Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Understanding Transistor

Status
Not open for further replies.
................
So "works" is a perfectly adequate term. If a beginner asked "how does a diode work?" the simplest answer would be "a diode works by only allowing electricity through in one direction".
................
Always insisting on absolute correctness is a crippling disability that I an sorry you are afflicted with. ;)

Hi Mr. RB,

it happens very often that the "truth" is something "in between".
That means: I agree with you - however, not in all parts.
For example, your "explanation" for the working principle of a BJT : Give it >0.6v at the base, and it turns ON. for my opinion is really not enough because it doesn't say anything. What means "it turns ON"?
Do you refer to switching applications only? Is it an "ON"-function like a relay? And what about the minus-pole of this voltage (a voltage exist between two nodes)?
On the other hand, I agree with you that "absolute correctness" (although not possible) is (a) not necessary and (b) can lead to formulas, which are not practical because one cannot identify the parameters with dominant influence. However, it is the task of a good engineer as well as a teacher to find the best trade-off between exactness and simplification.
(Coming back to the original problem: A statement like "the transistor turns on" is by far not enough to explain the working principle)
Regards
 
The problem of course when you get too pedantic is that it reduces ability. In most cases, the ability to communicate effectively with a beginner.

So "works" is a perfectly adequate term. If a beginner asked "how does a diode work?" the simplest answer would be "a diode works by only allowing electricity through in one direction".

That would have a number of errors to a pedantically imparied person like yourself, but is an extremely effective communication.

Always insisting on absolute correctness is a crippling disability that I an sorry you are afflicted with. ;)

Very well said Mr RB. For beginners, not everything needs to be spot on and in 100% agreement with absolute Scientific facts...Newbies want accurate and reliable SIMPLE help.

The rest of the learning comes much later if the person decides to stay here at ETO and dig deeper into the wonderful world of Electronics ;)

And then Ratch will amaze with his knowledge knowing that he has an attentive and alert audience....with the correct qualifications to understand all he is on about...

Just me and the way I look at life. Nothing against you Ratch.

Cheers,
TV Tech
 
Last edited:
I understand that but the OP is not the only reader.

Besides, the OP may re-read the thread once he is further up the learning curve.

Correct, but it is him I am replying to and not you. If others read it and gain from my analogy then something has worked. If you feel you can help better then go ahead.
Don't please just post one liners without contributing to helping the OP. It helps none other than your ego.

The skill of teaching is making something complicated align with something they can relate to. Some will learn, some won't. That's life in education.
 
You see now guys....the OP has buzzed off and will never check this thread again...Last Activity was Yesterday 02:43 PM.
sdmuashr has effectively abandoned ETO. Too scared to get involved with our outstanding cleverness at trying to outsmart each other :mad:

It doe's not help arguing amongst us "wise" folk here...the "we know it all"....folks.

The kid asked a simple question. And that deserved a simple reply. It really is that simple.

Another Newbie gone forever. Potential Einstein with the correct handling????

Cheers,
TV Tech
 
Last edited:
analogies are always a good way of getting a visual consept of wht is going on at first, then when a more detailed and exact understanding is needed later on it can be gained. There needs to be basics within the basics, You cannot lay foundations for a house if there is no solid ground.
 
... for my opinion is really not enough because it doesn't say anything. What means "it turns ON"?
Do you refer to switching applications only? Is it an "ON"-function like a relay? And what about the minus-pole of this voltage (a voltage exist between two nodes)?
...

If you have a better two-word term to give to a beginner I am happy to hear it. The phrase transistor "turns on" is quite standard in beginner teaching of transistor theory and although being incomplete in describing all forms of transistor biasing it is very beginner accessible, in the like of; "switch turns on" and "relay turns on".

WTP Pepper said:
...
The skill of teaching is making something complicated align with something they can relate to.
...

