### Author Topic: Current controlled device vs voltage controlled?  (Read 31771 times)

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#### RoGeorge

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##### Re: Current controlled device vs voltage controlled?
« Reply #25 on: February 14, 2017, 03:05:56 am »
Current controlled device vs voltage controlled? What is the difference?
Any voltage source can be seen as a current source, and any current source can be seen as a voltage source, so there is no real difference. They are equivalent. As long as you don't break the math, you can use whatever you like most, or whatever will ease your calculations.
See https://en.wikipedia.org/wiki/Source_transformation. This is sometimes called the Thevenin-Norton equivalence.

I saw in MrCarlson's lab that he said a transistor was a current controlled device and a FET? was a voltage controlled device?
I don't know if you saw that, or not. I guess you saw. Anyway, FET stands for Field Effect Transistor, and BJT (which I guess will be the other kind of transistor from your question) stands for http://bfy.tw/A4bU
Yes, people like to think about BJTs as current controlled devices (again, BJT means "normal" transistors, like NPN or PNP transistors) , and about FETs as voltage controlled devices. Because of Thevenin-Norton, it doesn't really matter how you want to see those transistors. Be careful here. Your teacher might want to see each device exactly as he, the teacher, teach you to see it. So, after all, no matter what Thevenin-Norton said, there is only one right answer: the one that your teacher is expecting from you.

I thought potential was potential. I know the difference in current and volts.
I have no idea what are you talking about.

But wouldn't this mean that a PNP was current controlled (Say it was used where you had the device then transistor then ground, so the pnp acts as a path to ground) and NPN was voltage controlled (you had the NPN then your load then ground so it feeding the load). You send voltage to the base of the NPN to turn on while current draw on the PNP turned it on.
No. For a device to be current controlled, it doesn't matter if the control current is going to the ground, or is coming from the load. It doesn't matter if you "push" a current into the Base of a NPN, or if you "pull" a current from the Base of a PNP. As long as you vary a current in order to control the transistor, then we are saying the transistor is current controlled.

In conclusion:
- What parameter are you tweaking when you want to control a PNP transistor?
- I am tweaking its Base current. Aha, a current! So this device is controlled by a current, so we call it a current controlled transistor.

- Now, what parameter are you tweaking when you want to control a NPN transistor?
- I am tweaking its Base current. Aha, a current! So this device is controlled by a current tweak, too, so a NPN transistor is also a current controlled transistor, like the PNP one.

- And finally, what are we tweaking when we want to control a FET transistor?
- Well, a Field Effect Transistor (AKA FET, or MOSFET) does not have a Base, in the first place. Instead of Base, the FET has a Gate. The Gate does not allow any current. Since the Gate has zero current, all we can tweak in order to control a FET is the voltage applied to its Gate. So, we say the FET is a voltage controlled transistor.

« Last Edit: February 14, 2017, 08:06:23 am by RoGeorge »

#### helius

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##### Re: Current controlled device vs voltage controlled?
« Reply #26 on: February 14, 2017, 04:58:03 am »
The gate of a FET is insulated from the channel, so current won't flow through it to the drain or source. But current still flows in and out of the gate because it has capacitance.

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#### LvW

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##### Re: Current controlled device vs voltage controlled?
« Reply #27 on: February 14, 2017, 08:52:41 am »
Any voltage source can be seen as a current source, and any current source can be seen as a voltage source, so there is no real difference. They are equivalent. As long as you don't break the math, you can use whatever you like most, or whatever will ease your calculations.
.......
Yes, people like to think about BJTs as current controlled devices (again, BJT means "normal" transistors, like NPN or PNP transistors) , and about FETs as voltage controlled devices. Because of Thevenin-Norton, it doesn't really matter how you want to see those transistors.

I think, we shouldnt mix circuit theorems and other mathematical manipulations/equivalences with real parts properties. The original poster was asking if the BJT is voltage or current controlled - thats all.

