Misconceptions and flaws noticed by the more educated...

[LvW]

This topic inspires a lot of religiosity and I'm not going to argue
with any of you about it (again), only to say that I was taught
another way and that offers a much more useful intuition of the
goings-on.

"....useful intuition.." ----- really?
Can this intuition help to explain
- how RE-feedback works?
- how current mirrors work?
- why we are using low-resistive base dividers?
- why the cross-over distortions in class-B stages look like the first part of the Ic=f(Vbe) curve?
- how the known Vbe-tempco of app. -2mV/K was calculated?
- how the E-field modification is responsible for the Early-effect?
-........

 

[KlausST]

Hi,

From reading all the posts I understand that there are different people with different targets.

The ones just want a simple explanation and a formula and they are satisfied. (Mainly practice oriented)
Others are interested in the details, in the correct physical explanation, in knowledge and science (mainly R & D).

So what's the correct way?
I assume either way may have it's benefit. There is no "one and only correct way" for all people in the world.
Thus we (me too) should learn to be more tolerant against each other.

Klaus

 

[LvW]

KlausST - in principle, I agree with you (of course, now a "however" follows).
However, in this specific case, I like to remind you on the original question (from Externet):
He was asking for some "misconceptiosn" during learning/teaching electronics and he was asking what
"should be changed in text books to facilitate understanding"

To me, there is, indeed, just "one and only correct way" to "facilitate understanding" and to desrcibe and explain the working principles of parts and circuits.
Of course, we always need simpolifications and have good reason to neglect secondary effects.
However, I think the situation is completely different
- in case of a simple claim (current control) without a single proofe.
- when there is another functional description (voltage control) supported by several explanations, observations and measurements.

This has absolutely nothing to do with religion (although some people treat it like this) - the only requirement is to be open for new and better insights.
(Of course, this applies also to me. I also have learned - and I did blindly believe without evidence - that the BJT would be current-controlled).

 

[vGoodtimes]

To the original topic, for me the issue was coursework trying to present concept after concept after concept while saving motivation for last. This makes a lot of sense for reference manuals, but the internet now provides this.

 

[KlausST]

Hi,

@LvW.
I agree with you.

@all
I just want to remind not to take the posts as personal offense.


Klaus

 

[LvW]

@all
I just want to remind not to take the posts as personal offense.
Klaus

Yes - I strongly support that comment.
I think it should be possible - in particular between technically oriented/educated people - to present, explain and discuss different views in a fair way.

 

[Slartibartfast]

When I hear the word "misconception in textbooks" there is one thing which comes immediately in my mind:

1.) I guess that in app. 50% of all textbooks the working principle of bipolar transistor is explained - no sorry: not "explained" - it is just claimed that the BJT would be a current-controlled device.
And the only "proofe" of this wrong statenment is the relation Ic=beta*Ib.

2.) At the same time, all the examples which are presented with the aim to show how a BJT-based amplifier stage is designed is based on a voltage-controlled model.
And - most surprisingly - the authors seem not to realize this contradiction.

(I am aware that this subject was discussed also in this forum already - in some cases with many emotions. It seems that some people who have learned that the BJT would be current-controlled are unable to accept a better - more correct - description. And this - in spite of the fact that there are many proofes for voltage-control)

You said that BJT can be controlled by voltage, but I can still argue that it's the voltage that dictate the current that goes to the base.

BTW, I'm not of those that are unable to accept a better explanation. I'm always open to better explanations

 

[schmitt trigger]

A joke, somewhat related to the subject, and hopefully will lighten up the mood.

A scientist and an engineer are together in the corner of a room. On the opposite corner, there is an exceedingly beautiful woman.
A wizard comes into the room and declares:

"I know that you both desire the woman, but only one can have it."
"The reward will come to the person who arrives at her first, with the following conditions: Your first move can only be as large as half the room's diagonal size. Your second move can only be half of the first move. The third move only half of the second, and all of the succeeding moves can only be half of the previous move."

The scientist starts crying. He knows it will take him an infinite amount of time to reach the beautiful woman.
On the other hand, the engineer is all smiles: "I can work with approximate values!"

The point I am attempting to drive across, is that engineers work all the time with simplifications, shortcuts and tolerances. However......It takes an engineer who both knows the theory well and has hands-on experience, to know exactly how much to simplify an analysis or what tolerance to use for a specific problem.

 

[ads-ee]

Schmitt trigger had to fix the punchline you had "the scientist is all smiles".

Personally I would tell the wizard to f**k off and walk straight across the room and chat her up (since I know the scientist will never make over there anyways and wizard's plan was to keep both of us occupied while they put the moves on her :wink

 

[betwixt]

I would take a far simpler approach - ask her to come to me.
That makes me a lazy %^&$", not a scientist, engineer or wizard!

Brian.

 

[Ratch]

You said that BJT can be controlled by voltage, but I can still argue that it's the voltage that dictate the current that goes to the base.

BTW, I'm not of those that are unable to accept a better explanation. I'm always open to better explanations

A BJT is a transconductance device (Vbe controls Ic). The base current is a result of the collector current, not the cause of the collector current. The physics of the BJT shows that. Can you prove it not being so?

Ratch

 

[c_mitra]

The scientist starts crying. He knows it will take him an infinite amount of time to reach the beautiful woman...

Poor scientist! He should immediately change into constant velocity mode from constant step time.

But does the scientist really believe than man (or woman for that matter) cannot stand on two legs?

(Everything should be made as simple as possible, but not simpler!)

 

[LvW]

You said that BJT can be controlled by voltage, but I can still argue that it's the voltage that dictate the current that goes to the base.

BTW, I'm not of those that are unable to accept a better explanation. I'm always open to better explanations

Yes - correct. This is always true - it is always the driving "voltage that dictates the current". That`s a basic rule.
However, what does this sentence say about the question regarding collector current? NOTHING.
Where is the logic in your statement?

 

[asdf44]

I'll use this as an example to re-iterate my point that a single model doesn't need to explain everything for everyone.

A BJT can be usefully modeled as voltage controlled -> Good for some things
A BJT can be usefully modeled as current controlled -> Good for other things.

The wise and educated understand both (and more) and understand when each model is useful.

 

[Ratch]

I'll use this as an example to re-iterate my point that a single model doesn't need to explain everything for everyone.

A BJT can be usefully modeled as voltage controlled -> Good for some things
A BJT can be usefully modeled as current controlled -> Good for other things.

The wise and educated understand both (and more) and understand when each model is useful.

Unless the models closely follow the physics of the device, they will only only give valid results in limited circumstances and need to be used cautiously.

Ratch

 

[c_mitra]

single model doesn't need to explain everything for everyone...

True.

But every model has an objective; it may be clearly stated in the beginning (but often not) and is supposed to highlight one aspect.

Some models may focus on visual aspects while some others may focus on functions.

For example simulations are models and each component is characterized by a limited set of parameters. The model is extremely useful because it lets you see how the set is going to function.

But there are other parameters that are often ignored by a (simulation) model. For example how a given device fails. Can you model how an actor today is going to look after 25 years?

More often than not, we use models in a very restricted sense: for example, we often model temp dependence as rate processes with a characteristic activation free energy....

Of course this model will NOT explain everything for every one. And if you are familiar with the underlying processes, this is going to be a very good tool.

In my personal opinion, I consider model as personal tools. One tool does perform one task well, well almost always but sometime it does fail. If you understand the mechanism, you will not be surprised.

get another tool that works for your application. There is nothing like a model of a model.