difference between bipolar and mosfet

[Kevin Aylward]

On Tue, 11 Jan 2005 19:57:25 GMT, "Kevin Aylward"

The rest of what you say here, is even more amazing.

---
Granted, but what you said you wanted to talk about wasn't the concept
of transconductance, it was that the "actual physics" of a BJT made it
a transconductance device. That being the case, what's nonsensical is
that almost anything can be called a transconductance device, so just
calling a BJT a transconductance device doesn't really say anything
about the physics behind it. Consider a simple potentiometer on the
bridge of a ship which is used to control the ship's speed. Is what's
being used to turn the screw a transconductance device? Sure. Does
it tell us anything about what's between the pot and the screw? No.
Same thing with a BJT. But dig a little deeper and we find that all
the voltage across the base-emitter diode is doing is pumping charge
through the diode,

No. Vbe is setting an *electric field* that attempts to accelerate
charges from the emitter into the base region. This is truly 101
physics.

that _current_ being what causes the collector
current to flow.

NO! NO! NO!. Absolutly not. You are wrong. It is not the flow of charge
in the base that *causes* collecter current. This has already been
explained in many posts.

Charge flows because of:

F=q(E+vxB)

That is, excluding magnetic effects, it is *Electric Fields* that make
chages move. Period. The flow of charge, excluding magnetic effects,
cannot make other charge flow, other then by the change in electric
field that such flow might cause.

For the umpteenth time. Applying an *electric field* to the base emitter
injects carriers from the emitter into the base region. Once the
carriers are in the base region, they are attracted by the *electric
field* of the collector and are swept up (collected) by the collector
due to this *electric field*. Some of the emitter carriers just don't
make it, and are picked up via the base terminal. This base current is
an *effect* not a cause, and is incidental to the base emitter *electric
field* injecting carriers.

To repeat, it is not the *motion* of base charge that *causes* the
*motion* of collector/emitter charge. It is the electric field at Vbe
that causes both base current and collector/emitter current, as it is,
now get this 101 physics, *ELECTRIC FIELDS THAT MAKE CHARGES MOVE*. End
of story. Period.

I have already given a link that diagrams this behaviour.

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/comp/active/BiPolar/page2.html

---


---
If that's true, then a BJT isn't a transconductance device because the
collector current isn't a _direct_ consequence of the base-emitter
voltage, it's once removed since the _direct_ consequence of the
base-emitter voltage is the base-emitter current.

No. This shows that base current is an effect, of an electric field
cause.

I am still amazed that after all these posts, you still don't understand
the basic operation of the bipolar transistor.

F=q(E+vxB)

Its that simple.

See above.

---
No, it isn't.

What I'm claiming is that when there is no charge being injected into
the base region, the NPN sandwich is as clearly delineated as
bread-ham-bread would be. BUT, (and it's a BIG but) when charge
starts being injected into the base region the ham starts to look more
and more like bread as more and more charged is pumped into it, with
the eventual result being that the ham looks and acts enough like
bread to take on the characteristics of bread. So, if the P type
material in the base gets enough electrons pumped into it to make it
look like N type material, then the battery connected from the
collector to the emitter will start seeing less and less resistance as
the base current gets larger and larger and will cause the collector
current to increase as the base-to-emitter voltage (and the base
current)increases.

And this is all wrong. I have already explained, a continuous resistance
would mean that the base is connected to that resistance, hence, no
transister action.

Quite frankly, as noted above, you simply don't understand how a bipolar
transistor works. Read and understand the above. Its the way it is. I
can tell you that if you gave this description in an academic
semiconductor physics class, you would get a zero grade.

For some reason, you have picked up an erroneous understanding of
transistor operation. You need to get to grips with the idea that you
have been mistaken on this issue. This isn't debatable. Its in all the
academic text books.

Look, actually produce a detailed *physics* explanation as to *how* a
flow of base charge can actually induce a flow of collector charge. This
should tell you something.

In all the semiconductor text books I have read, I have never seen this
done, so why you can claim that this is the case, is pretty far fetched
indeed.

And this is wrong.

Which isn't the case here. The error due to rbb' is usually small. It
doesn't change the bipolar from its fundamental physics description as a
voltage controlled device. Rbb' just makes the calculation of the
voltage a little more involved.

---


And yet in your last post it was:

..."this base current is simply a nuisance."

