difference between bipolar and mosfet

[Kevin Aylward]

Robert Monsen wrote:

Skeleton Man wrote:
Just wondering could someone explain fairly simply what the
difference is between a bipolar and a fet ? Can I put a bipolar in
place of a fet or vice versa ?

Regards,
Chris



The most basic difference is that a bipolar transistor requires
current at the control terminal (the base lead), whereas a mosfet
requires none. However, there are advantages to both in different
situations.
You generally cannot substitute a bipolar transistor for a fet,
because the circuit will not be designed to supply the required base
current.
MOSFETs have three leads, a source, a gate, and a drain. Bipolar
transistors also have three leads, but they are called emitter, base,
and collector. These leads roughly correspond to one another, ie, the
emitter is like the source, the base is like the gate, and the
collector is like the drain. Making the base (gate) more positive
(for NPN and N-MOSFETs) or negative (for PNP or P-MOSFETs) with
respect to the emitter (source) causes more current to flow from
collector (drain) to emitter (source).

This terminology is totally confusing,

Why?

The names have been specifically chosen to describe how the device
actually functions.

Charge carriers are sourced or emitted from the source/emitter. These
carriers are then drained off or collected by the drain/collector. The
gate or base *voltage* controls the flow of carriers. I will give you
that "base" is not on a par with "gate" in describing its function.

and, sadly, you just have to
get used to it if you want to talk about these things.

Once one understands the names, one will understand how mosfet and
bipolar actually function. If you don't understand why the names are as
they are, you wont understand how the devices function.

MOSFETs are used to construct CMOS devices, and are thus the main
transistor component to microprocessors. They are also good for
constructing huge power transistors, which are easier to control due
to the lack of required gate current.

Bipolar transistors are generally more useful for analog design, where
the lower noise, more easily predicted voltage requirements, and lower
control voltages are useful.

For a FET, the electrostatic field of charges on the control terminal
(the gate) is used to moderate the output. MOSFETs have a silicon
oxide layer that insulates the gate from the charge. JFETs use a
reverse-biased PN junction's depletion region to isolate the gate from
the source and drain. For bipolar transistors, the movement of charges
across PN junctions controls the output.

Here we go again... for bipolar transistors, it is the application of
*voltage* to the base emitter PN junction that controls the output
current. The movement of charges is irrelevant.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
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]

On Sat, 08 Jan 2005 10:13:28 GMT, "Kevin Aylward"
<salesEXTRACT@anasoft.co.uk> wrote:

Here we go again... for bipolar transistors, it is the application of
*voltage* to the base emitter PN junction that controls the output
current. The movement of charges is irrelevant.

---
The application of a forward voltage to the base-emitter junction of a
bipolar transistor will, of course, cause charge to flow between the
collector and emitter, but the movement of charges across the
base-emitter junction _is_ relevant, since without that movement there
can be no collector current.

In a MOSFET, however, the only movement of charge required to control
the drain-source current is that required to charge and discharge the
gate capacitance.

--
John Fields

 

[Robert Monsen]

Kevin Aylward wrote:

Robert Monsen wrote:

Skeleton Man wrote:

Just wondering could someone explain fairly simply what the
difference is between a bipolar and a fet ? Can I put a bipolar in
place of a fet or vice versa ?

Regards,
Chris



The most basic difference is that a bipolar transistor requires
current at the control terminal (the base lead), whereas a mosfet
requires none. However, there are advantages to both in different
situations.
You generally cannot substitute a bipolar transistor for a fet,
because the circuit will not be designed to supply the required base
current.
MOSFETs have three leads, a source, a gate, and a drain. Bipolar
transistors also have three leads, but they are called emitter, base,
and collector. These leads roughly correspond to one another, ie, the
emitter is like the source, the base is like the gate, and the
collector is like the drain. Making the base (gate) more positive
(for NPN and N-MOSFETs) or negative (for PNP or P-MOSFETs) with
respect to the emitter (source) causes more current to flow from
collector (drain) to emitter (source).

This terminology is totally confusing,


Why?

I'm not saything they are wrong, I'm just saying that it's a confusing
blob of information until you memorize it. Once you figure it out, you
can convince yourself that it makes sense, just like any terminology.

The names have been specifically chosen to describe how the device
actually functions.

Charge carriers are sourced or emitted from the source/emitter. These
carriers are then drained off or collected by the drain/collector. The
gate or base *voltage* controls the flow of carriers. I will give you
that "base" is not on a par with "gate" in describing its function.


and, sadly, you just have to
get used to it if you want to talk about these things.


