a question about transistors

[samcheetah]

i have read in books that in a transistor the voltage between two terminals control the current through the third terminal. like in a common emitter (or common source) amplifier the voltage between base and emitter (voltage between gate and source for the case of FET) controls the amount of current in the collector (or drain) terminal. this explains how transfer of resistance occurs in a transistor and why the device is named so.

some people dont like to explain "transistor action" like this. the transistor can also be thought of a current controlled device. in that case the current in the base controls the current in the collector.

now for my question.

in a common collector configuration what is the situation. the voltage between which two terminals is controlling a current through the third terminal. please explain. thanx in advance

 

[Russlk]

In a bipolar transistor, the current from base to emitter creates a current from collector to emitter that is Beta times the base current. The emitter current is Ib*(Beta+1) and the collector current is Ib*Beta.

 

[samcheetah]

In a bipolar transistor, the current from base to emitter creates a current from collector to emitter that is Beta times the base current. The emitter current is Ib*(Beta+1) and the collector current is Ib*Beta.

thanx but i already know that and i also said that in my post. thats the way of seeing it as a current controlled device. altho it is not a good way to visualize the "transistor action" it does provide answers to a few questions

my question was something else.

 

[Nigel Goodwin]

In a bipolar transistor, the current from base to emitter creates a current from collector to emitter that is Beta times the base current. The emitter current is Ib*(Beta+1) and the collector current is Ib*Beta.

thanx but i already know that and i also said that in my post. thats the way of seeing it as a current controlled device. altho it is not a good way to visualize the "transistor action" it does provide answers to a few questions

my question was something else.

Your question was fairly meaningless, a transistor IS! a current controlled device - and it is the only way to visualise the 'transistor action' (as you called it).

The voltage between base and emitter is pretty well constant, it's not voltage controlled - thinking of it as so will prevent you understanding how circuits work.

All three transistor configurations basically work in the same way, base current being amplified by the transistors gain to give the collector current, with the emitter current being the sum of both.

If you're talking FET's, they are voltage controlled, but bipolar transistors are current controlled.

 

[Roff]

One way of looking at a common collector (emitter follower) circuit is as a negative feedback circuit. The control voltage is not applied between two terminals of the transistor, but between the base and another node in the circuit (usually defined as ground). The emitter voltage will follow the base voltage with just enough offset (diode drop) to satisfy the condition Ib=Ic / beta.

 

[samcheetah]

a transistor IS! a current controlled device - and it is the only way to visualise the 'transistor action' (as you called it).

thats what u think of it. if u r satisfied with that explanation thats okay for u.

The voltage between base and emitter is pretty well constant, it's not voltage controlled - thinking of it as so will prevent you understanding how circuits work

no it isnt constant. thats the controlling factor. it is constant when we set the bias. ofcourse thats true. but that doesnt mean the base voltage doesnt control the collector current.

the way u r looking into a transistor has a flaw.

when u think of a transistor as a current controlled device u apply the equation

Ic = BETA * Ib

whereas everyone knows that BETA changes alot so we dont use the transistor in this way. one way to get rid of this is by including the emitter feedback resistor. in that case Ic is not equal to BETA * Ib.

and when u think of Ib the controlling factor there is also a need for it to be zero. because if Ib is zero Ic=Ie and thats what we want. so how can Ib be the controlling factor when we dont need it to be there in the first place.

now all that i have said is no ********. if u have the book Microelectronic Circuits u can see this explanation. and that book is the most used book by engineers all over the world

i have said that the current controlled behaviour is somewhat ok. if u want to visualize it that way its alright.

 

[Roff]

Ic = BETA * Ib

whereas everyone knows that BETA changes alot so we dont use the transistor in this way. one way to get rid of this is by including the emitter feedback resistor. in that case Ic is not equal to BETA * Ib.

Beta is defined as Ic/Ib. An emitter resistor does not change this fact.
Voltage control of collector current is highly useful in logarithmic converters and in current mirrors. It is useful to think of common emitter amplifiers (especially digital switches) as current controlled. Voltage and current are interrelated by the diode equation (Google search).

