difference between bipolar and mosfet

[Miles Harris]

If I didn't know the foundation was flawed, of course.

But you *do* know it's flawed, don't you - unlike the OP. You've
admitted as much to Kevin Aylward. And yet you're still prepared to
sell this flawed foundation to the OP!

It's a silly and pointless question so I'll decline, thanks.

 

[Miles Harris]

Not no. From:

http://searchsmallbizit.techtarget.com/sDefinition/0,,sid44_gci214200,00.html

"Transconductance is an expression of the performance of a bipolar
transistor or field-effect transistor (FET). In general, the larger
the transconductance figure for a device, the greater the gain
(amplification) it is capable of delivering, when all other factors
are held constant.

Formally, for a bipolar device, transconductance is defined as the
ratio of the change in collector current to the change in base voltage
over a defined, arbitrarily small interval on the
collector-current-versus-base-voltage curve. For an FET,
transconductance is the ratio of the change in drain current to the
change in gate voltage over a defined, arbitrarily small interval on
the drain-current-versus-gate-voltage curve.

The symbol for transconductance is gm. The unit is the siemens, the
same unit that is used for direct-current (DC) conductance.

If dI represents a change in collector or drain current caused by a
small change in base or gate voltage dE, then the transconductance is
approximately:

[snip]

What a lousy definition. Gate voltage and base voltage as referred to
above are totally misleading. The relevant values to concentrate on
are Vbe and Vgs; the potential difference applied directly across the
base/emitter junction (in the case of the BJT) and the PD applied
directly across the gate/source junction in the case of a FET.

 

[Miles Harris]

Can you point to where I advocated the beta model for him to study?

Here's what you wrote earlier:

"The problem which arises here, I think, is that the change in base
voltage required to affect a change in collector current is so tiny
that it becomes easier to consider what happens on the other side of
the change in base voltage. That is, the collector-to emitter current
change due to the base-to-emitter current change."

Clear enough?

 

[John Fields]

But you *do* know it's flawed, don't you - unlike the OP. You've
admitted as much to Kevin Aylward. And yet you're still prepared to
sell this flawed foundation to the OP!

---
What I told Kevin was that I had misstated something, not that I was
prepared to argue that the beta model was the be-all and end-all. If
you'd bother to go back and read my original post to the OP, instead
of playing your stupid posturing game, you might find that your entire
argument is baseless and, basically, noise.
---

 

[John Fields]

Not no. From:

http://searchsmallbizit.techtarget.com/sDefinition/0,,sid44_gci214200,00.html

"Transconductance is an expression of the performance of a bipolar
transistor or field-effect transistor (FET). In general, the larger
the transconductance figure for a device, the greater the gain
(amplification) it is capable of delivering, when all other factors
are held constant.

Formally, for a bipolar device, transconductance is defined as the
ratio of the change in collector current to the change in base voltage
over a defined, arbitrarily small interval on the
collector-current-versus-base-voltage curve. For an FET,
transconductance is the ratio of the change in drain current to the
change in gate voltage over a defined, arbitrarily small interval on
the drain-current-versus-gate-voltage curve.

The symbol for transconductance is gm. The unit is the siemens, the
same unit that is used for direct-current (DC) conductance.

If dI represents a change in collector or drain current caused by a
small change in base or gate voltage dE, then the transconductance is
approximately:

[snip]

What a lousy definition. Gate voltage and base voltage as referred to
above are totally misleading. The relevant values to concentrate on
are Vbe and Vgs; the potential difference applied directly across the
base/emitter junction (in the case of the BJT) and the PD applied
directly across the gate/source junction in the case of a FET.

---
LOL! So what's good for the goose isn't good for the gander?

Where did I just read this: "Nitpicking isn't going to help the OP."?

Unless otherwise stated, nitwit, that the emitter and the source are
the terminals to which the base voltage and gate voltage are
referenced is implicit.

 

[John Fields]

Here's what you wrote earlier:

"The problem which arises here, I think, is that the change in base
voltage required to affect a change in collector current is so tiny
that it becomes easier to consider what happens on the other side of
the change in base voltage. That is, the collector-to emitter current
change due to the base-to-emitter current change."

Clear enough?

---
Perfectly clear to me, and for a newbie who has to ask for the
difference between a BJT and a FET, an easy way to grasp that a tiny
change in base-to-emitter voltage will effect a change in
base-to-emitter current which will, in turn, cause a much larger
change in collector current.