Once one understands the names, one will understand how mosfet and
bipolar actually function. If you don't understand why the names are as
they are, you wont understand how the devices function.

Right, you have to understand how the device works in order to
understand the names of the terminals. Unfortunately, that is confusing
for beginners, who often want to simply build something simple, and get
confused by emitters, collectors, where which goes, whether PNP or NPN
should be used, etc.

MOSFETs are used to construct CMOS devices, and are thus the main
transistor component to microprocessors. They are also good for
constructing huge power transistors, which are easier to control due
to the lack of required gate current.

Bipolar transistors are generally more useful for analog design, where
the lower noise, more easily predicted voltage requirements, and lower
control voltages are useful.

For a FET, the electrostatic field of charges on the control terminal
(the gate) is used to moderate the output. MOSFETs have a silicon
oxide layer that insulates the gate from the charge. JFETs use a
reverse-biased PN junction's depletion region to isolate the gate from
the source and drain. For bipolar transistors, the movement of charges
across PN junctions controls the output.


Here we go again... for bipolar transistors, it is the application of
*voltage* to the base emitter PN junction that controls the output
current. The movement of charges is irrelevant.

Who cares? The point was that with a bipolar transistor, one needs
current into the base in order to pass current from collector to
emitter. It doesn't work without the current. This is a useful fact
which can often be exploited in circuits that simply want an on/off switch.

Whether it's 'right' is yet another matter. Newtonian physics is
'wrong', and based on incorrect physics, but for most things, it's OK to
use. This is also true of design using beta. Lighten up.

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

--
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]

On Sun, 09 Jan 2005 06:41:55 GMT, "Skeleton Man"
<invalid@guestwho.com> wrote:

so if I'm to understand correctly.. a bi-polar will pass current between
collector and emitter when a voltage is applied to the base ?

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

and a fet will do
a simmilar thing only doesn't require a current ? (at whichever terminal
corresponds to a base on a bipolar)

---
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.


--
John Fields

 

[Robert Monsen]

Skeleton Man wrote:

so if I'm to understand correctly.. a bi-polar will pass current between
collector and emitter when a voltage is applied to the base ? and a fet will do
a simmilar thing only doesn't require a current ? (at whichever terminal
corresponds to a base on a bipolar)

Regards,
Chris

More or less. That is why you can't just replace MOSFETs with bipolar
transistors. The bipolar transistor needs current into their base to
operate, and mosfet circuits will not be designed to supply it.

--
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.

 

[Ban]

Robert Monsen wrote:

Skeleton Man wrote:
so if I'm to understand correctly.. a bi-polar will pass current
between collector and emitter when a voltage is applied to the base
? and a fet will do a simmilar thing only doesn't require a current
? (at whichever terminal corresponds to a base on a bipolar)

Regards,
Chris


More or less. That is why you can't just replace MOSFETs with bipolar
transistors. The bipolar transistor needs current into their base to
operate, and mosfet circuits will not be designed to supply it.

You are wrong here, the base voltage of a bipolar will be below 1V whereas
the gate voltage of a Mosfet should be much higher, usually 10 to 15V. Logic
level FETs need at least +4V gate drive. A bipolar drive needs to be current
limited, not so a Mosfet. So usually you can replace a FET by a bipolar
transistor if you put a resistor in series with the base (if you are driving
low current apps like relays or LEDs).
--
ciao Ban
Bordighera, Italy

 

[Miles Harris]

On Sat, 08 Jan 2005 10:13:28 GMT, "Kevin Aylward"
<salesEXTRACT@anasoft.co.uk> wrote:

Robert Monsen wrote:
Skeleton Man wrote:
Just wondering could someone explain fairly simply what the
difference is between a bipolar and a fet ? Can I put a bipolar in
place of a fet or vice versa ?

Regards,
Chris



The most basic difference is that a bipolar transistor requires
current at the control terminal (the base lead), whereas a mosfet
requires none. However, there are advantages to both in different
situations.
You generally cannot substitute a bipolar transistor for a fet,
because the circuit will not be designed to supply the required base
current.
MOSFETs have three leads, a source, a gate, and a drain. Bipolar
transistors also have three leads, but they are called emitter, base,
and collector. These leads roughly correspond to one another, ie, the
emitter is like the source, the base is like the gate, and the
collector is like the drain. Making the base (gate) more positive
(for NPN and N-MOSFETs) or negative (for PNP or P-MOSFETs) with
respect to the emitter (source) causes more current to flow from
collector (drain) to emitter (source).

