BJT Common Emitter Large and Small Signal Research

LvW

Joined Jun 13, 2013
1,752
Hi Bordodynov, I am not sure if it will make much sense to continue this discussion.
I have got the impression that we both start from different views - and this will cause some misunderstandings.
Nevertheless, some short comments.
* Simulation results can never reveal which parameter (voltage or current) will be the cause of some effects.
These programs use formulas and calculate relationships forward and backwards.
But in this discussion - as far as I understand - we are discussing the question of cause and effect.

* When you speak about photodiode currents - this current will flow within a closed current loop.
Don`t you think that there will be an E-field driving the carriers which form the current? And how is this E-field produced?
What is your answer to the fact I have mentioned that photocells are combined in series for producing a larger voltage?

* Quote: "your statement about "Vbe-uncertainty (0.6 or 0.7 volts)" is wrong" .
I think, it is by far not sufficient simply to state that something is wrong - without any comment or justification.
I really do not understand such a "funny" comment.
It may be interesting to read about your early transistor experiments, but what do you want to show me?
Of course, you can assume (assume !) that it would be possible to "inject" a current into a circuit node (and calculate correspondingly), but that is not the physical truth - it is just a helpful "rule of thumb" !
A current cannot be "injected" - a current is always the RESULT of an applied voltage.

* Clear and simple question: Can you show me one or more circuit examples, which can be explained with BJT-current-control only? Up to now, nobody from the "current-control party" was able to present such a circuit.
On the other hand, I have no problems to show how and why it is the base-emitter voltage that determines the collector current.
(And that is the reason for my position which you call "obsession").
 

Bordodynov

Joined May 20, 2015
3,177
I agree that there is no point in continuing the discussion. If you had looked carefully at my diagrams, you would have seen that the base-emitter voltage was about 170mV, not the 0.6 - 0.7V you mentioned.
About the solar cells connected in series. Yes you have a knack for multiplication. But what happens if you close one of the solar cells? I had a panel of solar cells. It just so happened that one of the cells was in the shade. As a result, the voltage at the output dropped drastically. The panel was only giving out 10 mA. I was indoors and I was exposing the panel to the light from the window. This 10 mA was determined by the light inside the room falling on the solar cell in the shade. I simulated this. You can do this experiment yourself. About the light causing the voltage. What is the voltage on a shorted solar cell. Where did your voltage go. If you short the solar cell with an ammeter (in my case they are zero voltage sources, look at the schematics). The current is not created by the voltage, but by the electric field, and it can be present without voltage. And here is my final calculation of the solar cells connected in series. You may not believe my calculations showing that the solar cell is a source of current, which contradicts your views. Then I suggest you do an experiment. After all, practice is the criterion of truth!2021-06-02_14-22-06.png
 

LvW

Joined Jun 13, 2013
1,752
The great ones make mistakes, too. I have found mistakes in books by authorities. And just because a man has been telling the truth for a long time, doesn't mean he can't be wrong.
OK - here are some other "great ones" - and as you know they have explained and invented a variety of BJT-based circuits:

* The great late Barrie Gilbert ("base current must be seen as a defect"),
* The well-known "analog guru" Robert A. Pease,
* A. Paul Brokaw ("base current is a nuisance which will be neglected whenever it is practical to do so")
* Jim Williams (Editor of "Analog Circuit Design")
* Winfield Hill (co-author of Art of Electronics: "BJTs are transconductance devices, which means they are voltage controlled")
.
.
.
Perhaps these names with much more reputation than me can convince you?
In addition, there are very detailed publications from leading US- universities (Berkeley, Stanford, MIT) which clearly explain why BJTs are voltage controlled. Even the Early effect can serve as an evidence (as well as the popular tempco of -2mV/K)
 