- Now, what parameter are you tweaking when you want to control a NPN transistor?
- I am tweaking its Base current.
Sorry - but may I ask you: Did you ever design a BJT based amplifier stage?
What do you think about the following design steps

Design steps (common emitter stage with DC stabilization, Vcc given):
* Select IE (resp. IC) and collector/emitter resistors RC resp. RE,
* Calculate required base voltage VB - based on VE=IE*RE - and assuming a VOLTAGE VBE=(0.65...0.7) volts.
* Design voltage divider (rule of thumb: Divider current > 10*IB=10*IC/B).
* Voltage gain: G=-gm*RC/(1+gm*RE)
* Transconductance gm=d(IC)/d(VBE)=IC/VT
* Voltage gain does NOT depend on B (same DC quiescent point)

Question: Current control? Does everybody realize WHY we normally have select a divider current of of app. 10*IB ? (Answer: Because we want that the value of IB resp. the uncertainties/tolerances of IB play a minor role only and will cause just a small shifting of the desired DC operational point.)

Comment 1: IB was used only for providing the required voltage VB.
Comment 2: Unexpected IC increase reduces VBE (voltage feedback)

Fazit: Each designer and each newcomer/student should know WHY emitter degeneration (voltage feedback) is incoroporated - and HOW it works!

Attachement: The enclosed figures explain why it is sufficient to work with an estimate for VBE.

« Last Edit: February 14, 2017, 09:08:08 am by LvW »

#### Zero999

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##### Re: Current controlled device vs voltage controlled?
« Reply #28 on: February 14, 2017, 09:08:13 am »
BTW, for MOSFETs the insulating layer is typically SiO2.  The gate is polysilicon.
Yes, you're right. Polysilicon is a good conductor, which wouldn't be any good for an insulating gate. SiO2 is a good insulator.

Quote
Just to  , there are a lot of ways of looking at it, not just one true way.  Another way is "all transistors are charge controlled devices."  Yet another is "BJTs are current controlled" because the collector current is controlled by the emitter base current.
That's what you meant, wasn't it?

The original poster was asking if the BJT is voltage or current controlled - thats all.
I don't think the question was that simple. There seemd to be some confusion in the first post.

"I know the difference in current and volts. But wouldn't this mean that a PNP was current controlled (Say it was used where you had the device then transistor then ground, so the pnp acts as a path to ground) and NPN was voltage controlled (you had the NPN then your load then ground so it feeding the load). You send voltage to the base of the NPN to turn on while current draw on the PNP turned it on. "

The abbreviations are getting out of control. What do the abbreviations in those equations stand for? Biggest hurdle to learning is when you have to look up each abbreviation, then find you looked up the wrong one.  MrCarlson is never wrong!!!

Maybe I should just stick to organic chemistry, even though its technically a harder subject, for me its alot easier to understand then electronics.

I struggled with organic chemistry, got an E at A-level. There are so many different reactions to remember. My biggest problem was I lost interest in the subject.

What transistor circuit are you trying to understand?

Please post a link to the material you discussed in your first post.

#### RoGeorge

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##### Re: Current controlled device vs voltage controlled?
« Reply #29 on: February 14, 2017, 10:06:45 am »
Sorry - but may I ask you: Did you ever design a BJT based amplifier stage?
Yes.

I understand your view, and I already considered it. I am not saying you are wrong.
In the same time, I would like to keep my interpretation, even if you don't agree with it. After all, who knows, maybe tomorrow I will realize I was wrong, or vice versa.

Until then, it's OK to disagree.
That is why I will not continue this debate.

#### Codebird

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##### Re: Current controlled device vs voltage controlled?
« Reply #30 on: February 14, 2017, 10:17:15 am »
Quote
No real learning or teaching going on here, just ego exercises.

Disagreements about how to teach. There are those who advocate the 'throw them in the deep end' approach, where the questioner is given a complete and detailed answer and expected to understand it. There are also the 'shallow end first' who will present simplified, easy-to-understand answers even if they are not entirely correct, because they are much easier to grasp and can then provide a foundation for learning the more accurate model later. Neither approach is really superior to the other.

#### LvW

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##### Re: Current controlled device vs voltage controlled?
« Reply #31 on: February 14, 2017, 10:20:59 am »
That is why I will not continue this debate.
Wise decision - same to me, because in a technical discussion I dont like contributions like
........ rather than having a pissing contest?

#### Zero999

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##### Re: Current controlled device vs voltage controlled?
« Reply #32 on: February 14, 2017, 01:39:52 pm »
Quote
No real learning or teaching going on here, just ego exercises.

