Amplifiers, transistors and opamps

[fog37]

Hello Forum,

I have recently learned about transistors and their ability to control (amplify) other signals. BJT transistors amplify current. There are also FET transistors. Some devices are termed current-controlled and others are called voltage controlled.

Are there amplifiers that amplify voltage (voltage amplifiers) and current amplifiers? Is there such a distinction? If current is amplified, then also voltage seems to be automatically amplified (V=IR).

Is the generic simbol for amplifier a triangle? Isn't that the symbol for operational amplifier?

thanks,
fog37

 

[LvW]

I have recently learned about transistors and their ability to control (amplify) other signals. BJT transistors amplify current. There are also FET transistors. Some devices are termed current-controlled and others are called voltage controlled.

It is a common misconception that the BJT would be a current-controlled device. Surprisingly, this false statement even can be found in some textbooks.
Instead, you can trust some papers from leading US unversities explaining that the BJT is voltage controlled - and the base current is an unwanted but unavoidable byproduct.
Furthermore, it is very easy to proove on a circuit level (without going deep into the physics of the pn junction) that it is the voltage Vbe that controls the collector current.
I think, everybody who knows and understands the working principle of a pn junction (pn diode) cannot come to the conclusion that the pn junction within the transistor behaves completely different.

(PS: This question was intensively discussed already here in the forum (starting with reply#13) : https://www.physicsforums.com/threa...imagine-transistor-saturation-current.824127/

 

[anorlunda]

then also voltage seems to be automatically amplified (V=IR).

A transistor is nothing at all like a resistor, so V=IR does not apply. Also, a transistor conserves energy so that power = VI (over all three terminals) can not increase.

 

[fog37]

Hello LvW.

I have read that a BJT is a voltage controlled device too but most sources call it a current controlled device.

Could you give me your definition of voltage-controlled and current-controlled? In the BJT (voltage controlled) a voltage between base and emitter causes a large collector current to flow. So we have a voltage controlling a current. As you mention, the base current is very small and unwanted. Ideally it would be zero.
So, in general, there is a controlling circuit component and a controlled circuit component. How do the concept of voltage controlled and current controlled apply?

Voltage controlled device: the magnitude of either the current or voltage or resistance of the device is controlled by the voltage of a different device (controller).
Current controlled device: the magnitude of either the current or voltage or resistance of the device is controlled by the current of another different device (controller).

Do you have some practical examples of both types of device?

thanks
fog37

 

[meBigGuy]

Saying a BJT transistor is voltage controlled or current controlled makes no difference. The math for thinking of it as voltage controlled (Ie = Ies (e ^(Vbe/VT) -1) is pretty sweet, but Ic = beta*Ib isn't so bad either. You can increase the base voltage by forcing current or increase the base current by increasing voltage. Both will increase the collector current. Either view works. Most analog designers I know think of it as Voltage controlled. But, don't get hung up on one view over the other. They both have their uses.

Trying to separate voltage and current is impossible since they are very interrelated . Did the voltage cause the current, or did the current cause the voltage?
But generally, voltage amplifiers have high input impedance and low output impedance so the voltage becomes an easy way to express the performance.
Current amplifiers have low input impedance, and high output impedance, so vice versa.

If you look at amplifiers, there are voltage amplifiers, current amplifiers, current feedback amplifiers, and transimpedance amplifiers. They all have their applications and uses. Voltage amplifiers are probably the best known and generally understood. I suggest you search for current feedback amplifiers. And look at current-to-voltage and voltage-to-current converter circuits.
I find the most useful method is to enter something like "current to voltage amplifier" in google and then look at the images for one that seems useful.

AS you will quickl see, taking a standard op-amp and hooking it up differently can implement a lot of different "amplifications"

This page talks about current feedback and then has some good links under "See Also"
https://en.wikipedia.org/wiki/Current-feedback_operational_amplifier

 

[LvW]

"Many errors, of a truth, consist merely in the application of the wrong names of things." - Spinoza

I think, this sentence applies in particular to the subject of our discussion.
I agree to meBigGuy that - for practical applications and designing circuits - it makes no difference if we believe in current-control or voltage-control (note the term "believe"!) as long as we are following the correct design steps (and corresponding formulas).

