Amplifiers, transistors and opamps

[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?

 

[LvW]

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

As I have written in post#43:
This process stops when the force behind this effect equals the force within the E-field caused by the diffusion voltage. Because both forces have opposite directions.an equilibrium does exist (stopping the diffusion process).

 

[meBigGuy]

1. Does the creation of an E field by redistribution of charges result in a momentary magnetic pulse? Can someone calculate its magnitude?
2. Diffusion "pressure" is simply random thermal motion. A certain number of charges have enough thermal energy to cross the barrier, which increases the barrier. That barrier increase is what causes equilibrium.
3. It seems like a net charge flow in one direction would be a current, but it's hard to see thermal motion as a voltage. You can calculate an effective voltage (that's probably the barrier voltage that results) but was there an "applied" voltage?

4. No one has commented on my question about charge redistribution in a battery. When you insert electrodes into an electrolyte, charges move and create a voltage. Was there an "applied voltage" to move the charge?

5. What about moving the plates of a charged capacitor closer together. We have an increased voltage caused by moving charges, but no applied voltage to move them. It was caused by mechanical motion of the plates.

 

[LvW]

I think, your questions concern the DEFINITION of electrical current.
As we can see, charged carriers do not only move because they are exposed to an E-field.
Counter examples: battery (chemical process), pn junction (diffusion pressure),...

For this reason we should remember (or search for) a correct definition for the phenomenon we call "electrical current".
Perhaps it is necessary to add that movement of charges is called "current" only if this movement is caused by an electrical potential difference (resp. a corresponding E-field)?

 

[sophiecentaur]

creation of an E field by redistribution of charges result in a momentary magnetic pulse?

Isn't that basic EM theory? Photons are bound to be emitted whilst the electric energy state inside an object changes. The energy would be very very low of course.

Was there an "applied voltage" to move the charge?

etc
PS this is the kind of "classification" that I have problems with. The situation is the situation and just naming it one way or another doesn't really help. Naming things in this way determines the form of analysis that would be used and, sometimes, there are other, more suitable ways of analysing a situation.
If one chooses to look at a Broadcast Radio transmitter as a source of photons, for instance, the analysis will soon get out of hand. Jeez, the classical approach is hard enough!

 

[meBigGuy]

Photons are bound to be emitted whilst the electric energy state inside an object changes.

Now, that's precise science, yes it is! End of subject. You nailed it. We all fully understand now. NOT!

PS this is the kind of "classification" that I have problems with.

How is my asking if there was an applied voltage to move the charges (when it was diffusion and others are saying there needs to be voltage to cause current) in any way classification?
Do you even understand what is being discussed? Do you think you can make a meaningful contribution rather than useless potshots? (3 of them in that post)

Try presenting some science, physics, or other REAL technical information ,that relates to the thread. Try to stay on topic.

Maybe you could explain specifically how diffusion caused by thermal motion moving charges at the initial formation of a PN junction produces a magnetic moment that emits photons.
Maybe you could explain how moving capacitor plates closer together (charge redistribution) causes emitted photons.
Maybe you could explain how charge initial redistribution in a galvanic cell that is not supplying current causes emitted photons.

Thought not!

 

[sophiecentaur]

Now, that's precise science, yes it is! End of subject. You nailed it. We all fully understand now. NOT!
How is my asking if there was an applied voltage to move the charges (when it was diffusion and others are saying there needs to be voltage to cause current) in any way classification?
Do you even understand what is being discussed? Do you think you can make a meaningful contribution rather than useless potshots? (3 of them in that post)

Try presenting some science, physics, or other REAL technical information ,that relates to the thread. Try to stay on topic.

Maybe you could explain specifically how diffusion caused by thermal motion moving charges at the initial formation of a PN junction produces a magnetic moment that emits photons.
Maybe you could explain how moving capacitor plates closer together (charge redistribution) causes emitted photons.
Maybe you could explain how charge initial redistribution in a galvanic cell that is not supplying current causes emitted photons.

Thought not!

You are making this all far too personal. Please stop.
The fact is that you 'can' if you like, demand to talk in terms of "applied voltage" in any situation and there will be a long winded explanation which sticks to a model involving Volts and that will work. Classifying a problem in inappropriate terms does not help with understanding or with solving the problem, which is the point I was making. Inside the solid state, it is convenient to talk in terms of Energy Levels, Energy Bands and Potentials. Also, charges diffuse when there is a difference in potential in different places. The thermal aspect adds more complication. It's difficult stuff and the course I did was so long ago that I forget the details. My Kittel Solid State Physics book is still on the shelf behind me and I could get better acquainted with it all in time.
I don't actually understand what your "Maybe" comments actually mean.

That is very childish.

 

[Dale]

This thread is closed.