what is meant by fet-(field effect transistor)

 

Originally posted by vinayak@Mar 12 2006, 07:04 PM
what is meant by fet-(field effect transistor)

[post=14895]Quoted post[/post]​

mmmmmm well it is nice to see you want to jump in at the deep end. Not knowing your electronics background I will attempt to do this through analogy.

A FET differs from a BJT (Bipolar Junction Transistor) in as much as a FET is a votage controlled device and a BJT is a current controlled device. Having said this a BJT is still controlled by a voltage on the base but only is as much as to increase or decrease the base current which in turn increases or decreases the collector current.

A FET on the other hand uses the voltage on the gate of the FET control the drain current. It does this by "pinching" of the current flow from source to drain. If you consider a FET to be a hose with a variable clamp around it. As you tighten the clamp less water will flow.

That is the simple version. If you want to go into the electron flow and how holes and electrons move or the attraction and repulsion of electons then there has to be some online tutorials out there somewhere.

Hope that helps

PS. If you know how a triode valve works then you can consider a FET as a cold triode.

 

Originally posted by windoze killa@Mar 12 2006, 09:51 AM
....A FET on the other hand uses the voltage on the gate of the FET control the drain current. It does this by "pinching" of the current flow from source to drain. If you consider a FET to be a hose with a variable clamp around it. As you tighten the clamp less water will flow.....

[post=14898]Quoted post[/post]​

That's a good analogy.

 

think u got your answer?

 

In addition to the superb answer provided by windoze killa, you may wish to refer to The Art of Electronics by Horowitz and Hill which will give you a comprehensive overview of FETs (both MOSFETs and JFETs) without the excessive mathematical baggage that comes with learning about these devices.

AAC: Volume III deals with semiconductor devices that will explain in more detail about FETs. There is a lot of content in this section so you are best asking questions about sections you may not be familiar with here in the forums.

Dave

 

Originally posted by Dave@Mar 13 2006, 12:35 AM
In addition to the superb answer provided by windoze killa, you may wish to refer to The Art of Electronics by Horowitz and Hill which will give you a comprehensive overview of FETs (both MOSFETs and JFETs) without the excessive mathematical baggage that comes with learning about these devices.

AAC: Volume III deals with semiconductor devices that will explain in more detail about FETs. There is a lot of content in this section so you are best asking questions about sections you may not be familiar with here in the forums.

Dave

[post=14904]Quoted post[/post]​

WOW Thanks for the praise guys. I was just trying to make it simple. After teaching RF comms and multi layer PCB repair for 8 years I found that you can throw all the maths and theory you lke at a student but if you don't bring it down to their level they will never learn. Also it was technicians I was teaching and not engineers so there were no PHDs involved.

 

windoze killa you should come and teach at my uni! These formulas they through at us really makes things confusing

A lot of the lecturers tend not to lay good foundations down and rather jump straight into a lot of maths etc. They need to explain more on how these components work, so it will allow us to understand the formulas a lot easier. If you don't understand what is supposed to happen, the formulas just become useless numbers.

 

Originally posted by Hurdy@Mar 13 2006, 07:14 PM
windoze killa you should come and teach at my uni! These formulas they through at us really makes things confusing

A lot of the lecturers tend not to lay good foundations down and rather jump straight into a lot of maths etc. They need to explain more on how these components work, so it will allow us to understand the formulas a lot easier. If you don't understand what is supposed to happen, the formulas just become useless numbers.

[post=14944]Quoted post[/post]​

The University obsession of throwing every possible equation at students is what makes the subject of electronics more difficult than it needs to be; equations are alright when a student grasps the concept, however its tends to be the case that Universities wish to make a student know the equation before (and sometimes at the expense of) the concept. Also there seems to be too much emphasis in Universities on derivation of equations - all this tests is can you remember a sequence of instructions.

As for PhD's, my experience of working with people of such status has not always been a positive one, but then that's another story!