However, the situation changes if we
(a) ask for physical principles behind the formulas, or
(b) start thinking what we are really doing (and why !), or
(c) critically review the properties of the circuits we have designed.

Examples:
for (a): This involves carrier physics, which I don`t like to discuss at the moment (to involved for such a forum). On the other hand, we all know the "mystic" value of dVBE/dT=-2mV/K which tells us that - for a temperature caused increase in the collector current Ic - we have to reduce the base-emitter VOLTAGE by 2mV in order to keep Ic constant.

for (b): For BJT amplifier stages we are biasing the base terminal with a voltage divider which should be as low-resistive as possible (as allowed).
Why this design goal?
Because we try to make the base DC potential as "stiff" as possible which means: This potential should depend as less as possible on the base current (which has a very large tolerance). This clearly indicates that we are trying to use a DC VOLTAGE for biasing. However, we must find a trade-off between this "stiffness" and the resulting input impedance which shouldn`t be too small (loading of the signal source). Such a trade-off is easier to realize using, for example, the method of bootstrapping.

for (c): Because of several reasons it is always recommended to use an emitter resistance Re for negative feedback. This method changes/modifies many properties of the BJT stage - one of these changes is the signal input resistance that is increased (desired effect, of course).
From feedback theory we know that such an increase for the input resistance is to be observed only in case the feedback signal is a VOLTAGE. If the feedback signal is a current, the input resistance would be decreased.
From this it follows, that it is the base-emitter VOLTAGE that is influenced by feedback .
______________________
As another problem, the situation is even more complicated because of our technical language which sometimes is confusing (see Spinozas sentence).
As an example, we are using the term "current source" for a voltage source with an internal source resistance that is much larger than the connected load resistance. But in fact, it is NOT a current source. No current without voltage. Technically, there is nothing like a "current source". Each current needs a driving voltage (better: an E-field) - otherweise the charged carriers forming the current cannot move.

As to your question regarding examples for current-control:
In most cases, we are using this term if the controlling terminal has a very low input resistance (some ohms), for example current-feedback amplifiers which have a high-resistive (non-inv.) input node and a second low-resistive (inverting) input node.
As another example serves the classical current mirror where the current in the second (output) transistor is (nearly) identical to an externally injected current into the first (input) transistor. By the way: This equality of both currents is possible only because both base-emitter VOLTAGES are identical - another proove for voltage control!.

LvW

 

[fog37]

Thank you.

To keep things clear in my mind, how should I think of a current amplifier and a voltage amplifier? Is a current amplifier a device with low impedance that let's current pass through it and amplifies it?

Is a voltage amplifier always a large impedance device such that an applied voltage at the input is converted into a larger voltage at the output ports?

thanks,
fog37

 

[meBigGuy]

But generally, voltage amplifiers have high input impedance and low output impedance so the voltage becomes an easy way to express the performance.
Current amplifiers have low input impedance, and high output impedance, so vice versa.

 

[donpacino]

Thank you.

To keep things clear in my mind, how should I think of a current amplifier and a voltage amplifier? Is a current amplifier a device with low impedance that let's current pass through it and amplifies it?

Is a voltage amplifier always a large impedance device such that an applied voltage at the input is converted into a larger voltage at the output ports?

thanks,
fog37

what I am about to say assumes a continuous gain across different loads and ideal amplifiers (which takes into account what mebigguy said)

a voltage amplifier will always amplify voltage.
ex. input: 5 V
amplifer: gain of 5 V/V

output voltage with no load: 25V output current: 0A
output voltage with 10 ohm load: 25 V output current: 2.5A
output voltage with 500 Kohm load: 25V output current: 50uA

the same goes for current. Keep in mind REAL amplifiers won't be linear or perfect.

 

[sophiecentaur]

Saying a BJT transistor is voltage controlled or current controlled makes no difference. The math for thinking of it as voltage controlled (Ie = Ies (e ^(Vbe/VT) -1) is pretty sweet, but Ic = beta*Ib isn't so bad either.

Absolutely. If there is an equation that relates volts to volts, current to current or any other combination then, if you want to, you can use that to generate a name (Transistor = Transfer Resistor?) Just goes to prove my signature about categorising things.

 

[meBigGuy]

Actually, Sophie, you tend to categorize things more than anyone else on this forum (along with being the most negative poster). I always considered your signature to be the ultimate irony and have resisted dozens of situations when I wanted to tell you to read it yourself. I finally could not resist commenting on it. If no one "likes" this post I will consider myself suitably chastised.