Disagreements about how to teach. There are those who advocate the 'throw them in the deep end' approach, where the questioner is given a complete and detailed answer and expected to understand it. There are also the 'shallow end first' who will present simplified, easy-to-understand answers even if they are not entirely correct, because they are much easier to grasp and can then provide a foundation for learning the more accurate model later. Neither approach is really superior to the other.
The trouble is, in a situation like this, it just turns into a pissing contest and the person asking the question doesn't learn much. If people don't like others making statements like this, then they should accept others have different methods of teaching and people learn in different ways, rather than trying to convince others they're doing it wrong.

Personally I like a mixture of both methods. I appreciate being told that I'm being given a simplified explanation, rather than the full story, so I'm free to learn more in future.

As far the common emitter amplifier is concerned: I learnt the basics of that myself, before I went to college and I didn't worry about whether a BJT is voltage or current controlled. In fact I didn't even realise there was such a debate, until I started posting here.

#### MrAl

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##### Re: Current controlled device vs voltage controlled?
« Reply #33 on: February 14, 2017, 02:57:49 pm »
Hi,

This question can be answered in two basic ways:
1.  To the layman.
2.  To the theorist

To the layman, the BJT can be viewed as either current controlled or voltage controlled.  That's because in any given application one parameter may be MUCH more important than the other and the design gets a little easier if we do it one way rather than the other.

To the theorist, the voltage controlled BJT can apply but to be perfectly precise we have to limit that exact definition to the static case where we already have a voltage established.  That's because there is no way to establish a voltage without moving charge.  Also, i think (someone check this) that the current theory is that there is no question about what came first, the voltage or the current, as they both happen at the same time with no time lag between one or the other.  More current theory may have changed this view, but even if it did if the voltage comes first then the current can not be very far behind.  In practice it will never be measured by anyone who isnt in a laboratory trying to actually measure the difference.  Feel free to look this point up and see if you can verify or not.  But remember even if the voltage comes before the current it would have to be something like 0.001ps away from it or even less.

In many circuits however we also have the view of the BJT being "charge controlled', which brings in another view.  The Rutgers library has a book that goes into this in detail, and describes how to turn off a BJT really really fast by sweeping the charge carriers out of the base region.  What is that?  It's not explained on the basis of voltage it is explained on the basis of charge carriers because if you dont get them out of the base fast enough your transistor stays in saturation for too long.

My conclusion is because of all these views it is best to call a device "energy controlled" because except for the static case (which is not very interesting in practical circuits) we MUST move energy in order to change the characteristic of the device.

Still having said all that, the way we view the BJT mostly will depend on what design aspect we are targeting or what design formula we are using at the time.  We may end up using two views for the same project at some point.
« Last Edit: February 14, 2017, 03:00:49 pm by MrAl »

#### Zero999

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##### Re: Current controlled device vs voltage controlled?
« Reply #34 on: February 14, 2017, 03:07:51 pm »
That's because there is no way to establish a voltage without moving charge.  Also, i think (someone check this) that the current theory is that there is no question about what came first, the voltage or the current, as they both happen at the same time with no time lag between one or the other.
AC theory should answer that. It depends on the impedance. If it's inductive, the voltage comes first. If it's capacitive it's the current which is first.

In a transmission line current and voltage are anti-phase and which comes first, depends on where you take the measurement.

#### LvW

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##### Re: Current controlled device vs voltage controlled?
« Reply #35 on: February 14, 2017, 05:17:54 pm »
Disagreements about how to teach. There are those who advocate the 'throw them in the deep end' approach, where the questioner is given a complete and detailed answer and expected to understand it. There are also the 'shallow end first' who will present simplified, easy-to-understand answers even if they are not entirely correct, because they are much easier to grasp and can then provide a foundation for learning the more accurate model later. Neither approach is really superior to the other.

Hero999, with all respect - have you teaching experience? So - you really would give answers which are“not entirely correct“ - just because they are „simplified,  easy-to-understand answers“ ? Do you really think that students - who already have learned how a pn-diode works - are overstressed with the exponential function Ic=f(VBE) ?

If people don't like others making statements like this, then they should accept others have different methods of teaching and people learn in different ways, rather than trying to convince others they're doing it wrong. ...Personally I like a mixture of both methods. I appreciate being told that I'm being given a simplified explanation, rather than the full story, so I'm free to learn more in future.
Are we now discussing „teaching methods“ or do we try to find a suitable - and correct - answer to a pure technical question?

