🚀 go-pugleaf

Duncan Barclay wrote:
> 
> In article <fuNEGVAumei2EwR5@jmwa.demon.co.uk>,
>         John Woodgate <jmw@jmwa.demon.co.uk> writes:
> > <76b8i0$rqm$1@computer.my.domain>, Duncan Barclay
> > <dmlb@ragnet.demon.co.uk> wrote:
> >>Sorry, you are wrong twice. Both valves and transistors (BJTs and FETs)
> >>are voltage controlled current sources.
> >
> > This is no area for dogmatic statements. Not even, perhaps, for that
> > one!
> 
> But it is true irrespective of network "model".
> 
> The fundemental operation of valves, BJTs and FETs is to modulate a
> current carrying channel by one (or more) electrical fields.  Seems to
> be a VCCS to me.
> 
> > A true voltage-controlled device would have infinite input impedance -
> > approximately true for valves and FETs at low frequencies.
> >
> > A true current-controlled device would have zero imput impedance - not
> > true for BJTs.
> >
> > A true voltage source would have zero output impedance - only very
> > approximately true for earthed-cathode triodes: closer for
> > cathode/emitter/source-followers (earthed anode/collector/drain).
> >
> > A true current-source would have infinite output impedance -
> > approximately true for earthed-cathode/emitter/source pentodes, BJTs and
> > FETs.
> >
> > Controlled source models of active devices are useful in circuit
> > analysis because, although they are approximations, the models can be
> > made much less approximate by the addition of a few passive components.
> >
> > You can use *any* controlled-source model if you attach the appropriate
> > passives for correcting the approximations, but some choices of type
> > controlled source are clearly more appropriate than others in a given
> > case.
> 
> I said nothing about equivalent circuit models and don't have any
> problem with the above but it's not relevant.
> 
> The collector current of a transistor is govered by the base emitter
> voltage, look it any decent text book or reference any SPICE manual,
> Gummel-Poon or Ebers-Moll will help altavista.
> 
> Ic = Is * (exp (Vbe / (n * Vt)) -1)
> 
> Where Vbe is the base emitter voltage, Is is the saturation current of
> the device, n is a constant, Vt is kT/q about 26mV at room
> temperature.
> 
> The base current is due to the reverse leakage current from the
> collector-base junction, it is a secondary effect.
> 
> For FETs the equation is often quoted as
> Id = beta (Vgs-Vt)^2,
> this is just an approximation and not true apart from JFETs with a
> channel length of more than about 1um in Si or GaAs.
> 
> I take it you are actually talking about common-emitter/source
> _circuits_ and not transistors.
> 
> I do know what I am talking about, Chapter 2 of my D.  Phil thesis
> describes in detail GaAs MESFET modelling and I have spent the last
> three years designing BJT RFICs.
> 
> Transistors and valves are VCCS end of story.
> 
> Duncan
-----------------------------------------------------
That would be true if that were all there was to it, but what you have
actually stumbled on is a naming convention. We call things according to
what is most linearly responsible for output, and that is base current
in a BJT just as you have noted. There will always be voltages
associated with currents, and one COULD write the math to call a BJT a
voltage controlled voltage source, if one wished, but it would not
reflect precisely WHY we call them according to the simplist
mathematical relation. You seem as though you, have managed to learn
solid-state theory without learning the electronics practice and
nomenclature. I've came by that route, are you a physicist perhaps?? ;->
-Steve
-- 
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