 

[sophiecentaur]

Actually, Sophie, you tend to categorize things more than anyone else on this forum (along with being the most negative poster). I always considered your signature to be the ultimate irony and have resisted dozens of situations when I wanted to tell you to read it yourself. I finally could not resist commenting on it. If no one "likes" this post I will consider myself suitably chastised.

I have to admit to being negative and even grumpy at times. I do try to keep it under control.
There is a difference between attempted 'precision' and categorizing for its own sake - and then the resulting confusion when the chosen category causes a logical conflict. I read so many worried posts that have arisen just because someone has been told a 'rule' and then has found a exception.
I will try to practice what I preach, but I still see the message as a valid one. Could you give some example on this thread where you feel I have contravened my rule?

 

[fog37]

Hello,

I have done some reading and wikipedia has a good description on what an amplifier is. The goal of an amplifier is, given an input signal, to produce an exact but enlarged copy of it, the output signal. This is done by modulating a power source such that the modulation re-creates the same shape as the input signal.
There are the following types of amplifiers:

1. Voltage amplifier: An input voltage is amplified to a larger output voltage. The amplifier's input impedance is high and the output impedance is low
2. Current amplifier – input current produces a larger output current. The amplifier's input impedance is low and the output impedance is high
3. Transconductance amplifier – This amplifier responds to a changing input voltage by delivering a related changing output current
4. Transresistance amplifier – This amplifier responds to a changing input current by delivering a related changing output voltage

Some amplifiers can be realized with just a few resistors and a single transistors. In other cases operational amplifiers are used which have multiple transistors. At the end of the day transistors seem to be the main ingredient of an amplifier...

There is always a source connected to the amplifier (to its input) and a load connected to the amplifier output, correct?

In example above (reported below) for a voltage amplifier, the output impedance is
ex. input: 5 V
amplifer: gain of 5 V/V
output voltage with no load: 25V output current: 0A
output voltage with 10 ohm load: 25 V output current: 2.5A
output voltage with 500 Kohm load: 25V output current: 50uA

the impedance of the load affects the output current while the output voltage from the amplifier remains constant. But the load can change. The load is what we connect to the amplifier to receive the amplified voltage. The load (with its impedance) is not supposed to be part of the amplifier circuitry, correct? All the voltage amplifier does is to provide a potential difference. A transmission line has an impedance. I guess a device can have two impedances (an input and an output). I am familiar with the process of matching those impedances to the source or to the load to avoid energy reflections...

Thanks,
fog37

 

[donpacino]

Hello,

I have done some reading and wikipedia has a good description on what an amplifier is. The goal of an amplifier is, given an input signal, to produce an exact but enlarged copy of it, the output signal. This is done by modulating a power source such that the modulation re-creates the same shape as the input signal.
There are the following types of amplifiers:

1. Voltage amplifier: An input voltage is amplified to a larger output voltage. The amplifier's input impedance is high and the output impedance is low
2. Current amplifier – input current produces a larger output current. The amplifier's input impedance is low and the output impedance is high
3. Transconductance amplifier – This amplifier responds to a changing input voltage by delivering a related changing output current
4. Transresistance amplifier – This amplifier responds to a changing input current by delivering a related changing output voltage

Some amplifiers can be realized with just a few resistors and a single transistors. In other cases operational amplifiers are used which have multiple transistors. At the end of the day transistors seem to be the main ingredient of an amplifier...

There is always a source connected to the amplifier (to its input) and a load connected to the amplifier output, correct?

In example above (reported below) for a voltage amplifier, the output impedance is
ex. input: 5 V
amplifer: gain of 5 V/V
output voltage with no load: 25V output current: 0A
output voltage with 10 ohm load: 25 V output current: 2.5A
output voltage with 500 Kohm load: 25V output current: 50uA

the impedance of the load affects the output current while the output voltage from the amplifier remains constant. But the load can change. The load is what we connect to the amplifier to receive the amplified voltage. The load (with its impedance) is not supposed to be part of the amplifier circuitry, correct? All the voltage amplifier does is to provide a potential difference. A transmission line has an impedance. I guess a device can have two impedances (an input and an output). I am familiar with the process of matching those impedances to the source or to the load to avoid energy reflections...

