Transistors

[trash]

Transistors (Bipolar)

Transistors come in many shapes and forms. So we ll keep it simple
and look at only bipolars, and only one way of using a transistor.

Bipolar transistors are current devices, so it may be confusing to
try not to think of them in terms of voltage. But it can be said that
where the resistance doesn t change, more current means more voltage.

There are two types of bipolar transistors. PNP and NPN.
The way to tell the difference on a circuit diagram is that the little
arrow points “iN” or “outPut”. nPn or pNp.
If you can t remember it doesn t matter, you can always look it up. 🙂

On both types of transistor, the leg with the arrow is always the Emitter. The leg without the arrow is always the Collector and the
leg perpendicular to the thick line is always the Base.
They are often labled as C,B,E.

We ll start with a nPn. The arrow points outwards.
It can be said that these are positive biased.

A transistor can bet treated like a switch or relay. Current flowing
from the BASE to EMITTER, turns on the COLLECTOR to EMITTER path. That simple.

A more complex and true statement is:
When current is flowing from the BASE to the EMITTER, it will turn the transistor ON and let MORE current flow from the COLLECTOR to the EMITTER. (this is how an amplifier works).

If I set up two multimeters and measure the current flowing into the base of the transistor and the current flowing into the collector.
(assuming the emitter is zero volts)
This table shows what the relationship is in say milliamps..

Input Output
-1 0
0 0
1 10
2 20
3 40
5 50
6 55
7 60
8 60
9 60

You can see that the more current you put in, the more current it will let flow through the transistor. You can see that this transistor has 10 times the gain. You get 10 times more current out than you put it.
Of course the transistor has physical limits. The point comes where it just can t let any more current flow.

Transistors are like water taps, the more you turn them on, the more water they let flow. But you can only turn the tap on so much before no more water will flow. It s called “saturation”. (full throttle boys !)

A PNP transistor works just the same way, but in reverse.
They are negatively biased. The current flows from the emitter to the base which in turn lets more current flow from the emitter to the collector.

There is another characteristic that transistors have in common with diodes. They have a turn on voltage. It is typically 0.7 Volts.
This means that unless the voltage across the Base and the Emitter
will not allow any current to flow unless the voltage is greater than 0.7V. When transistors are used as switches, it is usally ignored.
When transistors are used as amplifiers, they are turned on a little
so that it isn t a problem. (it s called bias )

Some common transistors that you can find cheap and easy to play with are BC548 (NPN), BC558 (PNP), BC848 and BC558 are the same transistors
but in SMD packages.

There are many different transistors used for many different reasons.
Some are just simple switches, some are amplifiers.
Some are high frequency like radio or microwaves, some are high current for power, some are high gain for very small signals.
The back of the DSE catalog has a short list of some common transistors and their characteristics.

[betty.k]

good one, more useful stuff for my ever growing electronics 101 folder! but can you confirm this; popped into my local dse to buy a catalog and the guy told me they ve stopped making them. is this for good, or did he just mean for this year?:8ball:

[trash]

The catalog ? They must have become a full blown toy shop now. 🙁

You might be able to get hold of some old catalogs if you ask around. They have all sorts of useful stuff in them. A wanted ad in vkham.com will get you one if all else fails. Or a phone call to te NSW division of the WIA. They often have a stack to sell for $2.

[Crom]

Nice work trash!

Betty.k
You could try getting the Jaycar Catalogue instead? Still has useful info in the back.

Edited by – Crom on 25 November 2003 13:51:11

[Admin]

actually dick smith are not printing anymore catalogues…they are developing CD s…i gots one here!!!

[betty.k]

do they cost?:8ball:

[micro_Amps]

Of course they do, thats the whole idea.
:)uA

[Admin]

yep i think they will cost around $2.50 (yeah i know-ripoff) but then i can t complain i got it for free!!!

another bonus is that i work for woolies now (i bet you are thinking how the hell that relates to this) and after 1 month we get a discount card that can be used at all woolies owned stores. (woolies, Big-W, Tandy Dicksmith and there is some alcohol store too)

neway back to the original thread

[trash]

Wooohooo !! Fringe benifits.
Crappy fringe benifits for my job. I get $5 discount on my phone. I make my own alcohol, be buggered paying the humungus tax on it !

[Author]

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