As far the common emitter amplifier is concerned: I learnt the basics of that myself, before I went to college and I didn't worry about whether a BJT is voltage or current controlled. In fact I didn't even realise there was such a debate, until I started posting here.

OK - many people do not know about the BJTs working principle and they do not care. This is, of course, not a problem - however, when you were not aware of this „debate“ , how could you take part in this discussion and qualify my answers as „pissing contest“?
What do you think, could be the reason for the original question?
The questioner was confused because of different and contradictory explanations in different books (and, unfortunately, also in this thread).
I am afraid, he will be really disappointed because - as it seems - no pure technical knowledge exchange on a fair basis is possible.

#### rfeecs

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##### Re: Current controlled device vs voltage controlled?
« Reply #36 on: February 14, 2017, 06:17:42 pm »
Quote
Just to  , there are a lot of ways of looking at it, not just one true way.  Another way is "all transistors are charge controlled devices."  Yet another is "BJTs are current controlled" because the collector current is controlled by the emitter base current.
That's what you meant, wasn't it?

Actually, no.  I was referring to the argument that the way a BJT works is the collector current is controlled by the emitter current.  The base current is a side effect, the base-emitter voltage another side effect.  The amplification is due to the impedance transfer, turning a low impedance input into a high impedance output.  The "transfer-resistor" effect that the transistor is supposedly named after.

#### Zero999

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##### Re: Current controlled device vs voltage controlled?
« Reply #37 on: February 14, 2017, 07:22:44 pm »
Quote
Just to  , there are a lot of ways of looking at it, not just one true way.  Another way is "all transistors are charge controlled devices."  Yet another is "BJTs are current controlled" because the collector current is controlled by the emitter base current.
That's what you meant, wasn't it?

Actually, no.  I was referring to the argument that the way a BJT works is the collector current is controlled by the emitter current.  The base current is a side effect, the base-emitter voltage another side effect.  The amplification is due to the impedance transfer, turning a low impedance input into a high impedance output.  The "transfer-resistor" effect that the transistor is supposedly named after.
Oh I see what you mean now. Thanks.

Disagreements about how to teach. There are those who advocate the 'throw them in the deep end' approach, where the questioner is given a complete and detailed answer and expected to understand it. There are also the 'shallow end first' who will present simplified, easy-to-understand answers even if they are not entirely correct, because they are much easier to grasp and can then provide a foundation for learning the more accurate model later. Neither approach is really superior to the other.

Hero999, with all respect - have you teaching experience? So - you really would give answers which are“not entirely correct“ - just because they are „simplified,  easy-to-understand answers“ ?
I have no professional teaching experiance but I have had to teach others electronics in the past. As far as providing not 100% accurate information is concerned: yes I do that. It's known as abstraction. For example, we're taught about Kerckhoffs current and voltage, laws, which we later find are incorrect when we go on to study transmission lines.

Quote
Do you really think that students - who already have learned how a pn-diode works - are overstressed with the exponential function Ic=f(VBE) ?
It depends on what level they've studied a diode. At the very beginning we were just taught that VF is a virtually constant 0.6V and changes very little with varying current. Again, this was an abstraction. Later on we studied the relationship between VF, IF and temperature.

Quote
If people don't like others making statements like this, then they should accept others have different methods of teaching and people learn in different ways, rather than trying to convince others they're doing it wrong. ...Personally I like a mixture of both methods. I appreciate being told that I'm being given a simplified explanation, rather than the full story, so I'm free to learn more in future.
Are we now discussing „teaching methods“ or do we try to find a suitable - and correct - answer to a pure technical question?
As far the common emitter amplifier is concerned: I learnt the basics of that myself, before I went to college and I didn't worry about whether a BJT is voltage or current controlled. In fact I didn't even realise there was such a debate, until I started posting here.