Thanks,
fog37

one thing to add, amplifiers can also have a gain with a magnitude of less than one, which will attenuate the signal (make it smaller)

 

[sophiecentaur]

an exact but enlarged copy of it

One important thing about the 'enlargement' process is that the output signal should have more Power - or at least, more available. That may not involve increasing Voltage or Current, which could often be achieved using a passive method (e.g. transformer). This is mentioned in the above post.
Having said that, amplifiers can also be used to 'buffer' between two stages, so that the input circuits are not affected by variations in the output circuits.

 

[meBigGuy]

Sophie, I don't think your post in this thread was an issue, it just pushed my (admittedly oversensitive) button since you mentioned your sig. The router thread was a classic example of you classifying the OP and making assumptions about router complexity (but mostly not being supportive or educational). No one liked my post here, so maybe others don't agree. I can be over-reactive, I know. You quite consistently post useful and educational information. My primary goal when I post is to try to move my knowledge and the thread's content forward in an useful and educational way. If I think someone is not prepared, I *try* to point him to something that will demonstrate that in a useful way or ask questions that will make him realize what he doesn't know. (I'm sure there are plenty of contrary examples in 1600 posts)

Regarding this thread and amplifiers:
The load is technically not part of the amplifier, but can affect amplifier performance.
For example, real voltage amplifiers have an effective output impedance, so increasing the load (driving lower resistance) will cause more voltage drop within the amplifier (and more power dissipation). Similar effects exist with the source impedance. Amplifier performance usually is specified for a range of load and source impedances.
Regarding amplifier design: given the load range, feedback techniques may be used to reduce the effects of loading.

 

[sophiecentaur]

Sophie, I don't think your post in this thread was an issue, it just pushed my (admittedly oversensitive) button since you mentioned your sig. The router thread was a classic example of you classifying the OP and making assumptions about router complexity (but mostly not being supportive or educational). No one liked my post here, so maybe others don't agree. I can be over-reactive, I know. You quite consistently post useful and educational information. My primary goal when I post is to try to move my knowledge and the thread's content forward in an useful and educational way. If I think someone is not prepared, I *try* to point him to something that will demonstrate that in a useful way or ask questions that will make him realize what he doesn't know. (I'm sure there are plenty of contrary examples in 1600 posts)

Why are you using an unrelated post to continue with this personal stuff? Do you want me and others to take you seriously? If I were you, I would try to calm down about this. It cannot be good for your blood pressure.
Send me a PM if you want to continue this.

 

[jim hardy]

hmmmm

Trying to separate voltage and current is impossible since they are very interrelated . Did the voltage cause the current, or did the current cause the voltage?

indeed ,,

i = e^qv/kt and q&k are constants.

Absolutely. If there is an equation that relates volts to volts, current to current or any other combination then, if you want to, you can use that to generate a name (Transistor = Transfer Resistor?)

I've come to really look forward to posts by you guys, both of you are knowledgeable astute and eloquent.


Lavoisier on classification:

The impossibility of separating the nomenclature of a science from the science itself, is owing to this, that every branch of physical science must consist of three things; the series of facts which are the objects of the science, the ideas which represent these facts, and the words by which these ideas are expressed. Like three impressions of the same seal, the word ought to produce the idea, and the idea to be a picture of the fact. And, as ideas are preserved and communicated by means of words, it necessarily follows that we cannot improve the language of any science without at the same time improving the science itself; neither can we, on the other hand, improve a science, without improving the language or nomenclature which belongs to it. However certain the facts of any science may be, and, however just the ideas we may have formed of these facts, we can only communicate false impressions to others, while we want words by which these may be properly expressed.[3]

Please don't stop with your quips , Sophie , I've never seen one that was mean spirited or ad-hominem.
If we can't laugh at our mistakes, what will we do for fun when we get old ?

old jim

 

[LvW]

"Trying to separate voltage and current is impossible since they are very interrelated . Did the voltage cause the current, or did the current cause the voltage?"

Jim Hardy`s post has pointed my attention again to the above quote (from meBigGuy). I think, this question deserves a separate discussion. Of course, they are "interrelated" - but is this something like a "chicken-egg" case? I don`t think so: Voltage can cause current but not vice versa!