ISTM that you think beta is so evil that, if you had your way, any
mention of beta and all Ic VS Ib curves would be totally eliminated
from all the data sheets in the world. It ain't gonna happen, so you
might as well get over it.

Or not. It makes very little difference to me, one way or the other.

 

[Kevin Aylward]

Yes no.

From:

http://searchsmallbizit.techtarget.com/sDefinition/0,,sid44_gci214200,00.html

"Transconductance is an expression of the performance of a bipolar
transistor or field-effect transistor (FET). In general, the larger
the transconductance figure for a device, the greater the gain
(amplification) it is capable of delivering, when all other factors
are held constant.

{etc sniped.}

I have no basic problems with this quote, its all good stuff. However,
it has absolutely nothing to do with my point. It certainly has no
relevance as to why a transistor is a transconductance *device*.

"Transconductance" in the above is a pure and general mathematically
technique used to model a physical phenomena. It doesn't care whether or
not the phenomena is actually physically current controlled by a
voltage. My description that a bipolar is a "transconductance device" is
statement of its actual physics.

For a given emitter current, the base current will always be a small
fraction due to transistor action. If the npn junction were just a slab
of n type then there would be a direct connection from base to emitter
resulting in larger current. You would just have resisters connecting
base emitter and collector all together and therefore no hfe.

After all, the base-emitter
diode is just that, a forward biased diode operating on the far side
of the VI knee.

It isnt as far as the base circuit is concerned. It acts as a diode with
Ie/hfe

The intent, in both devices, is the same. That is to cause a
non-conductive region in a semiconductor to become conductive. In a
MOSFET it's accomplished by treating the channel like the plate of a
capacitor and making it _seem_ like it's composed of the same material
as the drain and the source by influencing the charge distribution in
it using the gate metalization as the other plate of the capacitor,
while in a BJT it's accomplished by forcing dynamic charge into the
base ["base region" if you like ] and using that charge flow to make
it seem like the base region material is becoming more and more like
the emitter and collector material as the base current increases.

I agree in a loose sense of "seems like", but it just doesn't behave in
the same way as a mosfet does in forming a same type channel.

---


---
Hardly. Here this newbie asks "What makes a BJT different from a
FET?" and you reply "If you put a voltage across the base and emitter
terminals of a BJT current will flow between the collector and
emitter, while if you put a voltage across the gate and source
terminals of a FET current will flow between the drain and the
source." So, while your description may be true, its utter simplicity
leads the newb to think they're the same same thing with differently
named terminals.

The above statement is with regard to the control of the collector
current only. It obviously needs a statement that "base current must
exist in real device".

Here is my original exchange with Skeleton Man:

<QUOTE>


---
Essentially, yes. But, the voltage applied to the base must force
charge through the base-emitter junction before collector current can
flow.
---


---
Yes, but it still requires current to charge the gate capacitance.
However, once that capacitor is charged up, current can flow through
the drain-to-source channel with no further current required into the
gate.
<END QUOTE>


Do you have a problem with that?
No.

So, what, its still something that ideally, would be zero.

Kind of like that we are different from corpses, in part, because we
are required to breathe, but that that breathing is simply a nuisance.

Yes.

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.

 

[John Fields]

Yes no.

---
Not yes no
---

{etc sniped.}

I have no basic problems with this quote, its all good stuff. However,
it has absolutely nothing to do with my point. It certainly has no
relevance as to why a transistor is a transconductance *device*.

"Transconductance" in the above is a pure and general mathematically
technique used to model a physical phenomena. It doesn't care whether or
not the phenomena is actually physically current controlled by a
voltage. My description that a bipolar is a "transconductance device" is
statement of its actual physics.

---

No, it's not. A true transconductance device is one in which no
current is required into the control electrode. The grid of a toob in
the region where no grid current is drawn more nearly approximates a
"true transconductance" device. Your admission that base current must
exist before collector current can exist makes the BJT a
"transresistance device", since the base current is inseparable from
the base-to-emitter voltage when the BJT is operating.

BTW, if it's singular, it's "phenomenon".
---

For a given emitter current, the base current will always be a small
fraction due to transistor action. If the npn junction were just a slab
of n type then there would be a direct connection from base to emitter
resulting in larger current. You would just have resisters connecting
base emitter and collector all together and therefore no hfe.