OK - many people do not know about the BJTs working principle and they do not care. This is, of course, not a problem - however, when you were not aware of this „debate“ , how could you take part in this discussion and qualify my answers as „pissing contest“?
What do you think, could be the reason for the original question?
The questioner was confused because of different and contradictory explanations in different books (and, unfortunately, also in this thread).
I am afraid, he will be really disappointed because - as it seems - no pure technical knowledge exchange on a fair basis is possible.
We don't know why the original poster asked the question. Very little background information was given. I don't think it's necessary to explain to them how a transistor works in the minutest of detail, down to what's going on with the charge carries in the semiconductor junction, however technically accurate it may be. All it would do is create more confusion. They already said they're confused about the difference between a PNP and an NPN transistor, so perhaps we should straighten that out before going any further?

It's quite likely a lot of what has been posted here is completely irrelevant to the original poster's question and will just cause more confusion.

#### LvW

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##### Re: Current controlled device vs voltage controlled?
« Reply #38 on: February 15, 2017, 08:59:57 am »
We don't know why the original poster asked the question. Very little background information was given. I don't think it's necessary to explain to them how a transistor works in the minutest of detail, down to what's going on with the charge carries in the semiconductor junction, however technically accurate it may be.

Who has done it?
In which contribution the transistor was described "in the minutest of detail, down to what's going on with the charge carries in the semiconductor junction" ?

The situation is as follows:
There was a question from raspberrypi with the title: Current controlled device vs voltage controlled?

Excerpts from some given answers:

"..IC = beta x Ib, a simplified approximation, .... but in simple form the above suffices."

"...they are best described as current controlled."

"...but they are useful simplifications."

To me, an engineer resp. somebody who wants to become an engineer cannot be satisfied with such vague statements. The problem is even more complex because the questioner will notice that in different books different claims (current-controlled, voltage-controlled) can be found - often without explanations or justifications.

That was the background of my answer to the questioner (post#7).
In this post, I gave a definite answer - according to my best knowledge and my experiences.
My reply was supplemented by some examples which should support the content of my contribution.

As an engineer, I expect that forum members who disagree with the contents of my answer, are able to present some counter arguments or - at least - give some technical comments to the purely circuit-oriented  facts I have mentioned (gain, transconductance, RE-voltage feedback, tempco -2mV/K...).

However, no such thing...
That`s somewhat disappointing.

#### MrAl

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##### Re: Current controlled device vs voltage controlled?
« Reply #39 on: February 15, 2017, 10:49:01 am »
That's because there is no way to establish a voltage without moving charge.  Also, i think (someone check this) that the current theory is that there is no question about what came first, the voltage or the current, as they both happen at the same time with no time lag between one or the other.
AC theory should answer that. It depends on the impedance. If it's inductive, the voltage comes first. If it's capacitive it's the current which is first.

In a transmission line current and voltage are anti-phase and which comes first, depends on where you take the measurement.

Hello,

That's not the same argument though as to which comes first.  You are saying that either voltage leads current or current leads voltage, which as you can see right away leads to a contradiction, because we can then ask the question, "So whcih is it?".

This argument is about having nothing to start with, no energy, and then trying to force a change of some kind.  To do that we have to separate charge and at the very start of that process it is believed that the electric field appears at the same moment that the charge is moved.  In other words, to move charge you need an electric field (void of any other input that is) and to create an electric field you need to move charge, so they both must happen at the same time with no delay between their appearance or start of their movement.

Also, if we energize an inductor with a step change we get a RAMP of current, not zero current, because the only time we see zero current is with zero voltage.  As soon as we get any voltage we get some current, even if it is very small.  This means simplified circuit analysis does not help understand this issue because at t=0 we think of having say 1 volt applied with zero current, but it is really t=0+ where we see the true effect.

#### LvW

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##### Re: Current controlled device vs voltage controlled?
« Reply #40 on: February 15, 2017, 11:31:49 am »
Also, if we energize an inductor with a step change we get a RAMP of current, not zero current, because the only time we see zero current is with zero voltage.  As soon as we get any voltage we get some current, even if it is very small.  This means simplified circuit analysis does not help understand this issue because at t=0 we think of having say 1 volt applied with zero current, but it is really t=0+ where we see the true effect.

Yes - completely agreed. While discussing time behaviour we must strictly distinguish between transient response and steady-state behaviuour.
AC analyses cannot tell us anything about delay ("first" or "second").  A phase of -30 deg is equivalent to a phase of +330deg.
« Last Edit: February 15, 2017, 04:09:33 pm by LvW »

#### danadak

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##### Re: Current controlled device vs voltage controlled?
« Reply #41 on: February 15, 2017, 12:10:51 pm »
Approximations vs inexact science.