There is no conflict here.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Miles Harris]

Ok, now I see. Your reading comprehension is so poor that you try to
excuse yourself by casting the blame for your ignorance on someone
else.

Nope. You quoted this half-baked definition so it's quite proper for
others to infer you're happy with it.

[snip juvenile insults]

It's always "snake oil" when you don't understand and think you're
being taken advantage of by someone who does, ain't it?

That's rich, coming from someone who doesn't even understand
transistor action. Your concept of it would have Shockley turning in
his grave.

BTW, did you catch the error I intentionally inserted into that little
series VS parallel LED power dissipation problem you were having so
much trouble with?

Translation: "Holy shit! I've made the kind of dumbass error in that
LED thread typical of a bonehead like me. Let's hope I can get away
with this half-baked excuse before Miles spots it and I get exposed as
an even *bigger* bonehead!"

 

[Miles Harris]

Actually, since you seem to think you've got all the answers and that
the OP has been so wronged, why don't you make things better by taking
about a minute out of your life and writing down everything you know
about beta for his benefit? I'm sure it would be thoroughly
appreciated if you could write up to his level.

It would, as you no doubt know, take me a great deal longer than one
minute to write out everything I know about Beta. But were I so
inclined, at least the OP would have an extensive, complete and
accurate grasp of the subject, rather than the false sense of
competence you attempted to instill in him by means of your own.
HTH.

 

[Miles Harris]

For the umpteenth time. Applying an *electric field* to the base emitter
injects carriers from the emitter into the base region. Once the
carriers are in the base region, they are attracted by the *electric
field* of the collector and are swept up (collected) by the collector
due to this *electric field*. Some of the emitter carriers just don't
make it, and are picked up via the base terminal. This base current is
an *effect* not a cause, and is incidental to the base emitter *electric
field* injecting carriers.

To repeat, it is not the *motion* of base charge that *causes* the
*motion* of collector/emitter charge. It is the electric field at Vbe
that causes both base current and collector/emitter current, as it is,
now get this 101 physics, *ELECTRIC FIELDS THAT MAKE CHARGES MOVE*. End
of story. Period.

Nicely put.
I doubt it'll be the end of the story, though. I suspect John Fields
may be a troll. No rational person could conceivably be so obtuse.

 

[John Fields]

On Wed, 12 Jan 2005 08:06:17 GMT, "Kevin Aylward"

The rest of what you say here, is even more amazing.

---
Well, Kevin, it's only analogy...
---

No. Vbe is setting an *electric field* that attempts to accelerate
charges from the emitter into the base region. This is truly 101
physics.

---
Yes, it is. I don't think anyone has said, yet, that it _isn't_ the
electric field which is making the electrons move, i.e. "pumping
charge", so It seems strange to me that you would think it has.
---

NO! NO! NO!. Absolutly not. You are wrong. It is not the flow of charge
in the base that *causes* collecter current. This has already been
explained in many posts.

Charge flows because of:

F=q(E+vxB)

That is, excluding magnetic effects, it is *Electric Fields* that make
chages move. Period. The flow of charge, excluding magnetic effects,
cannot make other charge flow, other then by the change in electric
field that such flow might cause.

---
I said that the flow of charge between the emitter and base causes
charge to flow between the emitter and collector, and you say that's
wrong because the flow of charge between the emitter and base causes
charge to flow between the emitter and collector [because the flow of
charge between the emitter and base causes the electric field between
the emitter and collector to be changed to the point where charge can
flow between the emitter and collector.] Now where was I wrong,
exactly?
---

For the umpteenth time. Applying an *electric field* to the base emitter
injects carriers from the emitter into the base region. Once the
carriers are in the base region, they are attracted by the *electric
field* of the collector and are swept up (collected) by the collector
due to this *electric field*. Some of the emitter carriers just don't
make it, and are picked up via the base terminal. This base current is
an *effect* not a cause, and is incidental to the base emitter *electric
field* injecting carriers.

---
It doesn't matter how many times you say it, if it's not right it's
not right.

If it weren't for the base current there would be no collector
current, so it _is_ a cause, regardless of what's causing _it_ to
happen.
---

To repeat, it is not the *motion* of base charge that *causes* the
*motion* of collector/emitter charge. It is the electric field at Vbe
that causes both base current and collector/emitter current, as it is,
now get this 101 physics, *ELECTRIC FIELDS THAT MAKE CHARGES MOVE*. End
of story. Period.

