FETs Vesus Bipolars, Why More Efficient?

D

Dr. Slick

Guest
Hi,

I've been using Mosfets for quite some
time now, and have enjoyed the greater
efficiency and gain that you get with them,
versus BJTs in VHF radio frequency applications.

I know the greater efficiency has partyly to do
with the much higher low frequency impedance
of the gate versus the diode of the base-emitter
junction, but i'd like to know the other reasons
if someone could explain them to me.

Thanks for your input.

Slick
 
In article <1d15af91.0411091335.52d72893@posting.google.com>,
Dr. Slick <radio913@aol.com> wrote:
Hi,

I've been using Mosfets for quite some
time now, and have enjoyed the greater
efficiency and gain that you get with them,
versus BJTs in VHF radio frequency applications.

I know the greater efficiency has partyly to do
with the much higher low frequency impedance
of the gate versus the diode of the base-emitter
junction, but i'd like to know the other reasons
if someone could explain them to me.

Thanks for your input.
Click to expand...

Here's one:

The storage effects in the bipolar makes you class C amplifier have a
wider conduction angle. In a MOSFET, the turn on and turn off speeds are
much more equal so the conduction time is delayed but not increased.

--
--
kensmith@rahul.net forging knowledge
 
On Wed, 10 Nov 2004 02:58:28 +0000 (UTC), Ken Smith wrote:

In article <1d15af91.0411091335.52d72893@posting.google.com>,
Dr. Slick <radio913@aol.com> wrote:
Hi,

I've been using Mosfets for quite some
time now, and have enjoyed the greater
efficiency and gain that you get with them,
versus BJTs in VHF radio frequency applications.

I know the greater efficiency has partyly to do
with the much higher low frequency impedance
of the gate versus the diode of the base-emitter
junction, but i'd like to know the other reasons
if someone could explain them to me.

Thanks for your input.


Here's one:

The storage effects in the bipolar makes you class C amplifier have a
wider conduction angle. In a MOSFET, the turn on and turn off speeds are
much more equal so the conduction time is delayed but not increased.
Click to expand...
Low Rds(on) for two - that and the aforementioned fast switching
work together to lower power dissipation... but *how much* does that
really help at VHF?
--
Best Regards,
Mike
 
On 9 Nov 2004 13:35:45 -0800, radio913@aol.com (Dr. Slick) wrote:

Hi,

I've been using Mosfets for quite some
time now, and have enjoyed the greater
efficiency and gain that you get with them,
versus BJTs in VHF radio frequency applications.

I know the greater efficiency has partyly to do
with the much higher low frequency impedance
of the gate versus the diode of the base-emitter
junction, but i'd like to know the other reasons
if someone could explain them to me.
Click to expand...
I think you meant to say "...much higher *high* frequency
impedance..."?
--

"What is now proved was once only imagin'd." - William Blake, 1793.
 
Paul Burridge <pb@notthisbit.osiris1.co.uk> wrote in message news:<fj34p01doaa15o5jfg7612vism2fg58uln@4ax.com>...
On 9 Nov 2004 13:35:45 -0800, radio913@aol.com (Dr. Slick) wrote:

Hi,

I've been using Mosfets for quite some
time now, and have enjoyed the greater
efficiency and gain that you get with them,
versus BJTs in VHF radio frequency applications.

I know the greater efficiency has partyly to do
with the much higher low frequency impedance
of the gate versus the diode of the base-emitter
junction, but i'd like to know the other reasons
if someone could explain them to me.

I think you meant to say "...much higher *high* frequency
impedance..."?
Click to expand...

No. The gate is like a capacitor in a FET, so at
higher frequencies, the reactance of the cap is small,
and you only have the ohmic (real) losses. At lower
frequencies and DC, the capacitive reactance is very
high. Xc=1/(2Pi*F*C), where F is the frequency in
cycles/sec.