Dave

 

Originally posted by Dave@Mar 13 2006, 08:07 PM
The University obsession of throwing every possible equation at students is what makes the subject of electronics more difficult than it needs to be; equations are alright when a student grasps the concept, however its tends to be the case that Universities wish to make a student know the equation before (and sometimes at the expense of) the concept. Also there seems to be too much emphasis in Universities on derivation of equations - all this tests is can you remember a sequence of instructions.

As for PhD's, my experience of working with people of such status has not always been a positive one, but then that's another story!

Dave

[post=14948]Quoted post[/post]​

It is unfortunate that there are poor teachers in all subjects, and many authors who make a living from it. I recall going on a management course many years ago and being asked to study a chapter on the subject of communication in Peter Drucker’s book ‘The Practice of Management’. It spanned about five pages and was full of what I call ‘nebulous waffle’ or modern management-speak. I had to re-read it three times before I understood what he was saying - it was that you should keep communications simple! I was quite surprised that my company was ‘taken in’ by such drivel!

About 30 years ago some colleagues and I decided that we really ought to get to grips with these newfangled ‘microprocessor’ things. The BBC computer was with us and micros were being introduced into new equipment. We felt that if we didn’t do something about it we would be left behind in the race for knowledge. The next step was to enrol in a course of evening classes at the local ‘College’ to learn all about the 6805.

Our introduction to the subject was nothing short of disastrous. Our instructor headed up an R&D in switched mode power supplies, and from all accounts was very good at his job. The trouble was lecturing in the evening was just a second source of income to him. English was not his first language and he had a heavy foreign accent and had not planned his lectures either. He had no handouts and just tried to explain things as they came into his head.

The first part of a micro he discussed was the stack pointer register – completely out of context because no one knew what it was! He would make several attempts at drawings on the board to explain things – only to rub them out each time. Because of his heavy accent he was hard to follow and by the time he had got it right at the 3rd or 4th attempt, the class had switched off.

Twenty x 2hour sessions later about the only thing I knew about the 6805 was that it had a stack pointer register. The rest went over my head. There was an attempt at writing machine code but it was just done parrot fashion. Everyone in the class felt the same.

A short while later I was approached by the head of training department in our own organisation, to say he was laying on a series of lectures on micros. The lecturer was to be an experienced trainer and was I interested? The lectures were to consist of a total of 9 hours theory and practice. I opted in. What a difference. The instructor started at the beginning by explaining how a group of transistors could be made to act like any type of gate by turning on some of the transistors, and so manipulating one bit of data. Which transistors were turned on and which were not constituted the ‘instruction’. The group of transistors were then increased to 8 groups to handle 8 bits at a time in a ‘register’. The explanations progressed smoothly and logically and everything he said immediately clicked. At the end of the course I had mastered the micro!

The point is there are good and bad teachers and tutorials. If you are at uni or college you cannot always choose so if you come up against problems then you can always ask in forums like this one. Here there will always find someone who can explain things to your satisfaction.

 

You make an interesting point pebe, about the poor quality of teachers/lecturers. This is an inherent problem, even in the world's highest educational institutes. The sad fact is that many educational institutes are more interested in getting "bums-on-seats" to get the income from governments (this is certainly the case here in the UK).

This is also related to universities in the UK now expecting all students to pay tuition fees and take out loans to pay for their education, whatever happen to investment in our future. On a side matter the problem with people studying for postgraduate qualifications, e.g. PhD and MSc, is that they pay good money (anything up to 10's of thousands of pounds) to study for the qualification - as far as the University is concerned this is the key factor; this means that the only route to postgraduate qualification level is to be born into a financial situation that allows for it. As a result, you tend to get a mono-type of people in the higher levels of engineering education, where their only experience of engineering is in solving equations and studying theories. Not to mention that someone who is from a poorer background, who incidentally could be smarter than any of their peers, would not be able to work towards these postgraduate qualifications, due to their background.

I can't comment on what its like in other parts of the world, and we may get it easy here in the UK, but I have fundamental issues with the education system. That said I'm a by-product of the very system I have problems with!!

