Understanding transistors.

  • #1
PrincePhoenix
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I was just curious about trying to understand how transistors work, but didn't understand it properly.
http://amasci.com/amateur/transis.html"

I found this site through google, which claims that most traditional explanations have contradictions and wrong concepts. And that this simple explanation is how a transistor really works.
I just want to know whether what is being said and explained here is true.

I am a high school student so I haven't formally read transistors in my course yet.

Thank you.
 
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Answers and Replies

  • #2
davenn
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interesting site haha

he is really arguing over minor things when it comes to the basic understanding of what is happening

So what flows inside of wires?

The stuff that moves within wires is not named Electric Current. Intead it is called Electric Charge. It's the charge that flows, never the current. And in rivers or in plumbing, it's the water that flows, not the "current." We cannot understand plumbing until we stop believing in a magical stuff called "current." We must learn that "water" flows inside of pipes. The same is true with circuits. Wires are not full of current, they are full of charges that can move.

yeah yeah, fair comment we call it current for "ease of understanding" the "Electric Charge" or moving electrons is a current

but it still goes back to a small current flowing (And we wont even go into conventional Vs electron current flow) between the Base and Emitter opens the Collector to Emitter path and controlling the amount of that flow controls the amount of flow between the Collector and Emitter.

for the avg non-physics hobbyist etc thats the main thing you need to know. You can then move onto the operation of FET's etc
In fact I really need to sit down and look into the actual operation of FET's from a basic point of view ;)

Maybe some one on here will teach both of us :)

Dave


Dave
 
  • #3
vk6kro
Science Advisor
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The collector current of a BJT transistor is proportional to the base current. That is, if you increase the base current there is a proportional increase in collector current.

The base current is changed by changing the voltage across the base emitter junction, but the collector current is not proportional to this voltage.

So, there is some truth in what that article says, but it is a fairly silly way of looking at it.

For example, you can change the base voltage from 0 to 0.4 volts and it will have no effect on the collector current, which will stay at zero.
So, how could you say that this voltage is controlling the collector current?

If you change the base current from 0 to 100 µA the collector current (on a transistor with a current gain of 50 ) will change from 0 µA to 5 mA. So, it is fair to say that the transistor is controlled by base current rather than base voltage.
 
  • #4
davenn
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Hi VK6RO
Dont know your name ;)

So, there is some truth in what that article says, but it is a fairly silly way of looking at it.

yeah my thoughts exactly, he's really just arguing terminology :)

ok so would you like to tell us about FET's then ? they are someting I have not really got into from an operation perspective


Dave
VK2TDN
 
  • #5
vk6kro
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You might like to read the Wikipedia article about FETs:

http://en.wikipedia.org/wiki/Field-effect_transistor

The best way to learn about them, though, is to get a couple and put them in circuits with a couple of power supplies.

I would suggest a junction FET like a MPF102 and a N-Chanel MOSFET.
In Australia, Jaycar have a BUZ71, but beware of using them without a series limiting resistor in the drain circuit. The drain current can get very high, very quickly.
 
  • #6
davenn
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hi from the east coast :)
so sick of the heatwave over here for the last 7 days :(
yeah time to do some serious reading up :)

Its crazy, have used them for yrs and yrs in various radio projects and at work in commercial audio amplifiers but havent really learnt what makes them tick :)

Dave
 
  • #7
PrincePhoenix
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Thanks for the help, vk6kro. That really helped.
 
  • #8
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The site mentioned has some truth as well as non-truth. The best info is from companies that make transistors. Peer-reviewed text books & publications from transistor makers & universities are trustworthy, reliable sources of info.

To search the web results in much truth & untruth. A beginner does not know which is which. Stick w/ peer-reviewed info & you will be fine.

The problem w/ web sites, is that the person publishing their viewpoint is convinced that they are right. Trying to tell them otherwise is futile. People who do not have the education & time spent applying it are unaware of how much they don't know. The operation of a transistor is pretty intense. To fully get it involves quantum mechanics which no mortal person can fully understand.

I'd advise all who wish to learn about the subject to go to a library of a uni w/ a good elec engr college. There should be some reference texts written at an undergraduate level. These are good sources. My recommendations are Hayt & Neudeck, & Horowitz & Hill. There are many others. Search National Semiconductor, Texas Instruments, International Rectifier, Fairchild, etc., & you should be able to download some technical papers detailing device operation.

I hope I've helped. BR.

Claude
 
  • #9
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So you have found Beatty's site.

I agree with VK6KRO that although he has much of worth to say, his abrasive style, railing against the establishment, makes it difficult to separate the worthwhile from the politicking.

Remember that all explanations / descriptions of transistor action are simplifications or models of the truth.
The trick is in selecting an appropriate model that behaves in the same way as the properties you are interested in and so allows you analyse what is happening and successfully predict what will happen if a particular change is made.

So if you would like to explain whether you are interested in how transistors act in electric circuits or the underlying physics of what happens within a transistor. The explanations will be quite different in these two broad cases.
 
  • #10
AlephZero
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ok so would you like to tell us about FET's then ? they are someting I have not really got into from an operation perspective

I haven't looked at the OP's link, but the basic operation of BJTs and FETs are fundamentally different.

As another answer said, in a BJT the collector-emitter current is controlled by the base current. What happens to the voltages depends on the complete circuit the transistor is in. For instance if you want to make a circuit that amplifies voltage rather than current, you can turn the current change into a voltage change by passing the current through a resistor.

On the other hand, in an FET the drain-source current is controlled by the gate voltage, and in many practical circuits the gate current is so small (e.g. 10^-10 or 10^-12 amps) that you can ignore it completely. because of this, FET circuits are usually very smilar to old vacuum tube (valve) circuits, except the working voltages are usually smaller.

BJTs and FETs have very little in common apart from both being called "transistors".
 
  • #11
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For example, you can change the base voltage from 0 to 0.4 volts and it will have no effect on the collector current, which will stay at zero.
So, how could you say that this voltage is controlling the collector current?

If you change the base current from 0 to 100 µA the collector current (on a transistor with a current gain of 50 ) will change from 0 µA to 5 mA. So, it is fair to say that the transistor is controlled by base current rather than base voltage.

The base-emitter voltage controls the collector current. The relationship is exponential. Going from Vbe = 0V to 0.4V doesn't have the same effect as going from 0.4V to 0.8V.

"Whenever the base-emitter junction is forward biased, some holes are injected from the P-type base into the N+ emitter. These holes are provided by the base current, Ib. Ib is an undesirable but inevitable side effect of producing Ic by forward biasing the BE junction."
p. 297, Modern Semiconductors for Integrated Circuits, Chenming Hu

It is more convenient to think of Ic as being proportional to Ib. Both views are correct.

From the same book:
"Because of the parasitic IR drops, it is difficult to accurately ascertain the true base-emitter junction voltage. For this reason, the easily measurable base current, Ib, is commonly used as the variable parameter in lieu of Vbe."
 

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