I like the discussion because those who use approximations, like Ic = beta x Ib
(linear region) are just as effective as designers as those that spend days (well for
me it would be days) solving Poisons equation and the various transport equations
with doping levels, also all approximations, in the transistor.

Even the basic notion does the existence of V depend on I or vice versa. V of course
is a result of charge differential, static, but the charge had to be moved in free space to
create it. Horse before or after the cart.

After all we currently do not have a clue where dark matter comes from, just that it seems
to constitute an appreciable amount of mass in the universe. What is its effect on transistor
action one could posit.

Lastly here is stated a oversimplified explanation that a bipolar can be viewed either as current
or voltage controlled. Seems reasonable.

http://www.ece.umd.edu/~neil/enee313/BJTnotes_05_6_10.pdf

Thanks ladies and gents, wonderful discussion. I am re-learning some stuff long forgotten
in my hummingbird sized brain pan.

Regards, Dana.
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer

#### LvW

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##### Re: Current controlled device vs voltage controlled?
« Reply #42 on: February 15, 2017, 04:32:34 pm »
Lastly here is stated a oversimplified explanation that a bipolar can be viewed either as current
or voltage controlled. Seems reasonable.
.............
Thanks ladies and gents, wonderful discussion. I am re-learning some stuff long forgotten
in my hummingbird sized brain pan.
Regards, Dana.

Dana - I think, it is a good news to hear from you that the whole discussion (although not always very aim-oriented) had a certain value for you.
May I suggest something to you? From the text you have provided - carefully read only the parts after the figure again and try to find an answer for yourself to the following two questions:
(1) What do they mean with the frequently used term "bias" (voltage or current ?) and
(2) The text gives a rough explanation how the transistor works and how the collector current is produced - what do you now think is the primary quantity which determines the amount of collector current (VBE or IB)?

#### MrAl

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##### Re: Current controlled device vs voltage controlled?
« Reply #43 on: February 16, 2017, 09:42:55 am »
Hello again,

I think i see now what the main problem in this issue is.  It seems to stem from the key word, "control", or "controlled".

When we think of controlling something we think of something that is changing and we want to force some sort of change such that we get the desired response.  For example, when we drive a car down a highway we want to maintain a certain speed even if we hit some hills and travel up hill or down hill.  We might want to maintain say 60mph and so we vary the way we push on the accelerator pedal in order to maintain that speed.  So we are varying something to obtain some desired effect and we have to do that because of external conditions that change that we dont have any control over.

This differs from a phrase like, "mode of operation".  Let me give a simple example how these might be different.
Say we have two coils energized by two currents, and the coils are facing each other with fields opposing.  We know that they will repel each other.  If we turn one around, we know they will be attracted to each other.  How do we know this?  It's because we understand that the principle mode of operation is due to the magnetic field, and notice here we dont have to mention the electric field because we are assuming static operation.  But even so, we can say that it is the magnetic field that is responsible for this repel or attraction and we dont mention the electric field.
The catch is that it is hard to call this situation, "controlled".  In other words, it is hard to say that the magnetic field is "controlling" the attraction or repulsion.  By the definition of the word control it implies some sort of action such as in the non static case.  So here we find it more appealing to just say that the magnetic field is 'responsible' for the result.

In this same way i find it hard to say that the voltage is 'controlling' the transistor, however i dont have a problem with saying that the voltage is the principle mode of operation.  That is, certain things would happen as long as we maintain the voltage at the exact same level throughout, and do not try to change the current.  The catch we could run into when trying to say that it is 'controlled' by the voltage is if something else changes that is NOT under our control, we loose that control and we must change the drive signal to make up for it, and that is really controlling the transistor, and that requires a change of energy just like the car when we hit a hill that goes up or down.
If we look at a constant current source made from a bipolar transistor we quickly see that there is feedback and that means the base current (and voltage) will be constantly changing in order to properly 'control' the collector current.

So in conclusion i can say that i dont like stating that the transistor is controlled by voltage, but i dont mind saying that the principle mode of operation is based on voltage (in the static sense).  This means we never have to ask the question, "how did the voltage get there".