---
I don't disagree with that.
---

No. This shows that base current is an effect, of an electric field
cause.

---
Again, I don't disagree with that.
---

I am still amazed that after all these posts, you still don't understand
the basic operation of the bipolar transistor.

---
And I'm more than equally amazed that you don't seem to be able deal
with analogy.
---

....

And this is all wrong. I have already explained, a continuous resistance
would mean that the base is connected to that resistance, hence, no
transister action.

---
I don't know why you keep claiming that I said that the thing is a
slab of of either P or N type material, when what I said was that it
starts to _look_ like that between the collector and emitter when base
current starts flowing. Much like the region between the cathode and
the plate of a vacuum tube starts to look like a resistor when the
tube is conducting. Is there a resistor between the plate and the
cathode? No. Does it _look_ like there is? Yes.
---

Quite frankly, as noted above, you simply don't understand how a bipolar
transistor works. Read and understand the above. Its the way it is. I
can tell you that if you gave this description in an academic
semiconductor physics class, you would get a zero grade.

---
Perhaps, but since this is sci.electronics.basic, a rigorous treatment
of the matter is seldom apropos. In this instance, a simple reply to
the OP (which, BTW, you said you had no problem with) and some
analogies which you either misunderstood or decided to take issue with
for your own reasons seems to have led us up to where we now sit.

I, for one, no longer have the desire to continue with this
"discussion", so I'll excuse myself and bid you good day.

Thanks,

John

 

[John Fields]

Nope. You quoted this half-baked definition so it's quite proper for
others to infer you're happy with it.

---
I _am_ happy with it, regardless of whether you choose to take issue
with it or not. After all, your objections to it are largely
unfounded in that from the context of the definition everything being
described was quite clear.
---

[snip juvenile insults]

It's always "snake oil" when you don't understand and think you're
being taken advantage of by someone who does, ain't it?

That's rich, coming from someone who doesn't even understand
transistor action. Your concept of it would have Shockley turning in
his grave.

 

[John Fields]

It would, as you no doubt know, take me a great deal longer than one
minute to write out everything I know about Beta. But were I so
inclined, at least the OP would have an extensive, complete and
accurate grasp of the subject, rather than the false sense of
competence you attempted to instill in him by means of your own.
HTH

 

[John Fields]

Nicely put.
I doubt it'll be the end of the story, though. I suspect John Fields
may be a troll. No rational person could conceivably be so obtuse.

 

[Kevin Aylward]

On Wed, 12 Jan 2005 08:06:17 GMT, "Kevin Aylward"

Indeed you did.

and you say that's
wrong because the flow of charge between the emitter and base causes
charge to flow between the emitter and collector

No I didn't. I specifically denied that. I said the *ELECTRIC FIELD*
between the base and emitter causes charge to flow from emitter to the
base region.

*THE FLOW OF CHARGE BETWEEN EMITTER AND BASE DOES NOT CAUSE THE FLOW OF
CHARGE BETWEEN EMITTER AND COLLECTOR*

I can't say it any plainer.

[because the flow of
charge between the emitter and base causes the electric field between
the emitter and collector to be changed to the point where charge can
flow between the emitter and collector.]

Now where was I wrong, exactly?

All of the above. I repeat what I said here.

"Applying an *electric field* to the base emitter
injects carriers from the emitter into the base region. Once the
carriers are in the base region, they are attracted by the *electric
field* of the collector and are swept up (collected) by the collector
due to this *electric field*. Some of the emitter carriers just don't
make it, and are picked up via the base terminal. This base current is
an *effect* not a cause, and is incidental to the base emitter *electric
field* injecting carriers."

*Nowhere* does this claim that the flow of charge causes another flow of
charge.

If it weren't for the base current there would be no collector
current, so it _is_ a cause,

Not at all. A correlation does not have to be causal. This is basic 101
statistics.

For example, suppose those that have cats have less stress, should we
recommend that people get cats? Or is it that less stressed people
simply have a side effect of liking cats.

regardless of what's causing _it_ to
happen.

No. This is why the tobacco industry had a legal claim against smoking
causing cancer. It might be that those more likely to get cancer also
had a side effect of just wanting to smoke.

One needs to show that a correlation is *actually* causal. In this case,
it is Vbe that causes both collector current and base current. Base
current does not cause the collector current, but it is correlated.


