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Author Topic: Transistors  (Read 326 times)
VK5FDAV
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« on: June 01, 2007, 04:30:06 AM »

Hi all, I am having trouble finding a decent explanation of how transistors work or more so how amplifiers work. All my googleing just turned up technical stuff about atomic structure of semiconductor material. Am I wrong in my understanding that transistors are just a type of non mechanical switch?
how do transistors take a signal and make it bigger? is it by adding dc to it?
Do I not understand the word "amplifier"?
I realy just want to know in simple terms how a simple transistor amplifier REALLY works.

73s.
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SSB
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« Reply #1 on: June 01, 2007, 04:47:38 AM »

Your question is a contradiction. If you REALLY want to know how a transistor works then you have to understand how a junction REALLY works.  But you don't want to know that so you are screwed.


Alex......
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N7NBB
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« Reply #2 on: June 01, 2007, 05:22:37 AM »

Rather than a "SWITCH"... Which, by the way would be an SCR, FET, and others... think of the "BASIC" transistor as a "variable resistor" limiting current flow.  The degree of limitation of the current flow is controled by a voltage applied to the BASE.  So the current FLOW TRHOUGH the JUNCTION can be made smaller or larger. Of course if you just slam a signal (voltage) onto the base all by itself, it WOULD just turn the transistor off (making it a switch) but by regulating the amount of signal reaching the base (done by a group of resistors in a configuration to form a voltage divider network) the transistor can be CONTROLLED in such a state that is neither completely ON or completely OFF.  A very small change in this BASE control (voltage) can result in a MASSIVE change in the CURRENT through the transistor... and THAT, is called "AMPLIFICATION"
The other poster(s) are correct.  If you want to learn HOW a transistor works, you will have to understand how voltage and current travel through the
 transistor's junctions... (yep that voltage is ELECTRONS and the properties of the electrons and the properties of the TYPES of junction materials DO make a difference.  For a QUICK PRIMER... go to
http://www.sci-toys.com/scitoys/scitoys/computers/solderless/amplifier.html THEN scroll most of the way down the page and read.."HOW IT WORKS"... hopefully that will answer your basic question... after that GOOGLE: "TRANSISTOR THEORY OF OPERATION" and get to know the basics of "WHY" (that other site works) and not just the "HOW".
Good Luck
Cam
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KB9CRY
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« Reply #3 on: June 01, 2007, 05:24:16 AM »

Try Wikipedia or try an Extra License manual or the ARRL Handbook.  Lots of explanations there.
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KD4RBG
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« Reply #4 on: June 01, 2007, 07:26:45 AM »

how do transistors take a signal and make it bigger? is it by adding dc to it?
------------------------------------------------------
Well, sort of, in the configuration I think you're talking about.

A transistor amplifier circuit in its simplest form takes a "drive" signal in on its base and varies a larger DC source and by so doing makes a larger "copy" of the supplied signal.

The British have the right idea, I think, because they call it a "valve".  Think about the water in your bathroom sink, a high point in the drive signal turns the water on higher pressure, a lower drive signal gets you a trickle.  No signal would turn it off, a maximum signal would turn it on full-blast.  Sometimes, this is inverted (reversed), by which I mean a high signal turns it off and no signal would make it run full blast, but that's at the discretion of the designer.

Yes, this is a massive oversimplification, but it is what was asked, so please don't start flaming, folks.  He didn't want an EE degree, he wanted a general idea.

Too much of the time we're saying "Go to Google" as if we were stockholders.  There's information there on the subject, but very little of it is for the laity.  I hear self-proclaimed experts say stupidly inaccurate things about general electronics all the time on the air and there are enough lies about *everything* online to make the whole body of work questionable.  In short, the Internet isn't peer-reviewed and we risk feeding new people to the wolves by saying "Go to Google" all the time.

They're coming here to be Elmered...if we're not ready to oblige, we shouldn't post here at all.  There are plenty of other forums.

Jeff/KD4RBG
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N3OX
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« Reply #5 on: June 01, 2007, 08:20:03 AM »

Some electronics lectures online:

http://www.du.edu/~etuttle/electron/elecindx.htm
http://www.phys.ualberta.ca/~gingrich/phys395/notes/phys395.html

If you're getting stuck on HOW a transistor does what it does, concentrate first on *WHAT* it does by looking at the collector current vs. collector-emitter voltage curves for different values of base current.