 

[sophiecentaur]

Nice words Jim - thanks. But I know I can be ` grumpy old sod at times.

 

[Baluncore]

No one liked my post here, so maybe others don't agree.

I did not like your post because that would unnecessarily divide the community into two hostile factions. The winner would be classification, with Physics the loser.

Meanwhile I'm going to keep trickling current into BJT bases while taking advantage of the greater current that flows as Ic. I will ignore Vbe wherever possible. I am quite happy to use current rather than voltage as the analogue. I prefer logic gates with only one input, but with many outputs.

We are taught to use oscilloscopes to look at voltages, so we rarely break a circuit to measure a current. The voltage or current dichotomy is like which came first, the chicken or the egg? The chicken is the best way the egg has of reproducing itself.

Voltage and current always dance together, sometimes one leads, sometimes the other.

 

[sophiecentaur]

Teaching any subject 'by numbers' is never going to give a good understanding. It may enable a soldier to operate a system and even solve some problems but, outside of the context of the 'numbers' that soldier cannot be guaranteed to deal with a situation. We get so many posts from worried students who see their simple school classifications let them down and it worries me.
My soldier is only an example. Many other examples are available.

 

[LvW]

Hi Baluncore - sorry, but I am not with you. I kindly ask you to answer two questions:

Meanwhile I'm going to keep trickling current into BJT bases while taking advantage of the greater current that flows as Ic. I will ignore Vbe wherever possible. I am quite happy to use current rather than voltage as the analogue.
.

Please, can you give me one single example (linear BJT operation) where you are allowed to "ignore Vbe" ?

Voltage and current always dance together, sometimes one leads, sometimes the other.

Please, can you give me one single real-world example (not usig an artificial ideal current source model) where we have current without driving voltage (with other words: where "current leads") ?
LvW
PS: I am happy to say that I fully support the first sentence of your reply.

 

[milesyoung]

Please, can you give me one single real-world example (not usig an artificial ideal current source model) where we have current without driving voltage (with other words: where "current leads") ?

This type of question has been discussed to death. There's always someone who has an idea of some example that proves voltage/current is _the_ driving force, whatever that means.

There's no causal relationship between voltage and current in EM, so it doesn't make any sense to argue one way or the other.

There exist mathematical relationships, circuit/element laws etc., and I'm personally happy to just leave it at that.

 

[LvW]

This type of question has been discussed to death. There's always someone who has an idea of some example that proves voltage/current is _the_ driving force, whatever that means.
There's no causal relationship between voltage and current in EM, so it doesn't make any sense to argue one way or the other.
There exist mathematical relationships, circuit/element laws etc., and I'm personally happy to just leave it at that.

I just have asked for a short example because I am still open to learn.

 

[Baluncore]

Please, can you give me one single example (linear BJT operation) where you are allowed to "ignore Vbe" ?

1. When Vbe is deliberately placed within the feedback loop of an op-amp.
2. When Vbe is canceled by the Vbe of another identical transistor such as in a balanced current mirror.
3. Integrated Injection Logic.

Please, can you give me one single real-world example (not usig an artificial ideal current source model) where we have current without driving voltage (with other words: where "current leads") ?

I am arguing that neither current nor voltage can exist alone.
Why do you challenge me to disprove my own statement?

 

[meBigGuy]

I apologize for being divisive.

Of course, they are "interrelated" - but is this something like a "chicken-egg" case? I don`t think so: Voltage can cause current but not vice versa!

In the most fundamental sense I think you are correct. But, inside the circuit it gets a bit gray. For example, when I increase the voltage across a resistive divider the current through the divider increases and the output voltage increases. Would you say "increased output voltage due to increased input voltage caused increased current through the second resistor". Or would you say "increased current due to increased input voltage caused a higher voltage at the output".

 

[meBigGuy]

Regarding transistor operation, both equations are correct, and both are useful depending on the situation.

Ie = Ies (exp(Vbe/Vt) -1)
Ic = beta * Ib

The first equation is used by the analog ASIC designers I work with (world class designers). They say that is how they view bipolar devices. They see Ib as an unavoidable side effect.

All of Baluncores examples can be looked at either way.
The whole point of a current mirror is that identical Vbe causes identical Ie. When I string 10 mirrors to 1 source, it is the identical Vbe that I'm attaching to multiple devices.