This terminology is totally confusing,

Why?

The names have been specifically chosen to describe how the device
actually functions.

True. The *really* confusing differences lie in the labels applied to
the regions of operation of the various devices. The terms linear,
saturation, ohmic, cut-off and so forth mean different things
according to the device under discussion. Beginners beware!

 

[Miles Harris]

On Sun, 09 Jan 2005 01:31:39 -0600, John Fields
<jfields@austininstruments.com> wrote:

and a fet will do
a simmilar thing only doesn't require a current ? (at whichever terminal
corresponds to a base on a bipolar)

---
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.

Now the OP will be confused by another over-simplification. It depends
on whether the FET is of the enhancement or depletion mode type. Your
statement is correct for enhancement mode FETs, but wrong for
depletion mode ones. Depletion mode FETs are 'normally on' and will
conduct fully with *no* applied gate voltage. You have to apply a
*negative* voltage to the gate to moderate the drain current. Enough
negative voltage will cut-off the drain current altogether. No doubt
*you* know this, but it should be pointed out to the OP.

 

[Robert Monsen]

Ban wrote:

Robert Monsen wrote:

Skeleton Man wrote:

so if I'm to understand correctly.. a bi-polar will pass current
between collector and emitter when a voltage is applied to the base
? and a fet will do a simmilar thing only doesn't require a current
? (at whichever terminal corresponds to a base on a bipolar)

Regards,
Chris


More or less. That is why you can't just replace MOSFETs with bipolar
transistors. The bipolar transistor needs current into their base to
operate, and mosfet circuits will not be designed to supply it.


You are wrong here, the base voltage of a bipolar will be below 1V whereas
the gate voltage of a Mosfet should be much higher, usually 10 to 15V. Logic
level FETs need at least +4V gate drive. A bipolar drive needs to be current
limited, not so a Mosfet. So usually you can replace a FET by a bipolar
transistor if you put a resistor in series with the base (if you are driving
low current apps like relays or LEDs).

You are right. It is possible, with circuit modification, to go between
enhancement devices.

Just to summarize:

For going from NPN to N-MOSFET (or PNP to P-MOSFET), you need
a) more gate voltage to turn it on. This
varies much more with mosfets than with NPNs. Given a
particular circuit, it may not be possible to fully turn on
a mosfet because of this.
b) a way to pull the gate to ground to turn it off, since
the NPN automatically turns off when the base no longer
is getting current, whereas the n-channel mosfet may float.

For going from N-MOSFET to NPN (or P-MOSFET to PNP), you need
a) A way to limit base current (which may be as simple as a resistor)
b) Far less base voltage (which the resistor may take care of)
c) Possibly much more current than the driving circuit can provide.

However, replacing a bipolar with a JFET or depletion mosfet is much
more difficult.

Also, the characteristics of these devices is completely different. A
BJT has a current gain which is exponential in voltage (or somewhat
linear in current), whereas current through a FET has a quadratic
relationship to gate voltage in the active region.

--
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]

On Sun, 09 Jan 2005 14:32:00 GMT, Miles Harris <mazzer@yahoo.com>
wrote:

On Sun, 09 Jan 2005 00:24:03 -0800, Robert Monsen
rcsurname@comcast.net> wrote:

More or less. That is why you can't just replace MOSFETs with bipolar
transistors. The bipolar transistor needs current into their base to
operate, and mosfet circuits will not be designed to supply it.

Yet another oversimplification. The bias requirements are *totally*
different and should not be studied by means of comparison with BJTs.
As Kevin Aylward said, it's better to nip these misconceptions in the
bud before they become entrenched views.

---
Well, Kevin seems to subscribe to the policy that one should learn to
run, then walk. That is, first throw Ebers-Moll at a newbie then,
later, beta. If you agree with that philosophy, then I'll have to
disagree with you both.

--
John Fields

 

[Miles Harris]

On Sun, 09 Jan 2005 15:04:52 -0600, John Fields
<jfields@austininstruments.com> wrote:

Have you paid any attention at all to the subject line? It reads:

"difference between bipolar and mosfet", not "How many different
flavors of FETs are there?"

