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Monday, January 08, 2007

Basics of Electronics: Transistor Time!

Woo! Transistors! Drool, drool. Ok, my childish reactions apart, transistors are the backbone of electronics as we know it. They represent a control structure, where you can control one part of the circuit with another (as a master-slave relationship). As an overview, a transistor has 3 terminals (I'm talking about BiJunction Transistors' (BJT) here), which are called the Emitter, Base, and Collector (EBC). By changing the potential difference between the Base and Emitter, the current between the Emitter and Collector can be varied. I'm going to use acronyms for EBC's from now on, because I'm lazy.

Generic NPN transistor diagram below:

Ok, we'll discuss the NPN (negative-positive-negative) transistor right now. The PNP transistor is nearly the same, except the polarities are reversed. So the NPN transistor consists of three regions, a N junction (see previous article for junctions), a P junction and a N junction. They are the Collector, Base and Emitter respectively. If you want to, you can look at the transistor as two diodes, with the P junction fused. You can take two PN diodes and try to make a transistor out of it, but it'd be really crappy, because the P junction must be very very thin for reasons we shall soon see.

Let's connect up our transistor to a battery, and try and understand how it works:

Firstly, note that the C is connected in reverse bias (i.e. the positive end is connected to it), with respect to the EC connection. The BE is in forward bias though. Remember the depletion zone (from the previous article) right? Well, its here too. By varying the potential difference between the BE, we are able to change the 'thickness' of the depletion layer. That allows more current to flow. Initially, the layer is pretty thick, and electrons can't flow from the E to the C, so there is no current in the EC circuit. Now, as we increase the potential between BE, the layer thins down. So now electrons are allowed to flow into the B region.

Remember that the EC in reverse bias? That means that the C can't give electrons because of the gap, but it does mean that it is very very eager to take electrons. Thus, it sucks up the electrons that enter the B region. The B region must be very thin so that the electrons are not absorbed by the B or taken away from the circuit.

There you have it folks, by varying the BE potential, you vary the depletion layer. This allows a current into the B region, which is taken up again by the C region (so the Collector collects the current). Through a model, the relation between the V(BE) and I(EC) is Ice is proportional to exp(Vbe) (exp is e power). There are other components called FET (field emmission transistors), which vary as Ice proportional to Vbe^2.

In the next article, we'll discuss Op-amps, Comparators and Logic Gates

(Did I catch yous in time for the PBs?)

9 comments:

TwIsTeR said...

Hey Arun ! Im glad u post sumthin bout transistors man! IAm simply in

Arun Chaganty said...

It seems as if your sentence is incomplete

Gr81 said...

Yipeee...
Well I gave Maths on Monday (pathetic, so no more talks about it here). Comp on Wed, Chem on Fri,Eng on Sat, (last but certainly not the least) Phys on Mon.

Thinking that CBSE datesheet is bad? Beat ours! :D

Anyway nice article! I'll post some doubts (when I read all my notes and books)

Enjoy!

Arun Chaganty said...

Drat, I thought I posted a comment. Well, I think its because of the blogger - google shift thingy. When are you shifting your blog rash?

Oh yeah, could you send me a gmail invite? (Please with puppy eyes). I kinda need it. I want to use a mail client (evolution). Hotmail and Rediff don't let me (besides rediff is screwing with me) Thanks in advance

Gr81 said...

Yeah obviously Gmail is way better than any other Mailing service...
Infact I suggest EVERYONE to Swith to Gmail (PS: Swithcing to Gmail is VERY Easy. Refer to its help for more info)
And yeah, u can always use returnpath.net to send all ur old mails to Gmail!

Plus Gmail gives host of features from Conversations to Inbuilt Chatting and User Friendly Labels, Feeds, POP, Auto Forwarding, Filters and 2 GB of Space! (I've used 985 MB already)

PS: I wasn't paid by Google/Gmail to write this comment. Its coz I LLLLLOOOOOVVVVEEE Gmail (Or better say - GOOGLE!)
:D

Arun Chaganty said...

985 MB?! What the heck do you do to get 985MB?

I thought you need an invite to get into gmail. Send me an invite already: studiesrule@rediffmail.com (I've just entitled my self to a heck of a lot of spam but WHO cares, I'm going to gmail (jump around hysterically))

Everybody loves google (except MS). They have a very anti MS pro-FOSS stance. They use Python, are composed of super smart asses. What more do you want?

shruti said...

Okie.. usually i just use this blog to get to other blogs easily but the post on diodes and this one on transistors really helped me.. please continue writing, esp about logic gates..

Sunita said...

is a pnp transistor more efficient than a npn.i got into a row with my external examiner in my final practicals insisting that it was not.

TwIsTeR said...

It depends on the circuit, wether PNP or NPN is better.

The advantage of a NPN over a PNP is that the main current carriers in NPN are electrons while those in PNP are holes. Thus PNP transistors have a larger response time. Hence they are not used in circuits which require rapid switching.

However,
the switching circuit I recently made used PNP transistors. I used those only after trying many other NPNs ... and found out that PNP provided better switching !

So it depends on where you use them. Both are cool ! lol