So elegantly said, and sums up the situation perfectly. Teaching is not about absolute correctness, it's about using the language most accessible to the learner that makes the learning faster and easier.

I suppose my wisecrack to Ratchit (and thanks for taking that in the good humour it was intended Ratchit) was saying that insistence on absolute correctness results in something like a "legalese" language. Complete with provisos and subclauses... More correct, for sure, but not more empowered in the task at hand; imparting some entry level useful knowlege to a beginner.
 
If you have a better two-word term to give to a beginner I am happy to hear it. The phrase transistor "turns on" is quite standard in beginner teaching of transistor theory and although being incomplete in describing all forms of transistor biasing it is very beginner accessible, in the like of; "switch turns on" and "relay turns on".
Perhaps the word "flow" would be better as it applies to both water & current.
 
If you have a better two-word term to give to a beginner I am happy to hear it. The phrase transistor "turns on" is quite standard in beginner teaching of transistor theory and although being incomplete in describing all forms of transistor biasing it is very beginner accessible, in the like of; "switch turns on" and "relay turns on".

Hello Mr RB,
thank you for your reply. May I ask you:
1.) Is there any necessity to use a „two-word“ term to roughly describe what a transistor is doing?
What happens after the transistor is „on“? I think, one should spend some more words to answer the original question.
Let me try it with one simple sentence:
A fixed dc voltage between collector and emitter allows a current to flow between these two nodes that can be controlled by another voltage between base and emitter, which may vary around a bias voltage of approx. (0.6-0.7) volts.
2.) Let me give another example related to BJT operation:
Based on the simple relation Ic=B*Ib (or ic=h21*ib) many people and even textbooks clearly state: The BJT is a current-controlled device.
However, knowing the physical background it is obvious that such a description is false.
Is there any good reason not to confront the beginner with the truth?
Is it really more complicated to explain the working principle of this voltage-controlled device? I don’t think so because it works for the FET.

3.) Summary: I think, simplification is good and necessary - it is even unavoidable – however, down to a certain degree only. I am afraid, if we to much simplify the physics of the electronic world the beginner does not feel the necessity – and is not motivated enough – to look „behind the curtain“. Why not leave him alone for a while with a lot of open questions?
According to my experience in educating engineers it is very important for beginners and students to know about their „gaps“. This is the first and most important step to fill the gap – in a second step - with knowledge.
 
I'm inclined to agree with Winterstone.

You can only take the water analogy so far. Sooner or later, you have to introduce the Physics.

I have attached a simulation of what Winterstone wrote - that the collector current is a function of the base/emitter voltage.

The Ic = β Ib is very useful, but it does not explain the fundamental operation of a BJT.

When I was at uni, our Electronics 1 lecturer introduced the BJT by saying that it is "a voltage controlled current source".
 
You can only take the water analogy so far. Sooner or later, you have to introduce the Physics.

This gives occasion to come back to the water analogy as given in post#4:

Quote: Start injecting a little water into the Base pipe and it starts the C to E valve to open.

This was a statement without any justification.
I think, everybody who has a certain "mechanical feeling" will agree that it seems impossible that a "small water-flow" has enough power to open the door in another valve - and even keeps it open - for a much larger water-flow in this valve. Isn't it rather a kind of mechanical force (equivalent to a voltage) that only can do the job? Analogy is good - as far as it helps to understand what's really going on.
 
Hi,

I have huge problem understanding circuit involving transistors. Therefore, I would be very thankful if anyone here can guide me to any online material or a book (preferrably low cost that can be bought) which is about the pratical use of transistors (BJT, FETs) and how they work in circuit about their biasing. The material should be down to earth.

I also saw capacitor being used in transistor circuit though I am not 100 % sure about their use with transistor.

Thanks

We seem to have lost sight of what the OP was asking.

The kid asked a simple question. And that deserved a simple reply. It really is that simple.

I would suggest rather than finding fault with the replies of others, you answer the OP's questions in the language/detail you think appropriate.