---
But, in fact, it _isn't_ a slab of N type material, it's a carefully
arranged sandwich made to become more or less resistive by changing
the electrical characteristics of the filling.
---

It isnt as far as the base circuit is concerned. It acts as a diode with
Ie/hfe

---
So what? The only thing hfe does here is to increase Vbe a little
because of the extra emitter current contributed by the collector.
Matter of fact, if you were to replace the collector silicon with a
resistor, like this:

+V
|
[R]
|
Vin>---+
|
+---Vbe
|
|A
[DIODE]
|
GND

and Vin were to be higher than the Vbe caused by the current flowing
through R and the diode, the diode would sink the extra current forced
through it by Vin and would still act like a diode.
---

The intent, in both devices, is the same. That is to cause a
non-conductive region in a semiconductor to become conductive. In a
MOSFET it's accomplished by treating the channel like the plate of a
capacitor and making it _seem_ like it's composed of the same material
as the drain and the source by influencing the charge distribution in
it using the gate metalization as the other plate of the capacitor,
while in a BJT it's accomplished by forcing dynamic charge into the
base ["base region" if you like ] and using that charge flow to make
it seem like the base region material is becoming more and more like
the emitter and collector material as the base current increases.

I agree in a loose sense of "seems like", but it just doesn't behave in
the same way as a mosfet does in forming a same type channel.

---
Well, of course it doesn't. If it did it would be called a MOSFET!^)
In fact, it doesn't even _form_ a channel; what it does form is a
region between and separating the collector and emitter which is
forced (by virtue of the current forced through the region) to become
more or less "P" like if the collector and emitter are made from "P"
type material or more or less "N" like if the collector and emitter
are made from "N" type material.
---

 

[Miles Harris]

LOL! So what's good for the goose isn't good for the gander?

Where did I just read this: "Nitpicking isn't going to help the OP."?

It's not nitpicking, it's vitally important. Vb and Vbe are two
different things. Your "definition" should have referred to Vbe.
Similarly inexcusably sloppy was the reference to Vgs as Vg.

Unless otherwise stated, nitwit, that the emitter and the source are
the terminals to which the base voltage and gate voltage are
referenced is implicit.

So all my old textbooks that specifically refer to Vbe and Vgs are
being unnecessarily pedantic, then? How are you then going to refer to
the 'absolute' values of gate voltage and base voltage (WRT ground)
without causing much confusion??
Tell me how a newbie such as the OP or anyone else reading this group
to learn about the subject is supposed to *know* that sometimes some
idiots write Vg and Vb when they mean Vgs and Vbe?
And personal insults are completely pointless, knob-head.

 

[Miles Harris]

Perfectly clear to me, and for a newbie who has to ask for the
difference between a BJT and a FET, an easy way to grasp that a tiny
change in base-to-emitter voltage will effect a change in
base-to-emitter current which will, in turn, cause a much larger
change in collector current.

That's the essence of the problem with your approach: easyness. Short
cuts are all very well provided they don't subvert fundamental
understanding along the way. Your 'easy' solution doesn't cut it in
this respect, I'm afraid.

ISTM that you think beta is so evil that, if you had your way, any
mention of beta and all Ic VS Ib curves would be totally eliminated
from all the data sheets in the world. It ain't gonna happen, so you
might as well get over it.

I think you owe the OP one further explanation since you've sought to
rely on your 'easy' way out. Kindly explain to him how, if he uses a
transistor with a beta listed as say 250 that he may in practice find
that his ratio of collector current to base current may be as much as
900:1?

 

[Kevin Aylward]

No. I have to say here, this is absolute nonsense. I cant believe I am
actually reading this.

The concept of transconductance is completely independent of whether or
not there is any control current.

The grid of a toob in
the region where no grid current is drawn more nearly approximates a
"true transconductance" device.

Sorry, mate, this is only *your* personal concept of a "true
transconductance" device. Somewhere you have picked up an erroneous view
without even thinking about it. The *only* requirement for a
transconductance is the output current is a direct function of a control
voltage. Any current at the control terminal is simply irrelevant.

Your admission that base current must
exist before collector current can exist makes the BJT a
"transresistance device",

No it don't. This is getting daft. The bipolar transistor is, to first
order, a voltage controlled current source. It is therefore a
transconductance device. Period.

since the base current is inseparable from
the base-to-emitter voltage when the BJT is operating.