Bordodynov

Joined May 20, 2015
3,177
I did not learn from these famous people and they are not an authority for me. There is Occam's razor. If I can design circuits (and good circuits) using the theory of current control of bipolar transistors, then I am entitled to consider it correct. Doesn't this remind you of the case of photons - is it a wave or a particle? You have one point of view I have mine. I concede that both are entitled to life. I'm on the side of what is simpler and more familiar to me. After all, it was you who started the discussion and imposed your point of view. You suggested that we stop and I agreed with you. Using an equivalent current source with a parallel resistor is preferable to a voltage source with a series resistor. When calculating in the first case, the matrix for the calculation will have an order of one less, and this speeds up the computer calculation. I choose simplicity and you choose truth (for you). For you the base current is an unfortunate fact, but for me it is a natural property. Just as in the germanium transistor circuit I showed, I neglected the base emitter voltage, so you can neglect the base current in the circuit with the base divider. What is there to discuss. And by the way, what about the solar cells, whose operation you do not understand?
 

Bordodynov

Joined May 20, 2015
3,177
Here is a comparison of how the transistors work when controlling voltage and current. Compare the nonlinear distortion.
In the old days, transformer amplifiers dominated. There was a rule of thumb. With a transformer, the resistance of the signal source was set to three times the input resistance of the transistor. What do you think. What is the signal source closer to in this case - a current generator or a voltage generator?
 

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LvW

Joined Jun 13, 2013
1,752
If I can design circuits (and good circuits) using the theory of current control of bipolar transistors, then I am entitled to consider it correct.
OK, let's get specific and design a classical common emitter amplifier:
1) From gain requirements we derive a rough value for gm*Rc (gm=slope of the voltage control characteristic Ic=f(Vbe)).
2) From this, we select corresponding values for Rc and Ic
3) we know about the temperature sensitivity of Ic caused by d(Vbe)/d(T)=-2mV/K and decide to use Re-feedback
4) Re causes a feedback voltage Ve (which - together with the base voltage Vb - will reduce Vbe for rising Ic)
5) Final step: Base voltage divider for providing Vb (divider current at least 10 times Ib). You know why?
__________________________
Question 1:
* At which step did we assume current control?
* Do you use another step sequence based on current control?
* Now - change the transistor (lower or higher beta value). Wouldn`t you expect that the voltage gain changes (same bias point, of course) ? But the gain remains constant (it is the transconductance which matters only!)

Question 2: Would you please describe and explain how current control is used in one of the circuits you have designed?

My conclusion:
I hope I'm not offending anyone when I say: I have the impression that many people use all the voltage control rules - without realizing it. They continue to believe in current through Ib.
However, suddenly when differential amplifiers or opamps come into play, it seems that transistors change their working principles immediately and the base currents are less important.
Funny situation.

Edit: My comment to your last post: It does not show or prove anything. You only have shown that the relation Ic/Ib is a linear one and that the relation Ic=f(Vbe) is nonlinear. This not new.
 
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LvW

Joined Jun 13, 2013
1,752
bias_new_page_001.jpg
This illustration shows what really happens when you use another biasing scheme (base current "injection"). When you want to see what an idealized current source (as in your last example) will do, continuously increase Vo and Rb to larger values.
 

Bordodynov

Joined May 20, 2015
3,177
You offend me. I wrote what I am so that you would have an idea of my qualifications. I'm going to disappoint you. The first two points are nonsense. I'll give you a simple rule. The gain of a common emitter stage is equal to the voltage across the resistor multiplied by 38. And this is virtually independent of the current. For example. You need Gain=50=> VRc=50/38=1.32. If you need to set the current to 1mA, then Rc=1.32k. But you can set the collector resistor Rc=1k. Then you need to set the current to 1.32V/1kOhm=1.32mA. This is the right way to do it.
And from this way it follows that the bigger the voltage the bigger the gain and at small supply voltages you can't get much gain with a resistor in the collector. This limitation can be avoided by using dynamic loads. I have developed many different dynamic loads and some have been implemented in practice. And I did it 35 years ago. And you are asking me questions about a schoolboy cascade. This cascade is widely used in textbooks. In practice, such amplifiers are used by beginners (mostly amateurs. I used direct-coupled amplifiers, which make better use of the amplifying capabilities and have fewer parts. I am sure if you need 1000 gain you will take two of these stages (with resistive dividers), which will indirectly characterize your qualification. I am a very skilled circuit engineer, and you offend me with your questions. I can draw a hundred variants of three-stage amplifiers, and you can?
 