 

[sophiecentaur]

Please, can you give me one single real-world example (not usig an artificial ideal current source model) where we have current without driving voltage (with other words: where "current leads") ?

If your "real world" means on a circuit board then I think that there is not an example. But if your real world can include a phenomenon out in space then I think you could say that a beam of charged particles, a long way from any other charges would constitute a current but there would be no PD between two points on their path and no measurable field, either, until you place some conducting objects in the path. That could produce a PD between the objects.
But this is only playing. milesyoung has supplied the pure answer to the pure question.

 

[Baluncore]

The solar wind is a current that arrives at the Earth. It is not the result of a voltage difference between the Sun and the Earth. It charges the Earth-Ionosphere capacitance to a high voltage. If the Sun stopped providing the proton current the E-I voltage would begin to fall. In an MHD generator, a voltage appears due to charged particle flow.

In a CRT, the previously accelerated electron beam is a current that causes a voltage to appear on arrival at the resistive phosphor screen. Consider an electron being accelerated by an electric field. As the electron accelerates, it's electric potential energy is converted to kinetic energy. As the electron passes through a zero potential electrostatic grid into a neutral space beyond, it carries kinetic energy without voltage. The electron beam is a current.

Quantised energy levels in ions can be specified in eV. The delivery of energy by a photon can lift an electron to a higher “voltage” level. Does the change in electron configuration constitute a current?
If the atom becomes fully ionised and an electron is lost from that atom, does it then a constitute a current? Where do you draw the line?

 

[meBigGuy]

I'm wondering how increasing the voltage across a capacitor then caused the current to flow into it.

But, I maintain that these are both correct
Ie = Ies (exp(Vbe/Vt) -1)
Ic = beta * Ib and its alpha variations

https://en.wikipedia.org/wiki/Bipolar_junction_transistor#Ebers.E2.80.93Moll_model

 

[LvW]

1. When Vbe is deliberately placed within the feedback loop of an op-amp.
2. When Vbe is canceled by the Vbe of another identical transistor such as in a balanced current mirror.
3. Integrated Injection Logic.
I am arguing that neither current nor voltage can exist alone.
Why do you challenge me to disprove my own statement?

Hi Baluncore - perhaps I misunderstood your term "ignore".. As to your examples:
1.) Can VBE be "ignored". As you know, the function Ic=f(Vbe) is the reason for the logarithmic transfer function
2.) OK - but is VBE canceled out? It is important that both VBE values give the same current Ic.

You see, my understanding of "ignore something" is somewhat different. That`s all. No problem.
Now I know what you mean.

Regarding the second point. Sorry - but I did not want to challenge you to disprove your own statements.
I just have asked for an example for a better understanding of your statements.
I am not a "youngster" - and in the past I have learned that - in physics and electronics - it is always good/necessary to give real-world examples while explaining rules, statements and formulas.

Hence, thank you for additional examples in your post#30. Now it is clear that you did not speak about "classical" currents in lumped electronic circuits (because we are in a thread about transistors and opamps), but about some other effects (solar winds, effects on quantised energy levels, ...

 

[sophiecentaur]

You see, my understanding of "ignore something" is somewhat different. That`s all. No problem.
Now I know what you mean.

He was, I'm sure, writing as an Engineer. A real project has so many variables that you grab the few that you can 'ignore' (get away with) with both hands. And it's still hard to design faultless electronics.

 

[LvW]

He was, I'm sure, writing as an Engineer. A real project has so many variables that you grab the few that you can 'ignore' (get away with) with both hands. And it's still hard to design faultless electronics.

Yes - that may be the case. I think, again my quotation (my post#6) does apply:
"Many errors, of a truth, consist merely in the application of the wrong names of things." (Spinoza).
Perhaps not "wrong" names - but different (simplifying) terms and formulas.
For example:
1.) The meaning of the word "ignore". Does it mean "forget at all" because it is unimportant? Or does it mean "does not appear in a formula"?
2.) The classical relation Ic = beta * Ib. Some people even believe that Ic is determined by Ib only (and NOT by Vbe) because Vbe does not appear in the formula.

 

[LvW]

In the most fundamental sense I think you are correct. But, inside the circuit it gets a bit gray. For example, when I increase the voltage across a resistive divider the current through the divider increases and the output voltage increases. Would you say "increased output voltage due to increased input voltage caused increased current through the second resistor". Or would you say "increased current due to increased input voltage caused a higher voltage at the output".

