basics of transistor

8/3/2019 Basics of Transistor

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BASICS OFTRANSISTORHow do transistor


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Transistor Symbols

This is the symbolused to represent an

"NPN" transistor.

This is the sysmbolused to represent an

PNP transistor.


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Transistors

Labels:

"B" (Base),"C"

(Collector)"E" (Emitter)


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So what have we learned?

1. A tiny amount ofcurrent flowing into "B"allows a large amount toflow from "C" to "E" sowe have an "amplificationeffect". We can control aBIG flow of current with aSMALL flow of current. Ifwe continually changethe small amount of

water flowing into "B"then we causecorresponding changesin the LARGE amount ofwater flowing from "C" to

"E".


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For example, if we measure the current flow ingallons/minute: Suppose 1 gallon/minuteflowing into "B" allows 100 gallons/minute to

flow from "C" to "E" then we can say that thetransistor has a "gain" or "amplification" factorof 100 times. In a real transistor we measurecurrent in thousandths of an Ampere or"milliamps". So 1mA flowing into "B" wouldallow 100mA to flow from "C" to "E".


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2. The amount of current thatcan flow from "C" to "E" islimited by the "pipe

diameter". So, no matter howmuch current we push into"B", there will be a pointbeyond which we can't getany more current flow from"C" to "E". The only way tosolve this problem is to use a

larger transistor. A "power"


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3. The transistor can beused to switch thecurrent flow on and off. Ifwe put sufficient currentinto "B" the transistor willallow the maximumamount of current to flowfrom "C" to "E". Thetransistor is switchedfully "on.

If the current into "B" is reduced to the point where itcan no longer lift the black plunger thing, thetransistor will be "off". Only the small "leakage"current from "B" will be flowing. To turn it fully off, we

must stop all current flowing into "B".


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Getting Technical

The difference between PNP and NPNtransistors is that NPN use electrons ascarriers of current and PNP use a lack of

electrons (known as "holes"). Basically,nothing moves very far at a time. One atomsimply robs an electron from an adjacent atomso you get the impression of "flow. In the case

of "N" material, there are lots of spareelectrons. In the case of "P" there aren't. Infact "P" is gasping for electrons.


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Getting Technical

A transistor doesn't "increase" current. Itsimply allows power supply current to passfrom collector to emitter* - the actual amount

depends on the (small) current allowed to flowinto its base. The more electrons you allowinto the base, the more flow from collector toemitter .


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Emitter. One of the three regions that form abipolartransistor. Under forward bias of theemitter-baseP-N junction, the emitter injects

minority carriers (electrons or holes) into thebase region where they either recombine ordiffuse into the collector. The flow of minoritycarriers from the emitter to the collector is

controlled by the base-emitter P-N junction,thus giving rise to signal amplification.
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Collector. One of the three regions that form abipolartransistor. The base-collector P-N

junction is usually reverse-biased so that

minority carriers that are injected into the basefrom the emitter are efficiently extracted intothe collector.
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Base. One of the three regions that form abipolartransistor. It physically separates theemitter and collector regions. Minority carriers

are injected from the emitter into the base,where they subsequently either recombine ordiffuse into the collector.
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Transistor

(1) the bipolar transistor (often called thebipolar junction transistor, or BJT),

(2) the field-effect transistor (FET).
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Thank you....

basics of transistor

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