Dave

 

Originally posted by Dave@Mar 15 2006, 01:26 AM
You make an interesting point pebe, about the poor quality of teachers/lecturers. This is an inherent problem, even in the world's highest educational institutes. The sad fact is that many educational institutes are more interested in getting "bums-on-seats" to get the income from governments (this is certainly the case here in the UK).

This is also related to universities in the UK now expecting all students to pay tuition fees and take out loans to pay for their education, whatever happen to investment in our future. On a side matter the problem with people studying for postgraduate qualifications, e.g. PhD and MSc, is that they pay good money (anything up to 10's of thousands of pounds) to study for the qualification - as far as the University is concerned this is the key factor; this means that the only route to postgraduate qualification level is to be born into a financial situation that allows for it. As a result, you tend to get a mono-type of people in the higher levels of engineering education, where their only experience of engineering is in solving equations and studying theories. Not to mention that someone who is from a poorer background, who incidentally could be smarter than any of their peers, would not be able to work towards these postgraduate qualifications, due to their background.

I can't comment on what its like in other parts of the world, and we may get it easy here in the UK, but I have fundamental issues with the education system. That said I'm a by-product of the very system I have problems with!!

Dave

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Well Dave, it is the same here in Oz. We have to pay a fortune to go to Uni and the government have to suck there fees out of us to. But I tricked them. I never went to Uni. I did all my training in the defence forces which is also where I did my teaching.

With respect to good and bad teachers you couldn't be more correct. I think there are 2 main things required to be a teacher, 1: Want to be there and 2: Be teaching at a level for the students. If you are teaching technicians like I did then you should be or have been a technician. It is just a waste of money and resources having an computer science PHD teach diode theory.

You may have noticed I did use the term lecturer. I do not consider these people "teachers" as all they do is stand out the front and waffle on. Most of what I have seen of them you don't get a question and answer session. You just take notes and work it out later.

I am so glad I have found this and a couple of other sites as I get a real kick out of helping people. Same reason I enjoyed teaching so much. You see a dumb thickhead come in one door and a not quite so dumb thickhead go out the other. I have also learnt a few new things as well.

And to all you Pomgolians over there in the mother land, I should be over there for a few weeks around June. Will be staying near Manchester.

 

Originally posted by windoze killa@Mar 15 2006, 10:35 AM
And to all you Pomgolians over there in the mother land, I should be over there for a few weeks around June. Will be staying near Manchester.

[post=15017]Quoted post[/post]​

At least you are staying in the better part of the UK!! All this London lark is no good for the visiting national. Be sure to take a look at our fantastic engineering heritage in Manchester, particularly go and see the Manchester Museum of Science and Industry should you get the opportunity.

Dave

 

Originally posted by Dave@Mar 18 2006, 11:10 AM
At least you are staying in the better part of the UK!! All this London lark is no good for the visiting national. Be sure to take a look at our fantastic engineering heritage in Manchester, particularly go and see the Manchester Museum of Science and Industry should you get the opportunity.

Dave

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I was there for about 3 weeks last year and hardly saw Manchester at all. Did the tourist thing over Middlesborough and Newcastle. Absolutely love the scenery and history. Durham was amazing.

 

Originally posted by windoze killa@Mar 18 2006, 09:31 AM
I was there for about 3 weeks last year and hardly saw Manchester at all. Did the tourist thing over Middlesborough and Newcastle. Absolutely love the scenery and history. Durham was amazing.

[post=15133]Quoted post[/post]​

Aye "Geordieland" is quite a place. The Geordies are great people, if a little difficult to understand!

Well if you're staying in Manchester, not only do you have the luxury of the fantastic engineering heritage and industrialisation of the city, but 1/2 an hours drive from the city you are in the Peak District National Park which is some of the most beautiful scenery in the UK. I'm sure you will find that Manchester has so much to offer, and since they have invested extensively in it recently, it has a very cosmopolitan feel to it.

Anyway I don't want to digress to much from the original topic here, if you want to chat about the UK start up a thread in the Off-Topic Lounge, and we'll have a chat there. It might be quite interesting, I know there are quite a few "Poms" who frequent these boards.

Dave