FETs Vesus Bipolars, Why More Efficient?

[Ken Smith]

In article <pan.2004.11.24.05.52.15.514376@example.net>,
Rich Grise <rich@example.net> wrote:
[...]

No, this crosses the line. FETs have theoretically no gate current at all,
which came as a major relief to us old toob jocks. They _actually_ _are_
voltage-controlled.

For the AC case , FETs have a gate current. At low frequencies, it looks
capacitive. If you go up to higher frequencies, a resistive component
in the gate impedance appears. This is why FET RF stages have a finite
power gain.

--
--
kensmith@rahul.net forging knowledge

 

[lemonjuice]

On Wed, 24 Nov 2004 14:50:37 -0500, Active8 <reply2group@ndbbm.net>
wrote:

On 24 Nov 2004 05:46:32 -0800, lemonjuice wrote:

On Tue, 23 Nov 2004 23:57:30 +0000, Paul Burridge
pb@notthisbit.osiris1.co.uk> wrote:


Kevin pointed out days ago, you need an electric field to get a
current flow.

Oh dear. Is Mike still here posting snake oil? You can get a current
flow from a *magnetic* field. Back to school with you, Mike!

Oh dear. Is Mike still here posting snake oil? You can get a current
flow from a *magnetic* field. Back to school with you, Mike!

Hey assholes. Faraday's law says that a changing magnetic field
gives an induced EMF, not a current flow, so fuck off.

and you believe that you can have an induced emf without charges in
movement? hint: Look at Poissons equation

yes and you can get current from light using a photodiode

Now you're going to say that light generates current. Wrong again.
Light absorbed in the active region creates an electron hole pair.
It's charge or electric potential again, that causes a current flow.
Now charge = electrical potential ? Explain please.

Charge is measured in coulombs and electric potential in volts... that
is Joules/coulomb
Photodiodes detect light by measuring the photocurrent.

take care

 

[Kevin Aylward]

Dr. Slick wrote:

"Kevin Aylward" <salesEXTRACT@anasoft.co.uk> wrote in message
news:<xIWod.24952$08.4646@fe2.news.blueyonder.co.uk>...


My point, once again, was that the majority
of texts DO use beta and current gain to describe
BJTs, but never to describe FETs, which have to
high of a DC gate impedance to use current gain,
so they use transconductance instead.


I agree that the majority of popular elementary texts do this.
However, they are wrong if they claim that a bipolar transistor is
current controlled. It just introduces much confusion. The most
appropriate model is a voltage controlled device with a non-linear
input resistance.


Hint: What's a current source? It's a voltage
source with a relatively high series resistance (as
compared to the resistance of the driven input
impedance.)


Yes. Which confirms my point.


But you CAN use a current source
to drive the base of a BJT.

Of course.

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.

 

[Active8]

On Thu, 25 Nov 2004 07:55:48 GMT, Kevin Aylward wrote:

Rhett Oracle wrote:
On Wed, 24 Nov 2004 14:50:37 -0500, Active8 wrote:

On 24 Nov 2004 05:46:32 -0800, lemonjuice wrote:

On Tue, 23 Nov 2004 23:57:30 +0000, Paul Burridge
pb@notthisbit.osiris1.co.uk> wrote:


Kevin pointed out days ago, you need an electric field to get a
current flow.

Oh dear. Is Mike still here posting snake oil? You can get a
current flow from a *magnetic* field. Back to school with you,
Mike!

Oh dear. Is Mike still here posting snake oil? You can get a current
flow from a *magnetic* field. Back to school with you, Mike!

Hey assholes. Faraday's law says that a changing magnetic field
gives an induced EMF, not a current flow, so fuck off.

yes and you can get current from light using a photodiode

Now you're going to say that light generates current. Wrong again.
Light absorbed in the active region creates an electron hole pair.
It's charge or electric potential again, that causes a current flow.

If you trace the chicken-egg thing all the way back, you'll find that
Magnetic was first.


No you wont. All of E&M is described by momentum exchange of photons.
Observing these photons in different frames of reference result in
different measurements. i.e. magnetic or electric or both.

What about after the fact, i.e. permanent mags? The photon action is

long over (no?), the iron is left magnetized, but you can still
measure it.

But I digress. The point was that PB is FOS.

--
Best Regards,
Mike

 

[Active8]

On 24 Nov 2004 14:16:06 -0800, lemonjuice wrote:

On Wed, 24 Nov 2004 14:50:37 -0500, Active8 <reply2group@ndbbm.net
wrote:

On 24 Nov 2004 05:46:32 -0800, lemonjuice wrote:

On Tue, 23 Nov 2004 23:57:30 +0000, Paul Burridge
pb@notthisbit.osiris1.co.uk> wrote:


Kevin pointed out days ago, you need an electric field to get a
current flow.