The subject line refers to MOSFETs, certainly. However, in the body of
his original post, the OP refers to FETs., which could just be
innocently (but wrongly) intended as an abbreviation for MOSFET, or it
could actually mean JFET. We don't know if the OP knows the difference
between JFETs and MOSFETs, so I think it's entirely appropriate to
point out they are different devices with different characteristics.
Had this not been pointed out, the OP might well assume that
everything that he has read here WRT to MOSFETs applies equally well
to JFETs - and of course it doesn't!

As far as confusion goes, I'm sure that your refusal to KISS have done
little to alleviate the OP's.

Your approach would lead the OP into a false sense of competence over
his grasp of the subject. At least now he has a much better idea of
the scope of this surprisingly complicated question and if he trawls
through all the replies and studies them intently, he will be a better
man for it. Plus he will have no unpleasant surprises further down the
line in his studies.

 

[John Fields]

On Mon, 10 Jan 2005 00:01:33 GMT, Miles Harris <mazzer@yahoo.com>
wrote:

On Sun, 09 Jan 2005 15:18:24 -0600, John Fields
jfields@austininstruments.com> wrote:

Well, Kevin seems to subscribe to the policy that one should learn to
run, then walk. That is, first throw Ebers-Moll at a newbie then,
later, beta. If you agree with that philosophy, then I'll have to
disagree with you both.

Then presumably you're the type of person who tells his kids Father
Christmas exists.

---
Beta doesn't exist? That's news to me!
---

All very nice and well-intentioned, but not fair on
the child when he finds out the real truth and starts to question
everything he's ever been told.

---
I rather doubt whether you have the "real truth" on tap to hand out,
and all children should be taught to question everything they've been
told no matter how much you may cherish the truth you think you're
being so benevolent in bestowing on them.
---

I'd sooner be straight with people right from the start.

---
Then why not try it?

--
John Fields

 

[John Fields]

On Mon, 10 Jan 2005 00:01:32 GMT, Miles Harris <mazzer@yahoo.com>
wrote:

On Sun, 09 Jan 2005 15:04:52 -0600, John Fields
jfields@austininstruments.com> wrote:

Have you paid any attention at all to the subject line? It reads:

"difference between bipolar and mosfet", not "How many different
flavors of FETs are there?"

The subject line refers to MOSFETs, certainly. However, in the body of
his original post, the OP refers to FETs., which could just be
innocently (but wrongly) intended as an abbreviation for MOSFET, or it
could actually mean JFET. We don't know if the OP knows the difference
between JFETs and MOSFETs, so I think it's entirely appropriate to
point out they are different devices with different characteristics.
Had this not been pointed out, the OP might well assume that
everything that he has read here WRT to MOSFETs applies equally well
to JFETs - and of course it doesn't!

---
Then, in fact, you were remiss in not further including _all_ FETs in
your "exposition" and expounding on them at length. After all, why
leave any tern ustoned?
---

As far as confusion goes, I'm sure that your refusal to KISS have done
little to alleviate the OP's.

Your approach would lead the OP into a false sense of competence over
his grasp of the subject.

---
My _carefully measured_ approach would allow the OP an initial grasp
of the subject matter which could later be broadened, if he chose to,
without the undue confusion forced on him by your rather heavy-handed
insistence that he eat more than he asked for.
---

At least now he has a much better idea of
the scope of this surprisingly complicated question and if he trawls
through all the replies and studies them intently, he will be a better
man for it. Plus he will have no unpleasant surprises further down the
line in his studies.

---
The point is that there really was no reason for him to have to trawl
through anything until you started pumping out your bilge. Plus, if
you think that you've shielded him from any unpleasant surprises
farther down the road because of your "contribution", I suggest you
have another think.

--
John Fields

 

[Kevin Aylward]

John Fields wrote:

On Mon, 10 Jan 2005 00:01:32 GMT, Miles Harris <mazzer@yahoo.com
wrote:

On Sun, 09 Jan 2005 15:04:52 -0600, John Fields
jfields@austininstruments.com> wrote:

Have you paid any attention at all to the subject line? It reads:

"difference between bipolar and mosfet", not "How many different
flavors of FETs are there?"

The subject line refers to MOSFETs, certainly. However, in the body
of his original post, the OP refers to FETs., which could just be
innocently (but wrongly) intended as an abbreviation for MOSFET, or
it could actually mean JFET. We don't know if the OP knows the
difference between JFETs and MOSFETs, so I think it's entirely
appropriate to point out they are different devices with different
characteristics. Had this not been pointed out, the OP might well
assume that everything that he has read here WRT to MOSFETs applies
equally well to JFETs - and of course it doesn't!

