Re: Op-amps and Transistors

From: John S. Dyson (toor_at_iquest.net)
Date: 09/15/04 

Date: Wed, 15 Sep 2004 02:02:14 +0000 (UTC)

In article <SFJ1d.13213$_66.12055@fe1.news.blueyonder.co.uk>,
        "Kevin Aylward" <salesEXTRACT@anasoft.co.uk> writes:
> Ken wrote:
>> On Tue, 14 Sep 2004 09:51:27 GMT, "Kevin Aylward"
>> <salesEXTRACT@anasoft.co.uk> wrote:
>>
>>
>>> *All* transisters are voltage controled. End of story.
>>> http://www.anasoft.co.uk/EE/index.html
>>
>>
>> Not according to your page either (Ib).
>
> 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.
>
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