 

[samcheetah]

ok so this is how the story goes

in a common emitter configuration without the emitter feedback resistor the following equations apply

Ib = (Vbb - Vbe) / Rb where Vbe is 0.7 for silicon

now Ic is BETA times Ib

and Vce = Vcc - Ic*Rc

now if u calculate the q-point using these equations it will be highly unstable because of the unstable value of BETA. it changes with temperature.

now if u add an emitter feedback resistor the operating point can be calculated without the BETA. so this operating point is independent of BETA and it will be stable to the changes of BETA due to changes in temperature. The following equations apply.

Ve = Vbb - Vbe

Ic = Ie = Ve / Re

Vc = Vcc - Ic * Rc

and Vce = Vc - Ve

as u can see from the above equations there is no Ib or BETA involved.

so when we need stability we dont see the transistor amplifying an input current. when u use the configuration without the emitter feedback resistor u can say that the transistor is a current controlled device and Ib is the control current. but with that we also need Ib to be zero so that Ic=Ie. so how can a thing be a controlling factor when there is no need for it.

now in the configuration with the feedback resistor Ib isnt involved in the calculation of the Q-point. with this we acheive stability. so by the exclusion of Ib we made a good amplifier. now do u think Ib is worthy to be called a controlling factor?

its actually the voltage that controls the current and i already told u people that this isnt my own thoughts.

my question was something different which nobody has been able to answer

PLZ READ MY QUESTION FIRST

why is it that whenever i post a question in this forum nobody seems to give me the answer.

 

[samcheetah]

go to

http://www.amasci.com/amateur/transis.html

 

[Nigel Goodwin]

my question was something different which nobody has been able to answer

PLZ READ MY QUESTION FIRST

why is it that whenever i post a question in this forum nobody seems to give me the answer.

Your question has already been answered, it's meaningless!.

A transistor is still current controlled, the current gain is what makes it work.

Adding a resistor in the emitter may gave an illusion that the base voltage is controlling it, but it's still just the base current. The reason for this illusion is that as the base current increases the collector and emitter currents increase as well. As there is a resistor in the emitter the voltage on the emitter will increase, reducing the base current, giving negative feedback.

In your formulas you ignored the most important part - Ie is not equal to Ic, Ie = Ic + Ib.

As I also mentioned previously, all three transistor configurations work the same - it's only the input and output which are different.

To recap:

Bipolar transistors require base current, they are current driven.

FET's don't require gate current, they are voltage driven.

Valves don't require grid current, they are voltage driven as well.

Try putting a microammeter in the base lead of a transistor, you can measure the current it's driven by!. Do the same for an FET and a valve, no current is displayed.

 

[samcheetah]

Your question has already been answered, it's meaningless!.

no it hasnt been and it does have a meaning if u once read it :lol:

In your formulas you ignored the most important part - Ie is not equal to Ic, Ie = Ic + Ib.

its not i who ignored it. thats why we use the feedback resistor i.e not Ib and BETA involvment

Bipolar transistors require base current, they are current driven

but at the same time we dont want that controlling current. right?? so how does that current control the transistor.

i know that there is some base current in every BJT circuit. but that doesnt mean that it is controlling the output. that is one of the major drawbacks of BJTs that it draws base current.

plz do have a look at the article

http://www.amasci.com/amateur/transis.html

i know most of u think that BJTs are current controlled devices (even my electronics teacher teaches us that). All over the world people consider the transistor as a current controlled device. and i dont have any problem with it

the problem is that the current controlled model has a flaw and it doesnt apply everytime.

plz read the article and my recent posts and then think about it.

dont try to shove this current controlling stuff down my throat. it is convention to think of the BJT as a current controlled device because it is easy to explain but that doesnt mean it is true[/quote]

 

[Nigel Goodwin]

its not i who ignored it. thats why we use the feedback resistor i.e not Ib and BETA involvment

You can ignore Ib for rough approximations (and I almost always do), but you can't ignore it for explanations of how it works. In many ways it's like using an opamp, you assume infinite gain and infinite input impedance, this makes calculations easier. With a transistor, as long as you operate it with decent levels of negative feedback, you can ignore Ib and beta values - but the beta value must be greater than the gain of the stage.