E.
 
Last edited:
Hi Eric,

I am afraid, I cannot agree.
In contrary, I think the whole thread is full of answers. And, more than that, the OP can learn something about the problems and the necessary degree of simplification. That is the basic job of an engineer and it is not a waste of time to read something about.

More than that, the question was (partly) answered already in post#2 (recommendation of a particular link)
What elese can we do? I think, it is not appropriate to explain the transistor principle in detail here in the forum.
Thus, we could search via google to find some pages explaining the basic principles - and recommend it to the OP.
However, why not suggest to the questioner to do the same? Did he never hear about Wikipedia?
I am not a wiki-fan but it is always good for some basic information.
 
I cannot resist to add another comment.
Several years ago - in another electronic forum - I have asked "Is such a forum dangerous for beginners"?
The background of this provocative question was the following: I am afraid that somebody who has a certain task to solve does not rely on (or even is not willing to use) his own capabilities for exploiting several sources of information (books, articles, web, ...perhaps brainstorming?.). Instead, it is easier to place a simple question to other persons in such a forum. In most cases, he will get an answer - however, I doubt if this will be a good base for better knowledge and understanding. ..... Perhaps I am to pessimistic.
What do you think about it?
 
This is a reasonable anology, but it needs qualification.

A BJT is a voltage controlled current source.

The collector current is determined by the charge in the base/emitter which is a function of the base/emitter voltage.

Certainly, the collector current is proportional to the base current (Ic = β * Ib) while the transistor is in the active region, but it is not fundamental to the BJT operation.

No it's not. A transistor is a base current controlled vs collector current device. How you deliver that current to the base maybe voltage controlled but it's the base current that affects the collector current.

A potential divider on the base may control the voltage on the base, but it's the base current that controls the C to E current.
Try getting a basic BC109 transistor to switch with a divider of 100M & 22M resistor to prove the point on its base with a 4.7K collector load. It won't work.

There will be insufficient current flowing through the base x its Hfe to cause the BJT to switch due to its required collector current..
 
I'm afraid you're wrong.

You need to study how a BJT works.

Did you see the attachments in my post # 30?
 
WTP Pepper,

No it's not. A transistor is a base current controlled vs collector current device. How you deliver that current to the base maybe voltage controlled but it's the base current that affects the collector current.

In the link below, notice the crucial sentence in the second paragraph, "The root cause of the this misconception is the inability to distinguish a functional relation from a causal relation." After you read the link, we can discuss it more.

Ratch

https://www.kevinaylward.co.uk/ee/voltagecontrolledbipolar/voltagecontrolledbipolar.html
 
The attachments below and in the next post show examples of where Ic = β Ib cannot be used to explain the behaviour of the BJT.

First some words of introduction.

The collector current is controlled by the charge in the base/emitter region. This charge is proportional to the base/emitter voltage.

However, you can’t measure the charge but you can measure the base/emitter voltage.

That is why the BJT is said to be voltage controlled.

You have to calculate the charge using fundamental BJT parameters.

The example below shows the simulation using LTSpice of a transistor being switched from cut off to saturation and vice versa.

There is a delay time before the collector current rises, and a storage time before the collector current starts to fall.

In this post, I’ll concentrate on the storage time. When the BJT is in saturation, there is an excess of charge in the base/emitter region.

When the BJT is to be switched off, the excess charge has to be removed before the collector current can fall as shown in the simulation.

The time taken for this to occur is called the storage time because the stored charge has to be removed in order for the transistor to switch off.
 
Last edited:
Delay time

There is a delay time when a BJT is to be turned on.

This is because it takes time for the charge in the base/emitter region to accumulate.

Again the Ic = β Ib is of no use.

More fundameltal calculations are required to estimte the delay time.

I used the .asc simulation that is posted above for this simulation. I just changed the time scale to highlight the delay time region.
 
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top