---
Then, in fact, you were remiss in not further including _all_ FETs in
your "exposition" and expounding on them at length. After all, why
leave any tern ustoned?
---

As far as confusion goes, I'm sure that your refusal to KISS have
done little to alleviate the OP's.

Your approach would lead the OP into a false sense of competence over
his grasp of the subject.

---
My _carefully measured_ approach would allow the OP an initial grasp
of the subject matter which could later be broadened, if he chose to,
without the undue confusion forced on him by your rather heavy-handed
insistence that he eat more than he asked for.

Your approch gave an inital *wrong* grasp of the subject, i.e. no grasp
at all.

---

At least now he has a much better idea of
the scope of this surprisingly complicated question and if he trawls
through all the replies and studies them intently, he will be a
better man for it. Plus he will have no unpleasant surprises further
down the line in his studies.

---
The point is that there really was no reason for him to have to trawl
through anything until you started pumping out your bilge.

No. There is no reason for not giving a *correct* description if that
correct description is simple.

To wit:

The bipolar transistor is a voltage controlled device,. Its collector
current is a direct function of its base emitter voltage. Incidentally
to this process, the base terminal requires current in order for the
transistor to work correctly.

If the reader can not understand such a simple idea, then there would be
no point in giving any description at all.

Plus, if
you think that you've shielded him from any unpleasant surprises
farther down the road because of your "contribution", I suggest you
have another think.

If you think that you have helped by reinforcing erroneous notions on
how the bipolar transistor operates, I suggest you have another think.


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

 

[Jamie]

Kevin Aylward wrote:

I dont see johns orginal post here


On Sun, 09 Jan 2005 15:18:24 -0600, John Fields
jfields@austininstruments.com> wrote:


Well, Kevin seems to subscribe to the policy that one should learn to
run, then walk.


I most certainly dont.

That is, first throw Ebers-Moll at a newbie then,

later, beta.


Not at all. One need only state that the collector current is a direct
function of base emitter voltage, and that when this voltage is applied,
there is some base current, which is typically much less than the
collector current.

This correct description is no more complicated that giving the *wrong*
base current controlled one.


If you agree with that philosophy, then I'll have to
disagree with you both.


I don't agree with a philosophy of giving false technical information,
if the correct information is just as easy to give.


Then presumably you're the type of person who tells his kids Father
Christmas exists. All very nice and well-intentioned, but not fair on
the child when he finds out the real truth and starts to question
everything he's ever been told. I'd sooner be straight with people
right from the start.


Yep. It is very rare that I think white lies are the way to go. In this
particular case, kids should informed from the outset that ideas such as
santa claus, god, elves, etc are simply made up fantasies.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
I would question your idea about GOD, but are you

really trying to me that Santa Claus is a myth ?
don't tell it so! who the hell has been eating
those cookies and drinking that milk then!

 

[John Fields]

On Mon, 10 Jan 2005 19:31:40 GMT, Miles Harris <mazzer@yahoo.com>
wrote:

On Mon, 10 Jan 2005 12:39:48 -0600, John Fields
jfields@austininstruments.com> wrote:

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

 

[Kevin Aylward]

John Fields wrote:

On Mon, 10 Jan 2005 08:19:50 GMT, "Kevin Aylward"
salesEXTRACT@anasoft.co.uk> wrote:

John Fields wrote:
On Sun, 09 Jan 2005 14:31:59 GMT, Miles Harris <mazzer@yahoo.com
wrote:

On Sat, 08 Jan 2005 11:11:22 -0600, John Fields
jfields@austininstruments.com> wrote:

The application of a forward voltage to the base-emitter junction
of a bipolar transistor will, of course, cause charge to flow
between the collector and emitter, but the movement of charges
across the base-emitter junction _is_ relevant, since without that
movement there can be no collector current.

Yes, but that's misleading. It's essential to concentrate on the
relationship between the applied voltage to the base/emitter
junction and the resultant collector current. The BJT is a
transconductance device and should be viewed as such.

---
It's only a transconductance device because of the voltage required
to force charge through the base-to-emitter diode,

No. They are a transconductance device because applying a voltage
across the base emitter junction injects carriers from the emitter
to the base *region*. This charge essentially *all* flows out of
the collecter, not the base terminal.

---
Not no.

