# Which are main purposes of bipolar transistors? [closed]

In school, we learned a lot about transistors but I still don't get its point. I understand gain and amplification of voltage or current or both, but I don't get it how can it be used for example as a switch. I really want to understand transistors because of their widely use. At least someone recommend any good book about transistors.

• Your question is to broad for this site and product recommendations (the book) are off topic. There is a vast amount of information and tutorials available on the web. Do some more study on basic circuits trying to understand how they work and come back with a specific question on a point that requires clarification. Include links and schematics in your question. – Transistor Aug 13 '16 at 12:08
• Speaking about switch applications you must know that, in principle, the term "switch" can mean (a) analog switch (like a relais, not possible with BJTs) or (b) voltage switch (between two fixed voltage levels, high/low). – LvW Aug 13 '16 at 12:23
• @LuKa: Can you edit your post to explain what you do understand about using the transistor as a switch and where you get lost? – Transistor Aug 13 '16 at 12:27
• @LvW: about that voltage switch, in school we learned about circuit that works as a switch with transistor and relay, and I never understood purpose of BJT in that circuit. Can you maybe explain? – Lu Ka Aug 13 '16 at 17:14
• As I have mentioned, the bipolar transistor (other than a FET) cannot be used as an "analog switch" (relay). Speaking about swiching applications of BJTs we mean Hi-Low switches. – LvW Aug 14 '16 at 7:56

I don't get it how can it be used for example as a switch

You need to think about the voltage "lost" from collector and emitter when connected to a load: -

Take the magenta curve - base current is 62 uA and, if the collector current was (say) 10 mA the volt drop across collector and emitter is about 200 mV.

You might say "that's not much of a switch" and you'd be somewhat correct. A BJT is an imperfect switch but, if you want to control a load resistor of 1 kohm on a 10 V supply, then using a BJT will allow 9.8 volts (in this example) to be supplied to the load.

So, you could force more base current in and you would get a smaller volt drop for the same load current. Some transistors will allow over 50 mA base current and can control over an amp with a small volt drop. Darlington transistors "lose" about 0.7 volts but can control tens of amps typically.

So, a BJT is imperfect as a closed contact but, it's pretty good as an open contact i.e. it can switch the load off with only the odd micro-amp still flowing. If you can live with 200 mV "lost" then it's a switch!

Maybe this website will help a bit more.

If you really want a better switch then use a MOSFET - good MOSFETs these days are capable of dropping only a few milli-volts with a 10 amp load.