Your argument above says that you do disagree.

Your argument above says that you do disagree.

---
I don't know why you keep claiming that I said that the thing is a
slab of of either P or N type material, when what I said was that it
starts to _look_ like that between the collector and emitter when base
current starts flowing.

Because this analogy must account for the fact that there is a direct
connection from the base to the emitter.

Much like the region between the cathode and
the plate of a vacuum tube starts to look like a resistor when the
tube is conducting. Is there a resistor between the plate and the
cathode? No. Does it _look_ like there is? Yes.

Its not like the tube. The tube has no connection from grid to cathode
or anode.

---


---
Perhaps, but since this is sci.electronics.basic, a rigorous treatment
of the matter is seldom apropos. In this instance, a simple reply to
the OP (which, BTW, you said you had no problem with) and some
analogies which you either misunderstood or decided to take issue with
for your own reasons seems to have led us up to where we now sit.

I, for one, no longer have the desire to continue with this
"discussion", so I'll excuse myself and bid you good day.

Fair enough.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Bob Myers]

Sigh. If I might interject a couple of thoughts, just to hopefully
redirect this little p***ing contest into something more productive.

Could everyone please note the sign on the door, here? This is
sci.electronics.BASICS. While a number of us here are, in fact,
Professional Electrical Engineers of Long and Revered Standing
(at least, we have a diploma and someone actually pays us to do this
stuff), the questions here are more than likely going to come from
people who do NOT have such a background, are never going to
get into these phenomena down to the quantum-mechanics level,
and whose questions will be more than adequately satisfied by the
simple, "classical" explanations.

Are those explanations, in many cases, "wrong" in the sense that
they give what is to some degree a false or misleading understanding
of the fundamental physics underlying the operation of these
devices? Of course they are. But so what? If you're trying to
answer a question posed by someone who does not now and likely
never will care about his or her ability to derive the ideal diode
equation from basic principles, these simple explanations are very
likely still going to be the right choice. I strongly suspect that just
about all of us who currently have some sort of "Engineer" title on
our business cards started with just the same level of understanding, and
it didn't stop any of us from getting where we are today. As long
as it is made clear that the explanation being given IS a simplified
look at things, and that later on - IF they choose to go further -
they will learn more about what is really going on, I for one do not
see the harm in starting out at this level, as opposed to effectively
dumping a graduate-level solid-state physics texts in their laps and
saying "there you go." That sort of approach, IMHO, does NOT
serve the purpose of this newsgroup.

All too often, we seem to have requests for such basic information
posted by someone who is quite clearly an amateur/hobbyist, followed
by a response by someone apparently interested primarily in
demonstrating their own command of the intricacies of the topic
in question, rather than actually saying something that the original
poster would find helpful. I would like to respectfully suggest that
such an approach is, to say the very least, idiotic.

We now return you to your irregularly-scheduled chest-beating...

Bob M.

 

[Anthony Fremont]

Sigh. If I might interject a couple of thoughts, just to hopefully
redirect this little p***ing contest into something more productive.

Could everyone please note the sign on the door, here? This is
sci.electronics.BASICS. While a number of us here are, in fact,
Professional Electrical Engineers of Long and Revered Standing
(at least, we have a diploma and someone actually pays us to do this
stuff), the questions here are more than likely going to come from
people who do NOT have such a background, are never going to
get into these phenomena down to the quantum-mechanics level,
and whose questions will be more than adequately satisfied by the
simple, "classical" explanations.

Are those explanations, in many cases, "wrong" in the sense that
they give what is to some degree a false or misleading understanding
of the fundamental physics underlying the operation of these
devices? Of course they are. But so what? If you're trying to
answer a question posed by someone who does not now and likely
never will care about his or her ability to derive the ideal diode
equation from basic principles, these simple explanations are very
likely still going to be the right choice. I strongly suspect that just
about all of us who currently have some sort of "Engineer" title on
our business cards started with just the same level of understanding, and
it didn't stop any of us from getting where we are today. As long
as it is made clear that the explanation being given IS a simplified
look at things, and that later on - IF they choose to go further -
they will learn more about what is really going on, I for one do not
see the harm in starting out at this level, as opposed to effectively
dumping a graduate-level solid-state physics texts in their laps and
saying "there you go." That sort of approach, IMHO, does NOT
serve the purpose of this newsgroup.