Yes - I would, of course, use the second version during a conversation and even in a written text.
And I am also using the term "current source" - knowing that such a circuit does not exist in electronics.
On the other hand, from time to time we should try to convince ourself what we are doing and what we are saying.
And - as you know - in real electronics nothing is correct:
* Are there really linear circuits?
* Does a "sinusoidal signal" really have no harmonics?
* Has a resistor only a resistive component?

And - as far as the current through the second resistor of a voltage divider is concerned - does the current not only flow because there is an E-field within the resistor?
So - does the flowing current produce a voltage or...?

 

[sophiecentaur]

I can't believe that any contributor to this thread would hesitate to use the assumption that an emitter follower amplifier, really does follow the base input volts or that the gain of a simple voltage amplifier is quite near enough the ratio of the top and bottom resistors . . . . .when appropriate. Since the bjt transistor has been made with a Beta in the hundreds, the feedback that it can provide on its own allows all sorts of 'liberties'. And that "doesn't mean your a bad Engineer" to quote, loosely, Rod Steiger.
Why are we still arguing here? There must be far more meaty matters for us to lock horns on.

 

[LvW]

Sophiecentaur, OK , I agree with you - as long as we all feel and think as engineers.
That means (taking your example):
While speaking about an emitter follower, we automaticcaly assume that our discussion partner knows what I know: The gain will never be unity but we neglect the difference between unity and the actual value and we treat the stage as if it had unity gain. But we know WHY we are allowed to make such a simplification.
But , for my opinion, the situation looks somewhat different when we are in the position to answer some questions from students or other newcomers.
I have several years experience in teaching analog electronics, and I think it is of fundamental importance to explain under which constraints (circumstances) some simplifications are allowed.
Again, taking your example, I never would tell the students that the gain of an emitter stage with RE feedback would be G=-(RC/RE).
I remember a corresponding discussion in another forum where somebody was completely lost because he had to calculate the gain with a capacitor across RE (is the gain infinite?).
Therefore, I think it is good engineering practice to give the formula G=-RC/(1/gm + RE) ) and to show under which conditions (1/gm<<RE) we can make use of an approximation. I hope that I could make myself understandable (in a language that is not my mother tongue).
This was my answer to your question "Why are we still arguing here? There must be far more meaty matters for us to lock horns on."
Thank you
LvW

 

[jim hardy]

what comes first in a depletion region? Does a potential gradient appear ? Chicken or egg ?
honest question I've never quite figured out.

 

[meBigGuy]

The emitter current of a transistor is determined by its base-emitter voltage. The changing of the depletion region comes first.

 

[jim hardy]

The changing of the depletion region comes first.

Changing ?

I guess i didn't express my dilemma, sorry about that

in post 38 lower picture ,
electrons have migrated from donor atoms in n material into receptor atoms in p material, before any external voltage was applied.
That was current , for an instant, but was it sans voltage ?

 

[sophiecentaur]

The electrons are moving (have moved) to the condition of least potential (as in all cases). When the device was manufactured, this was the situation and, I guess, if you could take a block of n and a block of p and bring them together instantly in perfect contact, there would be a flow of electrons to cause the depletion layer. Producing a diode by doping will achieve the same thing but 'gradually'.
If you want to talk in terms of Volts then I guess you can. It's a term used to describe the activity between two conducting substances. I'm not sure it gets us much further though. Chicken and Egg worries can be frustrating without advancing understanding.

 

[meBigGuy]

electrons have migrated from donor atoms in n material into receptor atoms in p material, before any external voltage was applied.
That was current , for an instant, but was it sans voltage ?

Yeah, I missed that. You are speaking of the re-adjustment of charge when the regions are "connected" creating the potential barrier. It is essentially a "chemical process", I think. Or is it simply charges attracting?

"What happens when you make a battery?" sort of thing. But not my area of expertise, for sure.

I think flow of free electrons and holes has been acknowledged.

 

[LvW]

Yeah, I missed that. You are speaking of the re-adjustment of charge when the regions are "connected" creating the potential barrier. It is essentially a "chemical process", I think. Or is it simply charges attracting?
.

