🚀 go-pugleaf

<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!

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.
-- 
Regards, John Woodgate, Phone +44 (0)1268 747839 Fax +44 (0)1268 777124. 
OOO - Own Opinions Only. You can fool all of the people some of the time, but 
you can't please some of the people any of the time.