Oh dear. Is Mike still here posting snake oil? You can get a current
flow from a *magnetic* field. Back to school with you, Mike!

Oh dear. Is Mike still here posting snake oil? You can get a current
flow from a *magnetic* field. Back to school with you, Mike!

Hey assholes. Faraday's law says that a changing magnetic field
gives an induced EMF, not a current flow, so fuck off.

and you believe that you can have an induced emf without charges in
movement? hint: Look at Poissons equation

PB doesn't know f'all about Poisson. He's blabbering as per normal.
IOW, he would guess that Faraday's Law of Induction relates induced
current (rather than emf) to time varying magnetizing current -
don't even ask him about Phi or d(Phi)/dt - he has probably never
read chapter one of a first year physics text. Had he read the EM
chapters, he might have read something similar to the following
quote from Halliday and Resnick regarding Faraday's law of
induction:

"The current that appears in this experiment is called an *induced
current* and is said to be set up by an *induced electromotive
force*.

yes and you can get current from light using a photodiode

Now you're going to say that light generates current. Wrong again.
Light absorbed in the active region creates an electron hole pair.
It's charge or electric potential again, that causes a current flow.
Now charge = electrical potential ? Explain please.
Charge is measured in coulombs and electric potential in volts... that
is Joules/coulomb
Photodiodes detect light by measuring the photocurrent.

Ok, I skipped steps. charge => electrostatic field => potential

difference between 2 or more points - you knew that.

You should be able to figure out that what I was trying to say above
is that light doesn't create current, it just creates an electron
hole pair.

--
Best Regards,
Mike

 

[lemonjuice]

yes, also from a difference in temperatures across conductors connected
in a pn junction

 

[Paul Burridge]

On 25 Nov 2004 08:22:29 -0800, "lemonjuice" <exskimos@anonymous.to>
wrote:

yes, also from a difference in temperatures across conductors connected
in a pn junction

Oh yes! and from certain dissimilar metals bonded together and heated.
Mike really *does* need to go back to school!
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Dr. Slick]

"Kevin Aylward" <salesEXTRACT@anasoft.co.uk> wrote in message news:<Vdgpd.4056$38.3182@fe2.news.blueyonder.co.uk>...

Hint: What's a current source? It's a voltage
source with a relatively high series resistance (as
compared to the resistance of the driven input
impedance.)


Yes. Which confirms my point.


But you CAN use a current source
to drive the base of a BJT.


Of course.

Then we agree. A common emmiter
BJT can be considered a Current-controlled
Current-Source, like almost every basic
electronic text claims.

Slick

 

[Kevin Aylward]

Dr. Slick wrote:

"Kevin Aylward" <salesEXTRACT@anasoft.co.uk> wrote in message
news:<Vdgpd.4056$38.3182@fe2.news.blueyonder.co.uk>...


Hint: What's a current source? It's a voltage
source with a relatively high series resistance (as
compared to the resistance of the driven input
impedance.)


Yes. Which confirms my point.


But you CAN use a current source
to drive the base of a BJT.


Of course.


Then we agree.

Oh?

A common emmiter
BJT can be considered a Current-controlled
Current-Source,

No, well, yes, one may consider it, but the consideration would be in
error.

like almost every basic
electronic text claims.

And as which, they are all wrong. Only the Bantam paperback, layman
equivalents do this. *All* advanced physics *text* books state quite
clearly that the transistor is voltage controlled, as indeed it is. You
simply can't even get started in deriving the transistor equations using
base current as the controlling variable. Its either current, or charge
as a function of the applied voltages. Its that simple.

Reference me one, and I mean only one, *academic* level physics text
book that derives transistor equations by reference to base current as
the controlling variable.

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.

 

[Rhett Oracle]

On Thu, 25 Nov 2004 04:32:46 -0500, Active8 wrote:
....

quote from Halliday and Resnick regarding Faraday's law of
induction:

"The current that appears in this experiment is called an *induced
current* and is said to be set up by an *induced electromotive
force*.

Ah, yes. Use the Force, Luke.

;^j
R.

 

[Active8]

On Thu, 25 Nov 2004 21:47:30 +0000 (UTC), Ken Smith wrote:

Better to explain that the ideal transistor would have an infinite HFE and
that all this base current stuff is because the transistor is less than
ideal.

Good point, and it sure helps to ignore base current at times.
--
Best Regards,
Mike

 

[Paul Burridge]

On Wed, 24 Nov 2004 06:42:13 GMT, Rich Grise <rich@example.net> wrote:

No, this crosses the line. FETs have theoretically no gate current at all,
which came as a major relief to us old toob jocks. They _actually_ _are_
voltage-controlled.