All too often, we seem to have requests for such basic information
posted by someone who is quite clearly an amateur/hobbyist, followed
by a response by someone apparently interested primarily in
demonstrating their own command of the intricacies of the topic
in question, rather than actually saying something that the original
poster would find helpful. I would like to respectfully suggest that
such an approach is, to say the very least, idiotic.

We now return you to your irregularly-scheduled chest-beating...

Hurray Bob! ) I started to write something along these lines, but
couldn't make it sound as nice as you just have. I think it's important
to also note that everyone's model is wrong in the strictest sense of
definition of correctness. They are all just models, and like all
models, they serve to be a "reasonable" facsimile of the real and
absolute truth; whatever that may be. They are not perfect or they
wouldn't be called models. I'm reasonably sure that there are people
that could come in and make a mockery of every model that has been
presented here by demonstrating their shortcomings and failures. I
suspect some of those people are actually present here reading this, but
not wishing to add fuel to the fire.

 

[Robert Monsen]

Kevin Aylward wrote:
<snip arguments for and against>

The fact that current through a bipolar transistor and a diode are
described by basically the same equation points to the fact that a
bipolar transistor is really just a diode, in which many or most of the
electrons that would normally escape through the base are diverted to
the collector by the geometry and chemistry of the device and voltage on
the collector. Since a diode is clearly a voltage controlled device, it
is clear that the voltage across the PN junction of the base to emitter
that causes electron flow.

However, saying that current through the base has nothing to do with
this is just wrong. Because of the fact that a diode is involved, the
only way to keep the voltage of the base up is to pull electrons out of
it through the base lead. The ratio of electrons out the base lead to
electrons out the collector is 'fairly' stable, enough to be printed in
datasheets as the famous hfe or beta parameter.

Because of this, the base current is yet another feature of the bipolar
transistor that can be used to roughly predict the collector current,
and thus to design circuits with. It is also easier to use, in my
opinion, owing to the fact that the relationship between base and
collector current is generally linear, within certain ranges.

On the other hand, in my experience, the voltage to current equation is
far more less use, since Is is not generally published in datasheets,
and is severely temperature dependent to boot (just like beta).
Predicting the current through the collector of a a 2N3904, given a base
voltage, is practically impossible. Predicting the current through a
2N3904 using beta is simple, if somewhat imprecise. By guessing a beta
of 100, one can easily see that 10uA through the base will give about
1mA through the collector.

Both beta and EM have their place in one's toolkit. Why toss out tools
that can be useful?

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.

 

[John Fields]

Nicely put.
I doubt it'll be the end of the story, though. I suspect John Fields
may be a troll. No rational person could conceivably be so obtuse.

---
Speaking of obtuseness, you seem to have (after all the ass-kissing)
missed the part that was missing. That is, the role that Vce plays in
generating the collector current.

Kevin states that: "It is the electric field at Vbe that causes both
base current and collector/emitter current"...

While it's true that Vbe causes Ib to flow and is, of course, a
contributor to the flow of Ic, the implication is that all of the
collector current flowing through the base region is due solely to
Vbe, and that with a given Vbe and no Vce, the emitter current will be
what it would have been had the collector been connected to a source
odf voltage. In other words, the collector current which would
normally have been making its way to the collector because of the
attraction of Vce drawing it away from the base will now make its way
to the base because of the lack of Vce. Such is not the case.


Consider this case with Ic flowing:

183mA--> 2N4401
[HP6285A]-----[FLUKE 8060A]----------C E-----GND
B
1.8mA--> |
[HP6216A]-----[WAVETEK 27XT]-----------+




and this, with no Ic flowing:

2N4401
C E-----GND
B
15.1mA--> |
[HP6216A]-----[WAVETEK 27XT]-----------+


Both power supplies were set up as voltage sources with no current
limiting and adjusted to give the readings obtained in the upper
example. The collector supply was then disconnected from the
collector and the reading shown in the lower example was obtained.
About a tenfold increase in base current because the collector current
was no longer causing a voltage drop across the base-emitter diode,
but certainly not the hundred-fold increase one would expect if Vbe
were the sole contributor to the cause of Ic.



BTW, since I find you mildly annoying I decided to take a look at your
posting history to see what you're about, and I found that other than
about the 20 posts archived at Google (where you also seem to have an
attitude) you seem only to have posted here, so welcome to the swamp.

