# Transistors basics NPN vs. PNP

1. ## Transistors basics NPN vs. PNP

I need to know what the basics are for simple to-92 switching transistors.

Ie. On a NPN is the base always a positive voltage, is the emitter always grounded, and is does the collector always have plus voltage?

If those facts are true is a PNP just the opposite of an NPN? Negative signal on base, Negative voltage pulse on the emitter and the collector grounded?

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2. ## Transistors basics NPN vs. PNP

There are three basic configurations of bipolar transistors: common base, common emitter and common collector. The common emitter is most popular and is usually drawn with the emitter grounded. For an NPN, the base must be approximately 0.7V more positive than the emitter to turn on the transistor.
For a PNP, the base must be approximately 0.7V more negative than the emitter to turn on the transistor.
Let us say that we want to use an NPN transistor to switch an light bulb on and off. We could connect the emitter to ground, the collector to one side of the light bulb and the other side of the light bulb to the positive power supply rail. Putting a small positive voltage on the base with a dropping resistor will turn on the bulb. Remove this small positive voltage and the bulb turns off.
(I am over simplifying here. If is actually the base current that turns on the transistor.)

How do we accomplish the same thing with a PNP? We hook the emitter to the positive voltage rail and connect one side of the light bulb to the collector. The other side of the light bulb goes to ground. Now we turn on the transistor by pulling the base low with a dropping resistor.
Do a google search for BJT or bipolar transistor and you will find loads of examples.

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3. ## Re: Transistors basics NPN vs. PNP

Simple, easy to read, well illustrated, absolutely free downloadable is "Electric circuits" series of 6 volumes which is right for you.

Just google on the internet or search right here on this forum for that document.

nguyennam

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4. ## Re: Transistors basics NPN vs. PNP

Ok, that makes sense, thanks again.

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