Ho hummm...

BTW, if it's singular, it's "phenomenon".

That's what I am claiming, you are claiming otherwise.

it's a carefully
arranged sandwich made to become more or less resistive by changing
the electrical characteristics of the filling.
---

Therefore you are claiming by this that it *is* a slab of resistance.


It is, if we neglect rbb'. The fact that a source has to supply current
is irrelevant.

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.

 

[John Fields]

That's the essence of the problem with your approach: easyness. Short
cuts are all very well provided they don't subvert fundamental
understanding along the way. Your 'easy' solution doesn't cut it in
this respect, I'm afraid.

---
What you're _really_ afraid of is that you'll be shown up for the
disingenuous piece of shit you really are, so you just keep on adding
fuel to the fire, buying time, waiting for what you think will be an
opportune moment to attack and "vindicate" yourself. Keep it up as
long as you like, I don't mind grinding you into the ground at all.
---

I think you owe the OP one further explanation since you've sought to
rely on your 'easy' way out.

---
Wrong again. I don't owe the OP anything and the "reasoning" behind
why you think I do is irrelevant.
---

Kindly explain to him how, if he uses a
transistor with a beta listed as say 250 that he may in practice find
that his ratio of collector current to base current may be as much as
900:1?

---
Well, since you once again don't seem to have the answer, I'll be so
kind as to elucidate _your_ dumb ass, and if he wants to pick up on it
good for him.

If you take a look at a typical data sheet for a transistor,

http://www.fairchildsemi.com/ds/2N/2N4401.pdf

and learn how to read it, you will have answered your own question.
Better that than I should lead you astray by just telling you the
reason and letting you take the easy way out, no?

 

[John Fields]

It's not nitpicking, it's vitally important. Vb and Vbe are two
different things. Your "definition" should have referred to Vbe.
Similarly inexcusably sloppy was the reference to Vgs as Vg.

---
_My_ definition? Show me.
---

So all my old textbooks that specifically refer to Vbe and Vgs are
being unnecessarily pedantic, then?

---
Certainly not. It's the job of textbooks to be as pedantic as possible
in order that some of the morons trying to glean meaning from them not
get confused and misinterpret what was being taught. Know what I
mean?
---

How are you then going to refer to
the 'absolute' values of gate voltage and base voltage (WRT ground)
without causing much confusion??

---
Unless otherwise stated, (and certainly for the purposes of this
discussion) the emitter and source are considered to be at ground,
i,e, 0V. When they're not, as would be the case with an emitter or a
source follower, then the schematic or text description would be
expected to make that clear.
---

Tell me how a newbie such as the OP or anyone else reading this group
to learn about the subject is supposed to *know* that sometimes some
idiots write Vg and Vb when they mean Vgs and Vbe?

---
Assholes like you will come along and offer their bile as
"corrections"?

Again, I'd be interested in having you cite examples where I wrote Vg
or Vs in this thread.
---

 

[John Fields]

No. I have to say here, this is absolute nonsense. I cant believe I am
actually reading this.

---
Believe it!-)
---

The concept of transconductance is completely independent of whether or
not there is any control current.

---
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, that _current_ being what causes the collector
current to flow.
---

Sorry, mate, this is only *your* personal concept of a "true
transconductance" device. Somewhere you have picked up an erroneous view
without even thinking about it. The *only* requirement for a
transconductance is the output current is a direct function of a control
voltage. Any current at the control terminal is simply irrelevant.

---
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 it don't. This is getting daft. The bipolar transistor is, to first
order, a voltage controlled current source. It is therefore a
transconductance device. Period.

---
See above.
---

Ho hummm...

---
Yes.
---

That's what I am claiming, you are claiming otherwise.

---
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.
---

Therefore you are claiming by this that it *is* a slab of resistance.

---
Yes, but only when there's enough charge flowing through the base to
allow collector current to flow. Otherwise it's more like a couple of
series-opposed diodes. See above.
---

It is, if we neglect rbb'.

---
Well, yes, everything takes on a different meaning if we start
selectively neglecting that which makes our argument untenable.
---

The fact that a source has to supply current is irrelevant.

And yet in your last post it was:

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

Maybe in your next post it'll be: "Base current doesn't exist." ?-)

 

[Miles Harris]

_My_ definition? Show me.

The definition you sought to rely on is *your* definition - de facto.