« Last Edit: February 16, 2017, 09:49:23 am by MrAl »

#### LvW

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##### Re: Current controlled device vs voltage controlled?
« Reply #44 on: February 16, 2017, 11:50:47 am »
MrAl - I can fully agree to the first two paragraphs of your last contribution.
However, I have some objections against the rest of the text. Let me explain:

Quote: In this same way I find it hard to say that the voltage is 'controlling' the transistor, however I dont have a problem with saying that the voltage is the principle mode of operation. That is, certain things would happen as long as we maintain the voltage at the exact same level throughout, and do not try to change the current.

I cannot agree. The question if we are able to „maintain the voltage at the exact same level“ has nothing to do with physical determined properties of a device.

Quote: The catch we could run into when trying to say that it is 'controlled' by the voltage is if something else changes that is NOT under our control, we loose that control and we must change the drive signal to make up for it, and that is really controlling the transistor,

I am afraid that the working principles of a device do not depend on any changes which might be not „under our control“. In this context, may I direct your attention to the pdf figures I have attached to my reply’27 ? I think, the figures (stabilization lines) give an answer.
For my opinion, it would be a good thing when each designer of a BJT amplifier stage would be able to know the function of each part he is using.

Therefore - again my question:
How can the primary reason for using an emitter resistor RE (stablizing IC against unwanted changes, uncertainties and tolerances) explained WITHOUT using the relation between IE (resp IC)  and the voltage VBE?

Interestingly, I did get no answer in this thread up to now..
(The same applies to all other technical facts - verified by measurements (!) - I have mentioned in this thread supporting voltage control).

MrAl - may I ask you: What is your answer?

« Last Edit: February 16, 2017, 11:55:11 am by LvW »

#### MrAl

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##### Re: Current controlled device vs voltage controlled?
« Reply #45 on: February 16, 2017, 02:44:29 pm »
MrAl - I can fully agree to the first two paragraphs of your last contribution.
However, I have some objections against the rest of the text. Let me explain:

Quote: In this same way I find it hard to say that the voltage is 'controlling' the transistor, however I dont have a problem with saying that the voltage is the principle mode of operation. That is, certain things would happen as long as we maintain the voltage at the exact same level throughout, and do not try to change the current.

I cannot agree. The question if we are able to „maintain the voltage at the exact same level“ has nothing to do with physical determined properties of a device.

Quote: The catch we could run into when trying to say that it is 'controlled' by the voltage is if something else changes that is NOT under our control, we loose that control and we must change the drive signal to make up for it, and that is really controlling the transistor,

I am afraid that the working principles of a device do not depend on any changes which might be not „under our control“. In this context, may I direct your attention to the pdf figures I have attached to my reply’27 ? I think, the figures (stabilization lines) give an answer.
For my opinion, it would be a good thing when each designer of a BJT amplifier stage would be able to know the function of each part he is using.

Therefore - again my question:
How can the primary reason for using an emitter resistor RE (stablizing IC against unwanted changes, uncertainties and tolerances) explained WITHOUT using the relation between IE (resp IC)  and the voltage VBE?

Interestingly, I did get no answer in this thread up to now..
(The same applies to all other technical facts - verified by measurements (!) - I have mentioned in this thread supporting voltage control).

MrAl - may I ask you: What is your answer?

Hi again,

Interesting reply.

First about the emitter resistor...
I will say what i have said all along for the practical case, and that is that the type of analysis depends on what we want to know.  In some cases it is easier to use a voltage based equation and sometimes it is easier to use a current based equation.
I could have asked an equally valid question which would depend on knowing the base emitter voltage more so than the current, and that is in the practical case of the 'normal' voltage reference diode.  We use voltage because we care more about the voltage than the current.  If we cared more about the current we might want to know what the base current is, and knowing the spread of Beta we can design something that is independent of Beta.  However, we also can not forget that although we dont care as much about the current, the current still assumes whatever level it needs to be for the given device and that current will change with the voltage change.

Second to the point of "The question of if we are able to maintain the voltage at the exact level..."...
I actually said what you are saying too, but i only had a difference with what we called it, "control" vs "mode of operation".
My question to you is if you look at the two coil case (or even just two magnets really except that seems like there's no way to control it anyway) where we have the two coils facing each other and lets limit to the case where they are repelled by each other.  We know it is the magnetic field that causes the repulsion, but do you think it is a good idea to call the effect of the magnetic field "controlling" something?  For example, two magnets hanging by two threads each, suspended and repelling each other, is the magnetic field "controlling" the distance of separation?