Bordodynov

Joined May 20, 2015
3,177
Give me the calculation of your cascade at 3V (two alkaline cells). I'll have a laugh. By the way, I made a radio receiver with two transistors powered by one alkaline cell (1.5V) . The receiver received long and medium waves without a band switch. This was possible because the input impedance of the first transistor had a negative capacitive component. A miniature variable capacitor was used for tuning.
 

BobaMosfet

Joined Jul 1, 2009
2,110
Hi, I want to study about BJT Common Emitter Large (or bias) and Small Amplifier Circuits.

My question is I could not find a decent sufficient resource neither from youtube nor from google. :(

I want to study about those common emitter large & small signal models, analysis, ac gain, input & output impedance.

Please, is there any resource you can recommend me?


After studying, I want to become knowledgeable about turning the BJT circuit into lage and small model as well...

Here is an example that wanted from us to find its small & large signal model, Ic and also Vout:
(This is just for an example)

View attachment 240123

I really have to study about those topics in this week but my teacher did not give me some sufficient sources to understand them.

If you help me out for finding sources, I will be able to study a lot and that will make me such a happy person. ^^
Start here:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

The Art of Electronics 3rd Ed.
Author(s) Horowitz & Hill
ISBN-10: 9780521809269

Youtube has an incredible plethora of BJT tutorials and information. I suggest you change your search terms to get better matches.
 

BobaMosfet

Joined Jul 1, 2009
2,110
In addition, here is a short remark regarding the small-signal model you have shown:
In the right part of the model there is a voltage-controlled current source ic=gm*vbe.
From the physics point of view, this is a "correct" model (reflecting the real behaviour of the BJT).
However, you should not be confused if you see another model with a current-controlled current source ic=beta*ib.
This model can also be used for calculations because of beta=gm*rbe.
This is absurd. From a real-world physics viewpoint voltage is not real. It is in fact a man-made concept to describe the ratiometric relationship between the only two physics aspects in question: the movement of negatively charged electrons, and the opposition to that movement.

In BJTs, voltage is only a descriptor of the direction (polarity) and magnitude of that polarity in order to control direction of current, not quantity. BJTs use a small current on the base to control a large current between the emitter and the collector. Period.
 

ElectricSpidey

Joined Dec 2, 2017
2,757
Personally I think the problem here is the confusion of a control mechanism vs the underlying physics.

Example:

In a car the steering wheel and pedals are the control devices, they bear only a indirect relationship to the physics behind the actual control of the car.

Take an LED, you control the brightness using current, but in reality it takes both current and voltage to emit light from an LED.

The same is true for a transistor, it will not operate without both current and voltage.

My point is...the underlying physics and a control mechanism are not the same thing. it is my opinion that when people refer to the "control" of an electronic device, they are not referring to the underlying physics, simply the method to make it do what they want.

I control my dog with a leash, what is the physics behind that...friction...tension? Frankly I don't give a crap.

:) Carry on...
 

LvW

Joined Jun 13, 2013
1,752
BJTs use a small current on the base to control a large current between the emitter and the collector. Period.
In short: This is wrong.
More than that - it can be (and was) proven that the collector curent is dermined by the voltage Vbe only. In the past, I have mentioned several effects, observations and theoretical derivations to show that the BJT is a transconductance device. There is really no shortage in examples. To state that the BJT would be current-driven is a pure belief without proof. Have you a proof?
It results from a misinterpretation of the formula Ic=B*Ib. Ib is just a byproduct (a part) of Ie=Ib+Ic, but it cannot control Ic. How should this be possible?