Build an amplifier and play with it.  Do some experiments.

Everybody likes to start with a basic explanation of transistor junctions but I think it's easier to start with the voltage and current relationships and later learn how the junctions make that happen.

73,
Dan

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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« Reply #6 on: June 01, 2007, 08:31:12 AM »

"Rather than a "SWITCH"... Which, by the way would be an SCR, FET, and others... think of the "BASIC" transistor as a "variable resistor" limiting current flow"

Except that a FET out of saturation is much more a variable resistor than a bipolar transistor is, and BOTH a FET and a bipolar transistor are used as switches.

A bipolar transistor used as an amplifier is more of a variable constant current source than a variable resistance... sounds awfully like an oxymoron, maybe I should say constant current source programmed by the base current...

- - - - -

Anyway, run through some lectures... see if they help you get a handle on things.  Learning electronics requires building on the basics you already know, so it takes some doing to find a treatment that starts at a point where it clicks with you.

A lot of people who already know electronics will recommend things to you that look like they make sense... that look like they start from the basics and are clearly written and comprehensive, but they're looking at it in retrospect.  

The Art of Electronics by Horowitz and Hill is a good example.

http://www.artofelectronics.com/

It's a faaaaaaantastic book, but I've seen people who don't know electronics at all just give up in despair when someone hands it to them and says "this is a good book to teach yourself from".

Just keep looking until you find something that clicks.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KA5N
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« Reply #7 on: June 01, 2007, 08:51:47 AM »

As mentioned transistors are valves and when used in circuitry all you have to do is apply Ohms Law to understand how they work.  Why they work is a different thing.  What is essential is understanding that a small current or voltage can control a larger current or voltage (amplification).
Also, where did the idea come from that everything has a simple one sentence explaination?  
Want to learn electronics, take a course at your local junior college, or find a live elmer that will instruct you.  If you are serious about learning something it takes effort on your part.
Allen
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K8AG
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« Reply #8 on: June 01, 2007, 09:36:21 AM »

People throw the word signal around without much precision or explanation.  If we assume that we are talking about a bipolar transistor, we need a current in a forward direction in the base/emitter junction to allow a larger current in the collector.  Both base and collector currents flow through the emitter.  But a little current forward through the BE junction allows a lot of current (reduces greatly the resistance to current) through the collector into the emitter.

If you truly understand DC Ohm's Law this will be clearer.  We are talking about currents with a bipolar transistor.  Other transistors use voltage at the low power end to allow current to pass.

Hope this helps some.

73, JP, K8AG
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N6AJR
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« Reply #9 on: June 01, 2007, 12:27:52 PM »

a typical transistor has 3 connectors.  call them an in, and out and a control.

now lets look at a water hose the same way

you have water in from the city pipe, and water out with the hose,spout or what ever and you have a handle of some sort controlling the flow.  

in a transistor the in and out junction is controled by the power on the 3rd leg. so typically you have a small voltage on the control leg, which is the signal you want amplified, controlling the bias on the junction.  the more signal on the control, the more power flows through the junction, so the 3rd leg acts like a faucet controlling the flow of the fire hose with a small input ( turn the knob) controls the output proportinally.

so there ya go.  not technicslly accurate, but in spirit, that is how it works.

a diode acts like a one way check valve. as in water can go in but the flap closes on back preasure to keep the water from going back doen the supply pipe.


and so on.. you mite find the local junior college has a course in basic electronis for not too much $$$
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VA3EP
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« Reply #10 on: June 01, 2007, 01:04:29 PM »

This is a book that I recommend for people I am elmering:

"Electronics For Dummies by Gordon McComb, Earl Boysen"

http://www.dummies.com/WileyCDA/DummiesTitle/productCd-0764576607.html

Very hands on orientation, a great starter book. Does not assume previous knowledge or deep math/physics knowledge.

There is a lot in there about how transistors work for amplifying and switching.


Eric
www.va3ep.net
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W8JI
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« Reply #11 on: June 01, 2007, 02:18:33 PM »

My sister went to Sweden and got a sex change.

She is a transistor now.
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N6AJR
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Posts: 9879




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« Reply #12 on: June 01, 2007, 03:35:12 PM »

ouch
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