While looking, I found this rather interesting post on this NG:

<QUOTE>

Didoes drop voltage not current. Silicon diodes (e.g. 1N4148 or 1N4001)
drop about 0.7V so one would probably be adequate; but two, in series, would
be safer.

Um, yeah, but they drop voltage _according_ to current! If they're
dropping 0,7V., they're not passing much current! Diodes are a crap
way to drop voltage unless the load is light and predictable!!!

miles
<END QUOTE>

which seems to belie your claim that you're an electrical engineer
with 35 years of experience.

Continuing on, we find, from Danny T:

<QUOTE>
http://www.kpsec.freeuk.com/components/diode.htm

says

"There is a small voltage across a conducting diode, it is called the
forward voltage drop and is about 0.7V for all normal diodes which are
made from silicon. The forward voltage drop of a diode is almost
constant whatever the current passing through the diode so they have a
very steep characteristic (current-voltage graph)."

--
Danny
<END QUOTE>


To which you replied:

The forward voltage drop is entirely dependent on temperature (the
junction temp. of the p/n junction; which is in turn dependent upon
the current passed.) Higher currents equals higher temp. equals lower
voltage drop. It's a well known effect which can eventually destroy
the diode altogether. The physics of diodes is actually more complex
than a lot of texts would have you believe.

---
Indeed, but if you think the forward voltage drop is _entirely_
dependent on temperature, you seem to have missed reading some of the
more fundamental ones.

For example, while it's certainly true that the voltage across the
junction can be described by:


kT / If \
Vf = ---- ln ( 1 + ---- )
q \ Ir /

and that when T is equal to zero at 0°K, Vf will be 0, you've
neglected to mention that current passing through the bulk resistance
of the diode, at any temperature, will cause a drop across the
junction which is dependent on the resistance and the charge flowing
through the diode.

More importantly, perhaps, you pooh-pooh'd Danny T's idea to use a
diode as a bad one merely because of your opinion, which was
erroneous. Diodes are _often_ used as voltage dropping elements in
the real world because of the small change in Vf caused by If.
Moreover, your example of the negative TC of a diode destroying it
would more closely describe a diode with a voltage source connected
across it allowing the diode to get into thermal runaway. Such a
condition would not happen with the load limiting the current through
the diode and the diode sized to carry the required current under the
required environmental conditions. Furthermore, depending on the
diode, above a certain current the tempco becomes positive, something
else you "neglected" to mention.

What's most disturbing, however, is that with Danny T admittedly being
a newbie and asking for information, you deliberately sidestepped the
issue when he presented you with the [valid] information he found
which supported Andrew Holmes' suggestion to use a diode in order to
keep from having to admit that you were wrong in stating that: "Diodes
are a crap way to drop voltage unless the load is light and
predictable!!!

For shame, sir! :-(

 

[John Fields]

Sigh. If I might interject a couple of thoughts, just to hopefully
redirect this little p***ing contest into something more productive.

Could everyone please note the sign on the door, here? This is
sci.electronics.BASICS. While a number of us here are, in fact,
Professional Electrical Engineers of Long and Revered Standing
(at least, we have a diploma and someone actually pays us to do this
stuff), the questions here are more than likely going to come from
people who do NOT have such a background, are never going to
get into these phenomena down to the quantum-mechanics level,
and whose questions will be more than adequately satisfied by the
simple, "classical" explanations.

Are those explanations, in many cases, "wrong" in the sense that
they give what is to some degree a false or misleading understanding
of the fundamental physics underlying the operation of these
devices? Of course they are. But so what? If you're trying to
answer a question posed by someone who does not now and likely
never will care about his or her ability to derive the ideal diode
equation from basic principles, these simple explanations are very
likely still going to be the right choice. I strongly suspect that just
about all of us who currently have some sort of "Engineer" title on
our business cards started with just the same level of understanding, and
it didn't stop any of us from getting where we are today. As long
as it is made clear that the explanation being given IS a simplified
look at things, and that later on - IF they choose to go further -
they will learn more about what is really going on, I for one do not
see the harm in starting out at this level, as opposed to effectively
dumping a graduate-level solid-state physics texts in their laps and
saying "there you go." That sort of approach, IMHO, does NOT
serve the purpose of this newsgroup.

