P
powersupplies
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"Kevin Aylward" <kevindotaylwardEXTRACT@anasoft.co.uk> wrote in message
news:nxm7c.1283$cw6.796@newsfep3-gui.server.ntli.net...
approach the transistor. I think what you and Winfield are
saying is that a transistor is a voltage controlled current
source because you are using the hybrid pi equivalent circuit,
whereas I am using the h-parameters equivalent circuit which
defines the transistor as a current controlled current source.
It seems that some books use hybrid pi, others use h parameters,
which is confusing. Apart from that, hfe is h21 of the h parameters
and implies a current controlled current source. Whether it is a good
way to describe a transistor, you may say - no, but hfe is a current
controlled current source.
news:nxm7c.1283$cw6.796@newsfep3-gui.server.ntli.net...
I now realise, thanks to Winfield's post that there are two ways toWinfield Hill wrote:
powersupplies wrote...
If you want a practical way to design a transistor circuit
you need to use the h-parameters equivalent circuit.
No, the hybrid model was abandoned in the early 60s, just
about the time I started serious designing. I spent about
three months religiously using h-parameters, searching out
transistors with h-parameter curves, etc., and failing to
get accurate results. Then Carver Mead, who was a friend
of my boss, came by for a visit and I got a defining-moment
15-minute lecture on Ebers-Moll and its variants. Looking
in various textbooks, I realized that a few described this
method, which a small part of the community had been using
for years. Anyway, many difficult design issues suddenly
became easy, and my circuit results started matching my
calculations. Over the last 40 years, the awful h-parameter
approach has thankfully been relegated to the dustbin by the
engineering community, so that rarely do we see anyone now
espousing its use, let alone arguing it's the only true way.
Also, do you know what is hfe? hfe is *defined* as a current
controlled current source.
No its not. Look, dude. You aint goanna win on this. Your
statement here is completely nonsensical. hfe is simple
the ratio of collector current to base current. There is
nothing implied about the base current controlling the
collector current.
YES, it does imply that - you are wrong.
No, the collector current "controls" the base current is much
more accurate statement, albeit with limited usefulness.
I like to use the phrase "the collector current is a *function* of the
base current". This doesn't imply any direct control, just a
correlation. We all know, that the fact that there is a relation between
lung cancer and smoking does not, by itself, mean that such a relation
is causal, i.e controlling. It takes more evidence to show a causal
relation. In the case of the transistor, Vbe causes both the base
current and collector current, hence the base current and collector
current will be correlated, but not due to each other.
Does having a pet reduce your stress, or is the nature of people that
have pets less likely to have stress?
B.t.w. did you manage to find the gain for the original circuit,
I am still struggling with it.
Kevin accurately told you what it'd be, using Va (the Early
voltage for the LM395 "transistor") and the transconductance
of the LM317 current source, but neither of these parameters
is available from the manufacturer, nor their manufacturing
spreads. But they are very easy to measure for a given part.
The key thing about a transistor, is that it is, essentially, a diode!
One puts a voltage across a diode, and this accelerating potential,
tries to accelerate carriers, ie create a current in it. This is pretty
basic physics. Its voltage that makes things do things. The base of the
transistor is simply a way of impressing a voltage across this basic
diode, without such (controlling) voltage actually taking much current,
so that the bulk of the current is supplied by the collector circuit.
All that matters is that the voltage gets there. The diode junction
current then must follow the diode equation, irespective of any current
that leaks out of the base.
This very common misnomer about about transisters being erroneosly
described as base current controlled really needs addressing in a full
page in a good book, so I'll leave that one to you Win for your next
edition.
approach the transistor. I think what you and Winfield are
saying is that a transistor is a voltage controlled current
source because you are using the hybrid pi equivalent circuit,
whereas I am using the h-parameters equivalent circuit which
defines the transistor as a current controlled current source.
It seems that some books use hybrid pi, others use h parameters,
which is confusing. Apart from that, hfe is h21 of the h parameters
and implies a current controlled current source. Whether it is a good
way to describe a transistor, you may say - no, but hfe is a current
controlled current source.