Because you have used the term "nonsense": Even from the energy point of view it is impossible (and it is nonsense to believe) that a small quantity can DIRECTLY control (with amplification) a much larger quantity of the same kind.
As you know, there are some attempts to explain the transistor function with a "water model". However, none of these models can work in reality - the energy balance is violated.

You have recommend "Art of Electronics". Good recommendation!

Here are some excerpts (2nd edition, 1989):
Page 79...80 : "...we said earlier that Ic=hfe*Ib. We thought of the transistor as a current amplifier...That´s roughly correct, and for some applications it`s good enough. But to understand differential amplifierrs, log converters, temperature compensation and other important applications you must thnk of the transistor as a transconductance device - collector current is determined by base-to-emitter voltage"...."For transistors it is important to realize that the collector current is accurately determined by the base-emitter voltage, rather than by the base current"....

I am sure, you will recognize some contradictions between your opinion and Horowitz/Hill.
 
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LvW

Joined Jun 13, 2013
1,752
My point is...the underlying physics and a control mechanism are not the same thing. it is my opinion that when people refer to the "control" of an electronic device, they are not referring to the underlying physics, simply the method to make it do what they want.
I know what you mean - and, in principle, I can agree.
However, the problem is as follows:
In some books (and also in this discussion) it is simply stated that the BJT would be current-driven (Ic is determined by Ib). This is just a statement without any proof - just because of the formula Ic=B*Ib.
But formulas can never say anything about cause and effect - they contain just relations, nothing else.
Now comes my point:
Over the last few years, I have been in contact with many students who wanted not only to learn but also to understand.
And these students have seen a contradiction between the statement "current control" (in some books) and practical design rules as well as characteristics of the transistor circuits, which all point to voltage control.
Two simple examples (out of many others):
1.) Class A operation of an amplifier stage with Vbe=0.6...0.7 volts - and class AB operation with Vbe=0.1...0.2 volts.
2.) Current mirror: Who can explain its function with current-control ?

And I think this contradiction must be discussed and solved - but this seems to be a problem.
(I`ve got the impression that this question nearly is a religious one. In one comment, somebody was even of the opinion that I have an "obsession".).
 
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Bordodynov

Joined May 20, 2015
3,177
I argue that the transistor can be driven by both current and voltage. Why only the common emitter circuit is considered? What about the common base circuit? Take a cascode amplifier with transistors in series (CE-CB). The lower transistor gives a control current to the second transistor. And the base-emitter voltage of the second transistor is a consequence of the output current of the first transistor. Here you have a current generator, which does not exist in nature. The input current of the common-base transistor is not minuscule at all and it is not an unfortunate misunderstanding, as you think the base current is.
 

LvW

Joined Jun 13, 2013
1,752
When you are open to continue the "discussion" it would certainly help if you would react upon arguments and questions before introducing new aspects (see my post #28).
 

LvW

Joined Jun 13, 2013
1,752
This is absurd. From a real-world physics viewpoint voltage is not real. It is in fact a man-made concept to describe the ratiometric relationship between the only two physics aspects in question: the movement of negatively charged electrons, and the opposition to that movement.
To make my main point clear once again:
I do not want to and cannot discuss here the philosophical aspects of a "physical" truth. I only want to discuss and eliminate the contradiction between theory - as contained in some books, some "obscure" web-contributions (and, unfortunately, also in this forum) - and measurable practice.
That is all.
I really would be surprised if engineers with a good practical background did not recognize such contradictions.

Even worse: In this and in some other discussions about the subject I very often have mentioned that there are many circuit examples which - I think - clearly show that the BJT behaves like a transconductance device. To my opinion, a good engineer should be curious and interested to discuss, to agree or to disagree to some new arguments.
However - nobody was interested in those examples. It seems as if one's own "world view" must not be destroyed.
 
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