Yes, they are, but they *do* run a very small reverse leakage current
in normal operation. I believe you're thinking of MOSFETs with your
theoretical no gate current at all statement. Being IGfets, they don't
conduct a gate bias current at all even when forward biased - as you
say, "theoretically" but in reality there can be, but it's
vanishingly small indeed.
Just trying not to forget what I *have* learned by re-iterating it at
every opportunity. ;-)
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Kevin Aylward]

Active8 wrote:

On Thu, 25 Nov 2004 21:47:30 +0000 (UTC), Ken Smith wrote:


Better to explain that the ideal transistor would have an infinite
HFE and that all this base current stuff is because the transistor
is less than ideal.

Good point, and it sure helps to ignore base current at times.

Indeed, except...

I once designed and built a rather large bipolar power amp. It had 15A,
150W transistors to drive the output bank of devices.

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.

 

[Active8]

On Fri, 26 Nov 2004 07:37:54 GMT, Kevin Aylward wrote:

Active8 wrote:
On Thu, 25 Nov 2004 21:47:30 +0000 (UTC), Ken Smith wrote:


Better to explain that the ideal transistor would have an infinite
HFE and that all this base current stuff is because the transistor
is less than ideal.

Good point, and it sure helps to ignore base current at times.

Indeed, except...

I once designed and built a rather large bipolar power amp. It had 15A,
150W transistors to drive the output bank of devices.

Sounds like Deep Purple. Maybe something like that guy Ray (?) from

the doors did.
--
Best Regards,
Mike

 

[Winfield Hill]

Dr. Slick wrote...

Reference me one, and I mean only one, *academic* level physics
text book that derives transistor equations by reference to base
current as the controlling variable.

The Point is:

MOSFET are insulated gates, there is theoretically no gate current
at DC.

You can't say the same for the base of a BJT.

Consider an npn differential amplifier with a 1mA current sink at
the two joined emitters, and a pair of 10k collector resistors.
Analyze the amplifier's gain using transistor beta theory, i.e.
the base current.


--
Thanks,
- Win

 

[Active8]

On 26 Nov 2004 12:24:59 -0800, Winfield Hill wrote:

Dr. Slick wrote...

Reference me one, and I mean only one, *academic* level physics
text book that derives transistor equations by reference to base
current as the controlling variable.

The Point is:

MOSFET are insulated gates, there is theoretically no gate current
at DC.

You can't say the same for the base of a BJT.

Consider an npn differential amplifier with a 1mA current sink at
the two joined emitters, and a pair of 10k collector resistors.
Analyze the amplifier's gain using transistor beta theory, i.e.
the base current.

200
--
Best Regards,
Mike

 

[Winfield Hill]

Active8 wrote...

If'n yer drivin' another diff stage. One side is showing a gain
of 174 with 3904s in PSpice. That's close to the 200 I perdicted.

Besides load impedance, one has to include Early effect for full
accuracy (spice models usually include an Early voltage estimate).

We're looking for Dr. Slick's answer, I'm curious to see what
he'll come up with.


--
Thanks,
- Win

 

[Winfield Hill]

Kevin Aylward wrote...

Winfield Hill wrote:

We're looking for Dr. Slick's answer, I'm curious
to see what he'll come up with.

I'm not

It appears Dr. Slick has retired from the scene.


--
Thanks,
- Win

 

[Paul Burridge]

On 24 Nov 2004 10:53:14 -0800, radio913@aol.com (Dr. Slick) wrote:

"Kevin Aylward" <salesEXTRACT@anasoft.co.uk> wrote in message news:<xIWod.24952$08.4646@fe2.news.blueyonder.co.uk>...


My point, once again, was that the majority
of texts DO use beta and current gain to describe
BJTs, but never to describe FETs, which have to
high of a DC gate impedance to use current gain,
so they use transconductance instead.


I agree that the majority of popular elementary texts do this. However,
they are wrong if they claim that a bipolar transistor is current
controlled. It just introduces much confusion. The most appropriate
model is a voltage controlled device with a non-linear input resistance.


Hint: What's a current source? It's a voltage
source with a relatively high series resistance (as
compared to the resistance of the driven input
impedance.)


Yes. Which confirms my point.


But you CAN use a current source
to drive the base of a BJT.

Permit me to explain. Kevin is believed to actually know all this
stuff; it's simply that he's incapable of explaining himself in simple
English (poor literacy). He even reckons he's some sort of expert. But
from what he writes here and elsewhere, it's certainly not apparent.
HTH.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Dr. Slick]

"Kevin Aylward" <salesEXTRACT@anasoft.co.uk> wrote in message news:<oh1qd.35524$F7.28298@fe1.news.blueyonder.co.uk>...

We're looking for Dr. Slick's answer, I'm curious to see what
he'll come up with.

I'm not

Tell you what, i'll give my answer as
soon as Kevin can convince Motorola, Philips, and
all the other corporations to stop including
hFE or Beta in their data sheets for bipolars.

Good thing you ain't holding your breath..

Slick