But just because you ignore them doesn't mean they aren't there, nor that they are what is making it work.

If you are using the transistor anywhere near it's limits you need to take them both into account.

Bipolar transistors require base current, they are current driven

but at the same time we dont want that controlling current. right?? so how does that current control the transistor.

i know that there is some base current in every BJT circuit. but that doesnt mean that it is controlling the output. that is one of the major drawbacks of BJTs that it draws base current.

plz do have a look at the article

http://www.amasci.com/amateur/transis.html

I did do, I refer you to an early line "my idea of how it works".

He seems mainly to "play with words", talking about "current doesn't flow", but charge does. In my opinion (and experience) there are two schools of thought - both basically the same. The older one, conventional current, flows from positive to negative - the more modern one, electron flow, is the actual flow of electrons (from negative to positive). It makes no real difference which you use, and you should be able to simply interchange between them without a second thought.

i know most of u think that BJTs are current controlled devices (even my electronics teacher teaches us that). All over the world people consider the transistor as a current controlled device. and i dont have any problem with it

the problem is that the current controlled model has a flaw and it doesnt apply everytime.

When you can show me a transistor working without any current entering the base I'll agree with you :lol:

plz read the article and my recent posts and then think about it.

dont try to shove this current controlling stuff down my throat. it is convention to think of the BJT as a current controlled device because it is easy to explain but that doesnt mean it is true

And it doesn't mean it's not true, simple ohms law relates voltage and current together - with a transistor you need a voltage to give it base current. But with an FET (or valve) you have voltage but no current - that's the crucial difference!.

 

[Optikon]

Your question has already been answered, it's meaningless!.

no it hasnt been and it does have a meaning if u once read it :lol:

In your formulas you ignored the most important part - Ie is not equal to Ic, Ie = Ic + Ib.

its not i who ignored it. thats why we use the feedback resistor i.e not Ib and BETA involvment

Bipolar transistors require base current, they are current driven

but at the same time we dont want that controlling current. right?? so how does that current control the transistor.

i know that there is some base current in every BJT circuit. but that doesnt mean that it is controlling the output. that is one of the major drawbacks of BJTs that it draws base current.

plz do have a look at the article

http://www.amasci.com/amateur/transis.html

i know most of u think that BJTs are current controlled devices (even my electronics teacher teaches us that). All over the world people consider the transistor as a current controlled device. and i dont have any problem with it

the problem is that the current controlled model has a flaw and it doesnt apply everytime.

plz read the article and my recent posts and then think about it.

dont try to shove this current controlling stuff down my throat. it is convention to think of the BJT as a current controlled device because it is easy to explain but that doesnt mean it is true

[/quote]


I read your orginal post and the article you've posted about how to visualize transistor action. I've seen this article a few times in various forms before. I think it arguably(sp?) does provide an alternative explanation if that is what you are looking for.

But please do not lose sight of a simple fact. The "conventional" explanation of how a transistor works has been in successful use for many many years. And if one has a model (mathematical or otherwise) that works, its ok to use provided it explains the phenomenon via the scientific method. If someone believes the conventional explaination is full of errors (like the article mentions), I'd like someone to point them out specifically (which, the article did not do) so we can look at it.

Truth is, the explanation of transistor action has been thought about and analyzed for much much longer than we've been around (well, maybe except for RON H :wink: ) so we've gotta stake some faith in that.

