to work?" \$\endgroup\$
    – JustJeff
    Jan 29 '12 at 18:20

The base-emitter junction is like a diode. When the voltage across it (Vbe) exceeds approximately 0.65V (can be as low as 0.55V and as high as 0.9V, check the datasheet for your transistor) it begins conducting.

The current (not voltage!) through the base emitter junction is amplified by the gain of the transistor, which is known as HFE. Ic(collector current) = Ib(base current) * HFE. Remember HFE is not constant for transistors, it varies from transistor to transistor and depending on temperature, previous usage, etc., so don't rely on it for controlled amplification. For the 2N2222 it is around 160, plus or minus 30.

By applying a base-emitter voltage exceeding 0.65V to the transistor you can use it as a switch.


simulate this circuit – Schematic created using CircuitLab

(It is an NPN transistor you want. 2N3904 or 2N2222 will do.)

If you want to use an LED which is not blue or white then use a 47 ohm resistor in series with it.

When you press the switch the LED will come on.

  • \$\begingroup\$ with a 2n2222, Vbe at saturation of 0.8V is not uncommon. Typical Vce at saturation is around 0.2V, so if you see more than 0.25 or so, you need more base current. \$\endgroup\$
    – JustJeff
    Oct 18 '10 at 22:10
  • \$\begingroup\$ That's why in my circuit I demonstrated connecting to +3V. No one voltage will fit for all transistors. Some transistors conduct well as low as 0.55V, and I've seen many bipolar transistor based circuits (like oscillators) that work well down to 0.5V. \$\endgroup\$
    – Thomas O
    Oct 18 '10 at 22:15
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    \$\begingroup\$ You should probably mention in 2nd paragraph that it's the current through the collector that is an amplification of the current through the base. I.e. collector current = base current * HFE. \$\endgroup\$ Oct 18 '10 at 22:39
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    \$\begingroup\$ +1 for the current comment in this answer. I feel like this was a major mistake during my learning of electronics (and not just limited to transistors). Current matters, you won't get anywhere without some electrons moving at some point. \$\endgroup\$ Oct 19 '10 at 18:53
  • \$\begingroup\$ Wait, what? You mean a transistor in "switch mode" acts NOTHING LIKE A SWITCH? I.e., it's the current applied to the base multiplied by the Hfe which governs the current south of the emitter, rather than whatever load is hanging off the emitter and the voltage across the collector and emitter? \$\endgroup\$ Oct 20 '10 at 17:37

NPN (and PNP) implies BJTs. No, you do not need to understand the bipolar transistor to great depth (is an advantage, of course). Just know and use standard circuits.

Easy to follow explanations and calculations for finding resistor values (or checking that the 1 kohm resistor is OK) for the switch you have are in "Transistor Switch". Standard small signal amplifier circuits are also listed on the same page.

alt text

A more detailed explanation can be found in "Transistor as a Switch".


Watch this cool video about transistors. It'll trick you into being interested about the alphabet soup.

You should really learn about the basic chemistry of p-type materials, n-type materials and doping. You can then visualize the potential differences and where the electrons are going to go without having to really memorize anything. Don't be lazy here. :)

Then an article like this will fill in the gaps.

Learn about the concepts behind a basic transistor/diode. Then all of the other acronyms will fall into place without any real effort on your part.

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    \$\begingroup\$ Note that the video you mentioned is pretty flawed, in that he confuses enhancement and depletion mode FETs with NPN and PNP BJTs. PNP transistors are not normally "on." \$\endgroup\$
    – John Miles
    Sep 14 '11 at 17:07

Those answers and comments that say, "I know you don't like it but you have to learn the alphabet soup," are wrong.

I was lucky enough to come across EXACTLY the video(s) you want some time ago. It teaches you how to USE transistors without the ubiquitous lesson of how they are made in silicon.

Despite what others say, you will learn BETTER if you leave out the abc soup until you have practical knowledge. I never really understood transistors before; I'd been taught to think of them as little switches (which is very misleading, at best). Now I know enough to comfortably put them into use. Here are the videos:

What is a transistor? How does a transistor work? Part 1

What is a transistor? How does a transistor work? Part 2


I'd propose starting step by step with something tangible. Chew on one case at a time.

You might start with the simple case of a switch and I'm sure you can find very simple examples by looking. Don't dive into an old book with CE bipolar amplifier biasing with half a dozen resistors, compensation and h parameters flown through on the first page written by someone who doesn't remember what it was like not to know all that stuff first. :)

If you look around, it should be easy to find some tutorials with BJT, JFET, MOSFET... Maybe skip P and depletion devices first as well. Mostly P (PNP) does look like a mirror image, once you have an idea how the N part works, it should be easy to relate to the P part. That way you won't get so much chance to be confused by negative voltages and currents and circuits drawn upside down (they really do all that).

Then you really need to look at the datasheet parameters like just how much current and voltage it can take safely, what's the ratio of base current (gate voltage) needed/taken for a given collector current, total power dissipation (voltage loss * current) etc.

Once you're done with switches, you might look at turning on/off only partially (amplifier, current control). All three types behave a bit differently. Then maybe see different typical circuits: regulators, current sources and mirrors, timers, logic gates, B and AB power amplifiers.

A bit of theory (multiplication, Ohm's law, diode...) is necessary, more will help you understand what's going on and predict things. But you should be able to jump in with ballpark values first. Use some cheap parts (with the datasheet, at least for the pinouts and type) and perhaps a simulator to try things out.

  • \$\begingroup\$ The MOSFET-link is dead. \$\endgroup\$
    – Gunnish
    Sep 8 '13 at 14:55
  • \$\begingroup\$ Thanks. I replaced it with another article. Less bare-bones, but same information and more. \$\endgroup\$
    – XTL
    Sep 27 '13 at 5:58

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