---
Certainly not. It's the job of textbooks to be as pedantic as possible
in order that some of the morons trying to glean meaning from them not
get confused and misinterpret what was being taught. Know what I
mean?

Yes, absolutely! It is often said that "a little knowledge is a
dangerous thing" and this is where that truism is most apparent; when
hobbyists like you get out of their depth and try to 'help' other
hobbyists - as we have seen here - and simply end up making yourselves
look stupid. You should be more diligent in how you study those
'pedantic' books. Considered entry-level evening classes?

Unless otherwise stated, (and certainly for the purposes of this
discussion) the emitter and source are considered to be at ground,
i,e, 0V. When they're not, as would be the case with an emitter or a
source follower, then the schematic or text description would be
expected to make that clear.

It's crass to expect someone who knows even less about electronics
than you do to *know* that.

Assholes like you will come along and offer their bile as
"corrections"?

All the time boneheads like you keep peddling their snake oil, yes.

 

[Miles Harris]

What you're _really_ afraid of is that you'll be shown up for the
disingenuous piece of shit you really are, so you just keep on adding
fuel to the fire, buying time, waiting for what you think will be an
opportune moment to attack and "vindicate" yourself. Keep it up as
long as you like, I don't mind grinding you into the ground at all.

You're simply not capable. I'm a professional electronics engineer of
some 35 years experience. You, OTOH, strike me as someone who has yet
to build their first crystal set. You're out of your depth and out of
your class, pal.

[snip about datasheets]

and learn how to read it, you will have answered your own question.
Better that than I should lead you astray by just telling you the
reason and letting you take the easy way out, no?

Listen, bonehead. *You* are the one who brought up the 'easy' solution
of Beta, so you explain to the OP how it so often falls apart in
practice. I never cited it, okay??
Sheesh!!!

 

[Miles Harris]

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.

ROTFLMAO!!! Hey, boner, I hope this is simply some crappy analogy
you've devised here and not directly quoted from the kind of textbooks
you've been studying!
BWAHAHAHAHAHAHAHAAAAA!!!

 

[John Fields]

The definition you sought to rely on is *your* definition - de facto.

---
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.
---

Yes, absolutely! It is often said that "a little knowledge is a
dangerous thing"

---
If that's true, then I suspect that you're extremely dangerous!
---

and this is where that truism is most apparent; when
hobbyists like you get out of their depth and try to 'help' other
hobbyists - as we have seen here - and simply end up making yourselves
look stupid.

---
Well, looks may be deceiving, but I suspect in your case that
stupidity is more than just skin deep.

Besides, what's wrong with being a hobbyist? Everybody has to start
somewhere, don't they?

What about you, what do you do?
---

You should be more diligent in how you study those
'pedantic' books. Considered entry-level evening classes?

---
Sure, and if you need some suggestions I'll be happy to post them for
you.
---

It's crass to expect someone who knows even less about electronics
than you do to *know* that.

---
The _obvious_ shouldn't need an explanation. And, since even _you_
seemed to understand the point, I don't see why you'd think that
anyone else would miss it.
---

All the time boneheads like you keep peddling their snake oil, yes.

---
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?

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?

No? Hmmm...

 

[John Fields]

You're simply not capable. I'm a professional electronics engineer of
some 35 years experience. You, OTOH, strike me as someone who has yet
to build their first crystal set. You're out of your depth and out of
your class, pal.

---
Yup, I usually hang out with the pelagic fish, but this time I thought
I'd come up to the shallows and see what the guppies are doing. What
a surprise, there you were in a school of them!
---

[snip about datasheets]

and learn how to read it, you will have answered your own question.
Better that than I should lead you astray by just telling you the
reason and letting you take the easy way out, no?

Listen, bonehead. *You* are the one who brought up the 'easy' solution
of Beta, so you explain to the OP how it so often falls apart in
practice. I never cited it, okay??
Sheesh!!!

---
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.

 

[John Fields]

ROTFLMAO!!! Hey, boner, I hope this is simply some crappy analogy
you've devised here and not directly quoted from the kind of textbooks
you've been studying!
BWAHAHAHAHAHAHAHAAAAA!!!

---
Since you obviously don't understand the analogy and are more
concerned with 'i' dotting and 't' crossing than you are with anything
technically interesting, I can't see what possible difference it would
make whether it accurately reflected reality or not. Either way, it
would be gobbledegook to you.