 

[samcheetah]

thanx Optikon.

u know why i like to view the transistor as a voltage controlled current source is that i have seen this explanation in the best books around e.g

Microelectronic Circuits by Smith/Sedra

and Electronic devices and circuits by Theodore F Bogart

and i do say that visualizing the transistor as a current controlled current source is a good thing. but my question was something else. i wasnt asking that is the transistor a current controlled device or a voltage controlled device. my question was

in a common collector configuration what is the situation. the voltage between which two terminals is controlling a current through the third terminal. please explain. thanx in advance

 

[tavib]

8)
In the bipolar transistor the base-emitter voltage control the collector current, the base current not have any controll over collector current, the base current is parasitic and in an ideal case must be zero.
The belive that the bipolar transistor is a current controlled device is incorrect, but seem to be very popular between peoples on this forum, for that some explanations about internal transistor working must be maked.
The transistor have 2 junctions: emitter junction(between B-E) and collector junction(between C-B). In normal operation the collector junction is reverse polarised and emitter junction is direct polarised.
For case of NPN transistor, the emitter junction will inject electrons from emitter N region into base region, and gaps from base region into emitter, but because the emitter is very heavy dopped and base is light dopped the electrons injection is more more higher than the gaps injection. This injection is controlled by voltage over the emitter junction like every PN junction. When the electrons from emitter go into base region they will be into region rich in gaps and have tendency to recombine with this gaps, but because the base is very tick this recombination is very low and allmost all electrons arrive to collector junction border. At this border they are accelerated by reverse voltage of junction to go into collector region where form the collector current. The same reverse voltage will bloc the gaps from base and electrons from collector to cross the junction, so if is no electrons injection from emitter region through base then is no current through transistor. This is the simple explain on how transistors really work.
Now you can view that the B-E voltage controll the electrons injection and controll the collector current. The base current is give by gaps injected from base to emitter and from electrons that have recombined into base, so the beta factor is higher if emitter region is more higher doppen relative to base dopped, and if base region is more tick than before.
The base current appear from unwanted fenomens and is a secondary effect of main one, it have no effect over collector current controll.
But because the transistor need to be feeded with a current into base for compensate the base charge that flow from base into emitter, only APPARENTLY the base current controll the collector current.

I hope I'm clear enough in my explanations.

 

[samcheetah]

thanx alot tavib. It was a very nice explanation. thats what i have been trying to tell everybody. but most of the people opposed me. i dont know why some people are too skeptical :?

my question still remains unanswered

 

[Oznog]

In response to your question:

"in a common collector configuration what is the situation. the voltage between which two terminals is controlling a current through the third terminal. please explain. thanx in advance"

Yes you can look at it that way. It is mathematically valid, and we do often say these BJTs have higher "voltage gain" over MOSFETs. Howevever, it is an extremely complicated picture over the picture of current gain.

You've got an NPN BJT transistor with a rated current gain of 50. Now let's say the lot variability is +/- 10%, and the gain varies another +/- 10% over the temp range you might use it at. This are easy factors to work with. +1mA of base current = +50mA collector current.

Now base current vs base-emitter voltage follows the sharp curve of a diode; it IS a diode. A small change in Vbe creates a high change in Ibe. From the very weakest current imaginable to the strongest the base can take, it's around 0.3V to 1V. But it's extremely unreliable to predict what voltage makes what current from part-to-part and under different conditions. Within the same range of mfg variations and temp, a particular voltage could easily mean 1mA on one part at one temp or 20mA on another part at another temp!

You will never find a scheme which applies a fixed base-emitter voltage to drive a BJT on any commercial circuit. You'll usually see the solution of a resistor in series with the base or the emitter of sufficient size to make it an issue of base current versus signal voltage.

 

[tavib]

8)
In any transistor configuration the Vbe voltage controll the collector current Ic, in your case of common collector the BE voltage will controll the collector current Ic, but because a very small Vbe variations will produce a large Ic variation, seem that Vbe voltage is allmost constant(Vbe=0.6-0.7V and have only 10-50mV variations for mA range current), so if B voltage increase the emitter voltage increase with the same variation minus the small variation of BE voltage. But this small variation of BE voltage is the factor which produce the Ic variation so on emitter resistor will drop the emitter voltage.

And the collector current vs BE voltage follow the sharp curve of a diode.
Not the base current, Oznog.