All too often, we seem to have requests for such basic information
posted by someone who is quite clearly an amateur/hobbyist, followed
by a response by someone apparently interested primarily in
demonstrating their own command of the intricacies of the topic
in question, rather than actually saying something that the original
poster would find helpful. I would like to respectfully suggest that
such an approach is, to say the very least, idiotic.

We now return you to your irregularly-scheduled chest-beating...

 

[Kevin Aylward]

Kevin Aylward wrote:
<snip arguments for and against>

The fact that current through a bipolar transistor and a diode are
described by basically the same equation points to the fact that a
bipolar transistor is really just a diode, in which many or most of
the electrons that would normally escape through the base are
diverted to the collector by the geometry and chemistry of the device
and voltage on the collector. Since a diode is clearly a voltage
controlled device, it is clear that the voltage across the PN
junction of the base to emitter that causes electron flow.

However, saying that current through the base has nothing to do with
this is just wrong.

Not in this context it isnt.

Because of the fact that a diode is involved, the
only way to keep the voltage of the base up is to pull electrons out
of it through the base lead. The ratio of electrons out the base lead
to electrons out the collector is 'fairly' stable, enough to be
printed in datasheets as the famous hfe or beta parameter.

No its not stable. Hfe varies all over the place. In contrast, the gm
equation of a transistor is the same for all transistors.

Any decent design has to be done such that the huge variations in hfe
are overcome.

Because of this, the base current is yet another feature of the
bipolar transistor that can be used to roughly predict the collector
current, and thus to design circuits with.

No it most certainly cant be used to predict the collector current. For
switching circuits it is used to calculate the *minimum* base current
required, with the collector current being set by the collector loop not
the base loop.

For ac circuits, hfe is designed out.

It is also easier to use,
in my opinion, owing to the fact that the relationship between base
and collector current is generally linear, within certain ranges.

Ho hum...you don't do much transistor design do you?

This is a non starter.

On the other hand, in my experience, the voltage to current equation
is far more less use, since Is is not generally published in
datasheets, and is severely temperature dependent to boot (just like
beta).

Oh dear...in your experience...well, yet again, at the risk of sounding
arrogant, my experience is rather more extensive then yours in the
design of complex transistor level circuits. I have been professionally
designing very large transistor count i.c. and board level circuits for
well, some time now.

The gm equation is absolutely indispensable. It forms the heart of
serious transistor level design. For example, see
http://www.anasoft.co.uk/EE/bipolardesign1/bipolardesign1.html

e.g.

re = 1/40Ic

Av = Rc/re

max gain Av=Va/Vt

or the design of current mirrors, multipliers, bandgap voltage
references. The list is truly endless.

Predicting the current through the collector of a a 2N3904,
given a base voltage, is practically impossible.

Sure, if you just apply a raw voltage for DC conditions, but one doesn't
usually do this. For ac conditions, ic= vb.40.Ibias

Predicting the
current through a 2N3904 using beta is simple, if somewhat imprecise.
By guessing a beta of 100, one can easily see that 10uA through the
base will give about 1mA through the collector.

Oh dear, this approach is useless for anything but switching circuits,
and as noted above, it doesn't determine the collector current. This is
shown here
http://www.anasoft.co.uk/EE/bipolardesign3/bipolardesign3.html

Both beta and EM have their place in one's toolkit. Why toss out tools
that can be useful?

No one is claiming that beta has no use, one is pointing out that the
bipolar transistor is a voltage controlled device.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Miles Harris]

Never mind the bullshit, can you find it? Yes or no? Or are you
going to slither out of it with another one of your lame-ass dodges?

Oh, your assignment question? Listen, Junior, that's for your teacher
to explain to you. I owe you no such favors. Stick with your studies,
though; you may make it one day - if *you* work through your own
homework instead of trying to trick others into doing it for you.

 

[Miles Harris]

Are those explanations, in many cases, "wrong" in the sense that
they give what is to some degree a false or misleading understanding
of the fundamental physics underlying the operation of these
devices? Of course they are. But so what?

"Of course they're wrong, but so what?"????
The mind boggles.
I do appreciate the dilemma, though. On the one hand we'd like to
convey a complete understanding of the subject to the questioner. On
the other hand, we suspect that if we did so, they'd find it all too
much, be turned off and simply find another hobby to pursue.

