Op-amps and Transistors

Hi Kevin,

This misconception is quit a crucial one, I would love to see one such tutorial on your site covering this issue.

Most of the leading texts including; Electronics Principles by Albert Paul Malvino, Electronic Devices & Circuit Theory by Robert Boylestad claims transistor as current controlled voltage source. I don't really understand why they pass on this vague concept, is this for the sake of the readers or even they are also misleaded(shouldn't say like this)?

Yes, I can remember you once already clarified my misconception, but there are many more guys to be??

Best regards,

AM

The 'beta' model is a current-controlled current source. And it's all you need for the majority of transistor applications. Hardly anybody uses discrete transistor voltage amps any more, so transconductance need not be considered in many cases, and then likely only in the first stage.

John

Old "normal" transistors uses change in small current to control a large flow of current.

There are many types of transistors today. Some uses change in voltage to control a large flow of current.

Oh? and what transister would these have been? I know of no transisters that are not voltage controlled.

They *all* use voltage to control current. Whether or not the current is large is a matter of definitions. Incidentally, a flow of current is a flow of flow of a substance, which is rather poor grammar. Current means a flow of something already.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

All normal transistors like 2N2222, BC547, BC650, BC141 and many, many more. The big current flowing trough the transistor is controlled by the current (not voltage). I know that because I constructing many electronic circuits since the 60:s and do so even today. Ther are curves in the data books that tells the relationships, and that's I'm using.

For the 'technologist' who wants a slightly better view of what transistors do (or moreso, how they act), I suggest looking at Kevin's website. He will probably reply more detailed URLs, but he has written a few 'tutorial' documents that seem to be quite helpful.

IMO, it is VERY VERY important to avoid the Beta=Ic/Ib model as being definitive. I do apparently disagree with Kevin that I believe that the 'Beta' model can be useful for a few (VERY VERY few) purposes, while Kevin seems to believe that it is useful for even fewer purposes :-).

I sure wish that people would take the physical model (the diode equations along with Ebbers-Moll or Gummel-Poon) more seriously... They'll have LOTS less trouble for non-trivial practical circuits.

In the cases where the Beta model is sufficient, the transistor isn't really doing anything very useful :-).

The real problem with accepting the 'Beta' model as SOMETIMES being useful is that too many people will believe that it is more useful than it really is. In a nuanced fashion, the Beta model can be helpful, but it is only better than absolutely nothing.

John

Yep, Bipolars are physically voltage controlled.

Ic = (approx) Is (exp(Vbe/Vt) - 1);

(I might be slightly wrong, but the form of the controlling equation is that of an exponentional.)

Essentially, the associated Ib is a kind of leakage current, and only by happenstance is approximately linearly related to Ic. However, that equation (Ic=B*Ib), isn't really a very useful relation except in VERY limited conditions. The 'Beta' model really dies a horrible death in dynamic conditions, but is fairly bad for anything except for estimating the required Ib given a collector current... The proper (voltage controlled) model actually works really well for HF applications, while the Beta model just doesn't work for its limited purposes except for simple DC.

One problem with Beta is that it an important parameter, but confuses the non-engineer 'technologist' about it's actual limited physical applicability. Beta is helpful for some things, but so are Cbc, Is, etc.

If you want to judge the importance of 'Beta', think about it as equvalent to the 'Q' of an inductor. It measures a necessary evil, but isn't really a defining attribute.

John

Well, I wouldn't go that far;-) I am really just trying to correct this incredible widespread misconception that surrounds basic transistor operation. Its potential differances that make charge move.

Base current is obviously something that needs to be accounted for. Right off the bat, if your doing a power amp, you have to work out the drive current from Iload/Hfemin.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

{snip}

Ho hum...Dont try and each your granpa to suck eggs.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

Ho hummm again. What part of "controlled" do you have trouble with? Look, dude, the first order equation for transistor operation, derived in any elementary text book on semiconductor physics, is:

Ie = Io(T).(exp(Vbe.q/KT) - 1)

Ic ~= Ie

Now, prey tell me, where is the base current in this equation?

The fact that some base current flows dose not mean that the base current is a *controlling* factor. You need to appreciate the difference between causal relations and correlated relations.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

Not according to your page either (Ib).

My guess is that beginners tend to see the approximately constant Beta as implying that 'Ic = Beta * Ib' is the controlling relationship.

I like to show the uselessness of Beta by trying to simulate a transient circuit (even with the intrinsic capacitances included.) The Beta model is just wrong. (Well, not quite WRONG, but so insufficient as to being wrong :-)).

John

Ken wrote: ?

The transistors do what they do regardless of how we model them. Current control is algebraically easy and a good enough approximation for many purposes. Voltage control is very accurate but mathematically more difficult.

In the bigger and grand schemes of things, there may well be many models for the same phenomena, but this is in a different context then is being used here.

You still miss the point. The transistor is a voltage controlled derive, irrespective of the engineering model. This is getting exasperating. Its applying a *voltage* to the base and emitter that injects carriers into the base, that are then swept up by the collector. The base current is simply irrelevant. That is, the flow of base current does not *cause* transistor action. Therefore, the transistor is *not* "controlled" by base current. Its that simple!!!

Kevin Aylward snipped-for-privacy@anasoft.co.uk

NO, the base emitter voltage turns on the diode they form, the amount of current that flows through the junction determines the collector current. Because we are dealing with a diode junction we can use the relationship between voltage and current in a diode to model with transconductance equations. We also use transconductance model with voltage controlled current devices like mosfets. BUT, mosfets have no beta/hfe because they are voltage controlled devices...

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No, its yes. The transistor is voltage controlled. Period. End of story.

Yes. That is what I stated.

Yes. That is what I stated.

Yes. That is what I stated.

Yes. So whats your problem?

Kevin Aylward snipped-for-privacy@anasoft.co.uk

It must be your destiny to feel that way.

When you come right down to it, all currents are controlled, and generated, by voltage. I think a certain Herr Ohm said something about this.

cap

Exactly. Fundermenatlly, there is a charge that produces a voltage differance. This voltaege is usually referred to as an accelerating voltage as it is this that accelerates other charges. The flow of charge is an effect. However..., there is a somewhat subtle complication in that all effects are due to an exchange of motion at a deeper level:-)

Kevin Aylward snipped-for-privacy@anasoft.co.uk