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.

that charge changing
the electrical properties of the base material to more closely
approximate those of the collector and emitter. That is, when
charge is injected into the base-to-emitter diode of a PNP
transistor, the "N" type base material becomes more and more "P"
like as more and more current is forced through it, with the result
that the transistor starts looking more and more like a single
piece of low-resistance "P" type material as more and more current
flows through the base-to-emitter junction.

This is not an accurate description of the bipolar transistor. This
description is more relevant to operation of the mosfet. The npn
junction simply does not act like a slap of N type. If it did, base
current would be huge.

---
yes, were it not for the current limiting resistance external to the
base the base current could become huge.

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.

---

That being the case, collector current will
flow when base current does, and will increase with increasing base
current until the transistor goes into saturation. Of course it's
the base-to-emitter voltage which makes the whole thing happen,

Indeed it is.

but what
_I_ think is misleading is to burden an inquirer with too much
detail too soon.

This is not too much detail at all. Its can't get any simpler. vbe
controls the collector/emitter current. End of story.

---
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
On Sun, 09 Jan 2005 06:41:55 GMT, "Skeleton Man"
invalid@guestwho.com> wrote:

so if I'm to understand correctly.. a bi-polar will pass current
between collector and emitter when a voltage is applied to the base ?

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

and a fet will do
a simmilar thing only doesn't require a current ? (at whichever
terminal corresponds to a base on a bipolar)

---
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.

. After all, the question wasn't "How are
the BJT and the FET alike?" it was "How are they different?".

They are different, in part, in that the bipolar requires base
current, but that this base current is simply a nuisance.

---
Hardly a _nuisance_; it won't work without it.

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
salesEXTRACT@anasoft.co.uk
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]

On Tue, 11 Jan 2005 07:39:01 GMT, "Kevin Aylward"
<salesEXTRACT@anasoft.co.uk> wrote:

John Fields wrote:
On Mon, 10 Jan 2005 08:19:50 GMT, "Kevin Aylward"
salesEXTRACT@anasoft.co.uk> wrote:

No. They are a transconductance device because applying a voltage
across the base emitter junction injects carriers from the emitter
to the base *region*. This charge essentially *all* flows out of
the collecter, not the base terminal.

---
Not no.

Yes no.

---
Not 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.

---

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".
---

that charge changing
the electrical properties of the base material to more closely
approximate those of the collector and emitter. That is, when
charge is injected into the base-to-emitter diode of a PNP
transistor, the "N" type base material becomes more and more "P"
like as more and more current is forced through it, with the result
that the transistor starts looking more and more like a single
piece of low-resistance "P" type material as more and more current
flows through the base-to-emitter junction.

This is not an accurate description of the bipolar transistor. This
description is more relevant to operation of the mosfet. The npn
junction simply does not act like a slap of N type. If it did, base
current would be huge.

---
yes, were it not for the current limiting resistance external to the
base the base current could become huge.

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.
---

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

---
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.
---

That being the case, collector current will
flow when base current does, and will increase with increasing base
current until the transistor goes into saturation. Of course it's
the base-to-emitter voltage which makes the whole thing happen,

Indeed it is.

but what
_I_ think is misleading is to burden an inquirer with too much
detail too soon.

This is not too much detail at all. Its can't get any simpler. vbe
controls the collector/emitter current. End of story.

---
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".

---
Then it's not quite as simple as "Vbe controls the collector/emitter
current.", is it?
---

Here is my original exchange with Skeleton Man:

QUOTE
On Sun, 09 Jan 2005 06:41:55 GMT, "Skeleton Man"
invalid@guestwho.com> wrote:

so if I'm to understand correctly.. a bi-polar will pass current
between collector and emitter when a voltage is applied to the base ?

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

and a fet will do
a simmilar thing only doesn't require a current ? (at whichever
terminal corresponds to a base on a bipolar)

---
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.


. After all, the question wasn't "How are
the BJT and the FET alike?" it was "How are they different?".

They are different, in part, in that the bipolar requires base
current, but that this base current is simply a nuisance.

---
Hardly a _nuisance_; it won't work without it.

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

---
It is, in an IGBT.

--
John Fields

 

[Miles Harris]

On Mon, 10 Jan 2005 16:45:58 -0600, John Fields
<jfields@austininstruments.com> wrote:

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]

On Mon, 10 Jan 2005 18:08:01 -0600, John Fields
<jfields@austininstruments.com> wrote:

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?