Perhaps the answer is to provide the simpler explanation, based on the
questioner's level of knowledge, but spell out the caveat that there
is more to the topic than has been explained in the follow-up. IOW,
tell the questioner that the answer provided is sufficient for their
current purposes, but they may need to take more on board as they
advance in their studies.

 

[Miles Harris]

I, for one, no longer have the desire to continue with this
"discussion", so I'll excuse myself and bid you good day.

Translation: "My ass can't take no more whuppin' so I'm outa here!"

 

[Miles Harris]

On Wed, 12 Jan 2005 15:20:16 -0600, John Fields

[snip pointless, uncalled-for lecture on base current but maybe some
newbie can make use of it]

BTW, since I find you mildly annoying I decided to take a look at your
posting history to see what you're about, and I found that other than
about the 20 posts archived at Google (where you also seem to have an
attitude) you seem only to have posted here, so welcome to the swamp.

I've been posting to Usenet for many years, but normally use
X-no-archive or my nickname or whatever. Some of the views I put
forward on political matters are sadly not regarded as acceptable
these days, so when needs must....

While looking, I found this rather interesting post on this NG:

<QUOTE>


Um, yeah, but they drop voltage _according_ to current! If they're
dropping 0,7V., they're not passing much current! Diodes are a crap
way to drop voltage unless the load is light and predictable!!!

miles
<END QUOTE>

which seems to belie your claim that you're an electrical engineer
with 35 years of experience.

Really? So what is it you're taking issue with? Taken as a
generalization for most silicon diodes I can't see real problem with
it.

Continuing on, we find, from Danny T:

<QUOTE>
http://www.kpsec.freeuk.com/components/diode.htm

says

"There is a small voltage across a conducting diode, it is called the
forward voltage drop and is about 0.7V for all normal diodes which are
made from silicon. The forward voltage drop of a diode is almost
constant whatever the current passing through the diode so they have a
very steep characteristic (current-voltage graph)."

--
Danny
<END QUOTE>


To which you replied:

The forward voltage drop is entirely dependent on temperature (the
junction temp. of the p/n junction; which is in turn dependent upon
the current passed.) Higher currents equals higher temp. equals lower
voltage drop. It's a well known effect which can eventually destroy
the diode altogether. The physics of diodes is actually more complex
than a lot of texts would have you believe.

---
Indeed, but if you think the forward voltage drop is _entirely_
dependent on temperature, you seem to have missed reading some of the
more fundamental ones.

For example, while it's certainly true that the voltage across the
junction can be described by:


kT / If \
Vf = ---- ln ( 1 + ---- )
q \ Ir /

and that when T is equal to zero at 0°K, Vf will be 0, you've
neglected to mention that current passing through the bulk resistance
of the diode, at any temperature, will cause a drop across the
junction which is dependent on the resistance and the charge flowing
through the diode.

More importantly, perhaps, you pooh-pooh'd Danny T's idea to use a
diode as a bad one merely because of your opinion, which was
erroneous. Diodes are _often_ used as voltage dropping elements in
the real world because of the small change in Vf caused by If.
Moreover, your example of the negative TC of a diode destroying it
would more closely describe a diode with a voltage source connected
across it allowing the diode to get into thermal runaway. Such a
condition would not happen with the load limiting the current through
the diode and the diode sized to carry the required current under the
required environmental conditions. Furthermore, depending on the
diode, above a certain current the tempco becomes positive, something
else you "neglected" to mention.

What's most disturbing, however, is that with Danny T admittedly being
a newbie and asking for information, you deliberately sidestepped the
issue when he presented you with the [valid] information he found
which supported Andrew Holmes' suggestion to use a diode in order to
keep from having to admit that you were wrong in stating that: "Diodes
are a crap way to drop voltage unless the load is light and
predictable!!!

For shame, sir! :-(

It can hardly be described as "disturbing" FFS.
Excuse me for not having the time or inclination to wade through your
entire post and answer every individual point; but if you're trying to
suggest that I'm as guilty as you in giving 'easy' and expedient
explanations to newbies that don't reflect the full picture, then I
guess I'd have to plead guilty. None of us are perfect.
If you don't like it, sue me.

 

[Miles Harris]

Beta is simply collector current divided by base current. At the OP's
current <G> level of understanding, that's all he needs to know, so
why would it take you longer than a minute to come up with that?

You've pre-empted my considered view on the matter (see my posting of
earlier today further up the thread).