Some people call this phenomenon (re-adjustment of charged carriers) "diffusion current" because it is really a process of diffusion.
The cause (the force) behind this process is the tendency of the different charge concentations to redistribute uniformly.
This process stops when the force behind this effect equals the force within the E-field caused by the diffusion voltage (potential barriere). Both forces have opposite directions.

To answer Jim Hardy`s "dilemma": I wouldn`t see any "chicken-egg" problem behind this effect because
(a) the mentioned "diffusion effect" (current ?) surely is not a current in the classical sense (steady state current caused by a voltage),
(b) the development of the diffusion voltage can be described by a clear sequence of cause and effect .

 

[jim hardy]

The cause (the force) behind this process is the tendency of the different charge concentations to redistribute uniformly.
This process stops when the force behind this effect equals the force within the E-field caused by the diffusion voltage (potential barriere). Both forces have opposite directions.

Thanks, that makes sense.

I've long thought we electricals should have something akin to enthalpy for our charges
to help with effects like Seebeck, Thompson, Peltier

maybe in my next life.

 

[Baluncore]

I raised the question of quantum chemistry earlier in post #30.

Quantised energy levels in ions can be specified in eV. The delivery of energy by a photon can lift an electron to a higher “voltage” level. Does the change in electron configuration constitute a current?
If the atom becomes fully ionised and an electron is lost from that atom, does it then a constitute a current? Where do you draw the line?

To answer Jim Hardy`s "dilemma": I wouldn`t see any "chicken-egg" problem behind this effect because
(a) the mentioned "diffusion effect" (current ?) surely is not a current in the classical sense (steady state current caused by a voltage),

LvW; Your argument is self referential.
By arbitrarily claiming “(steady state current caused by a voltage)” as the only “classical current” you are defining the answer to the question and so eliminating the entire voltage/current/chicken/egg discussion.

I consider any “moving electron” to be a current. Making a PN junction will generate a magnetic pulse due to electron movement, (diffusion?).
Does a change in energy from PE to KE then back to PE constitute a change in eV~oltage? Is energy synonymous with voltage for an electron?

 

[meBigGuy]

I'm kind of lost. What exactly are we discussing? Whether electrons can move without an applied voltage? Is that meaningful (as in does it matter)? Somehow groups of charges got separated and when you bring them physically together things move, like closing a switch.

What happens in a battery to produce voltage?

 

[LvW]

LvW; Your argument is self referential.
By arbitrarily claiming “(steady state current caused by a voltage)” as the only “classical current” you are defining the answer to the question and so eliminating the entire voltage/current/chicken/egg discussion.

OK - if you like, forget my "arbitrarily claiming" about "classical current". That`s not the main point.
In post#40 J. Hardy has formulated a question which I have tried to answer.
Quote J. Hardy:
electrons have migrated from donor atoms in n material into receptor atoms in p material, before any external voltage was applied.
That was current , for an instant, but was it sans voltage ?

I think, for answering this question it is not necessary to discuss if this electron movement is a (classical) current - or not.
The main point is that this movement is not caused by an external voltage but by diffusion pressure, OK?

 

[Baluncore]

The main point is that this movement is not caused by an external voltage but by diffusion pressure, OK?

I am questioning everything. Is diffusion pressure not an internal voltage?
If following contact, diffusion results in a momentary flow of current, then there will be a reason for the advantage in such a change of the charge distribution and then to maintain that new equilibrium.
I am suggesting that the quantum energy states available on the two sides before contact are slightly different. To an electron, that energy difference, measured in eV, appears as a voltage. When contact is made, the momentary magnetic pulse produced by the charge redistribution, carries away some of the excess energy.

 

[LvW]

I am questioning everything. Is diffusion pressure not an internal voltage?
.

At first, I think it is a good practice to "question everything".
However, in this particulöar case...
Here is what wikipedia says (although I do not rely too much on this "knowledge source):

Diffusion is the net movement of molecules or atoms from a region of high concentration to a region of low concentration. This is also referred to as the movement of a substance down a concentration gradient.

I don`t think that the "diffusion pressure" is some thing like a voltage (unit V) - although the result is similar (movement of charged carriers).

 

[Baluncore]

Diffusion is the net movement of molecules or atoms

But in this case it is electrons moving, not atoms.
What equilibrium is reached that stops the "diffusion" continuing throughout the material?