So i agree that in order to understand the characteristic behavior we may use a voltage oriented formula, i just dont think it is a good idea to call it "controlling" just like with the magnetic field and the two coils or two magnets.

You can look up the definition of 'control' and see what you think.  I was going to ask if you could find other instances where it makes sense to call it controlling (other than the transistor) but i am not sure if i would be willing to accept them unless they made sense.

For the MOSFET, it is called, "voltage controlled", and most of us agree to that even though we can NEVER change the output without adding or taking away something physical from the input, which as we all know must be in the form of a current.
It is also interesting that this is the sole thing that causes so much confusion to people just getting into the field of electronics.  When they go to use a MOSFET, they almost always think that they can drive it with a 1 megohm resistor even if it has to switch at 100kHz.  What causes that confusion?  It's the description of the MOSFET as being "voltage controlled" and having high input impedance (another misnomer).

I realize that it is sort of a convention of sorts though to call something voltage controlled.  But i would bet we can not find one practical circuit that does not vary the voltage at the gate or base and thus the current as well.

So what do you say about the two coil or magnet example and the magnetic field?  Are the two magnets "field controlled" or not?

BTW you have kept this interesting :-)

« Last Edit: February 16, 2017, 02:51:06 pm by MrAl »

#### Cerebus

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##### Re: Current controlled device vs voltage controlled?
« Reply #46 on: February 16, 2017, 04:17:42 pm »
Orthodoxy and iconoclastic adherence ...

I think you'll find that "iconoclastic" means almost exactly the opposite of what you think it means.
Anybody got a syringe I can use to squeeze the magic smoke back into this?

#### Cerebus

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##### Re: Current controlled device vs voltage controlled?
« Reply #47 on: February 16, 2017, 04:20:48 pm »
The gate of a FET is insulated from the channel, so current won't flow through it to the drain or source. But current still flows in and out of the gate because it has capacitance.

Ahem, JFETs aren't insulated, only MOSFETS (or IGFETs if you prefer that term).
Anybody got a syringe I can use to squeeze the magic smoke back into this?

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#### 2N3055

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##### Re: Current controlled device vs voltage controlled?
« Reply #48 on: February 16, 2017, 07:20:02 pm »
This is simple misunderstanding.

"current controlled" and "voltage controlled" were never meant to be exact technical and physical explanation of exact physical phenomena inside transistor.

It is a simplification, from introductory texts, to draw attention to difference between bipolar and field effect devices. And it was mentioned only in regards of static operation.

My professor used to say that bipolar was called "current controlled" to help us remember that in a circuit we will have to account that at static operating point we will have a current going into the base (or out of ) as opposed to "voltage controlled" field effect devices that will not draw (or source) any mentionable current from the gate. And in making a circuit , we have to be mindful of that and account for it.

And that is all.  At AC all bets are off, so we have real models for that...

One "current controlled" device I know of is magnetic relay...

And current and voltage is chicken and egg question.. They are inextricable from each other..

#### LvW

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##### Re: Current controlled device vs voltage controlled?
« Reply #49 on: February 16, 2017, 08:54:04 pm »
This is simple misunderstanding.

"current controlled" and "voltage controlled" were never meant to be exact technical and physical explanation of exact physical phenomena inside transistor.

It is a simplification, from introductory texts, to draw attention to difference between bipolar and field effect devices. And it was mentioned only in regards of static operation.

My professor used to say that bipolar was called "current controlled" to help us remember that in a circuit we will have to account that at static operating point we will have a current going into the base (or out of ) as opposed to "voltage controlled" field effect devices that will not draw (or source) any mentionable current from the gate. And in making a circuit , we have to be mindful of that and account for it.

And that is all.  At AC all bets are off, so we have real models for that...

One "current controlled" device I know of is magnetic relay...

And current and voltage is chicken and egg question.. They are inextricable from each other..

2N3055 - I am afraid you are in error.
Are you aware that you did nothing than to repeat a claim of your professor - without any attempt to justif your "belief" (it is nothing else).

Smf