Slick
 
In article <1d15af91.0411101127.177fb0f2@posting.google.com>,
Dr. Slick <radio913@aol.com> wrote:
[....]
No. The gate is like a capacitor in a FET, so at
higher frequencies, the reactance of the cap is small,
and you only have the ohmic (real) losses.
Click to expand...
Don't forget those inductances.
--
--
kensmith@rahul.net forging knowledge
 
On 10 Nov 2004 11:27:48 -0800, radio913@aol.com (Dr. Slick) wrote:

No. The gate is like a capacitor in a FET, so at
higher frequencies, the reactance of the cap is small,
and you only have the ohmic (real) losses. At lower
frequencies and DC, the capacitive reactance is very
high. Xc=1/(2Pi*F*C), where F is the frequency in
cycles/sec.
Click to expand...
Mea culpa. I thought you were referring to the signal impedance across
the B/E diode in a BJT.
--

"What is now proved was once only imagin'd." - William Blake, 1793.
 
Paul Burridge <pb@notthisbit.osiris1.co.uk> wrote in message news:<u4t6p0pv7jtdsimd88jb959fkgm437pge2@4ax.com>...
On 10 Nov 2004 11:27:48 -0800, radio913@aol.com (Dr. Slick) wrote:

No. The gate is like a capacitor in a FET, so at
higher frequencies, the reactance of the cap is small,
and you only have the ohmic (real) losses. At lower
frequencies and DC, the capacitive reactance is very
high. Xc=1/(2Pi*F*C), where F is the frequency in
cycles/sec.

Mea culpa. I thought you were referring to the signal impedance across
the B/E diode in a BJT.
Click to expand...

Well, i kind of am, in the sense that the base-emmiter
diode has significant current across it past the threshold
voltage, whereas the FETs have more of a capacitor for the
gate-source where very little DC current flows.

That's why a BJT is more like a current-controlled-
current-source, and the FETs are more like voltage-controlled
-current-sources.


S
 
kensmith@green.rahul.net (Ken Smith) wrote in message news:<cmupu4$2d2$4@blue.rahul.net>...
In article <1d15af91.0411101127.177fb0f2@posting.google.com>,
Dr. Slick <radio913@aol.com> wrote:
[....]
No. The gate is like a capacitor in a FET, so at
higher frequencies, the reactance of the cap is small,
and you only have the ohmic (real) losses.

Don't forget those inductances.
--
Click to expand...
yeah, the series parasitic inductance too, but
you have to get into UHF or microwaves before
they get bad.


S.
 
In article <1d15af91.0411111200.59a478be@posting.google.com>,
Dr. Slick <radio913@aol.com> wrote:
kensmith@green.rahul.net (Ken Smith) wrote in message
news:<cmupu4$2d2$4@blue.rahul.net>...
In article <1d15af91.0411101127.177fb0f2@posting.google.com>,
Dr. Slick <radio913@aol.com> wrote:
[....]
No. The gate is like a capacitor in a FET, so at
higher frequencies, the reactance of the cap is small,
and you only have the ohmic (real) losses.

Don't forget those inductances.
--

yeah, the series parasitic inductance too, but
you have to get into UHF or microwaves before
they get bad.
Click to expand...
Depends on what you call bad. With higher power MOSFETs (BLF276 IIRC) the
input impedance hits zero angle before 200MHz so at 100MHz, it will be
effecting the impedance.

--
--
kensmith@rahul.net forging knowledge
 
In article <1d15af91.0411111158.5ac0b07e@posting.google.com>,
Dr. Slick <radio913@aol.com> wrote:
Paul Burridge <pb@notthisbit.osiris1.co.uk> wrote in message
news:<u4t6p0pv7jtdsimd88jb959fkgm437pge2@4ax.com>...
On 10 Nov 2004 11:27:48 -0800, radio913@aol.com (Dr. Slick) wrote:

No. The gate is like a capacitor in a FET, so at
higher frequencies, the reactance of the cap is small,
and you only have the ohmic (real) losses. At lower
frequencies and DC, the capacitive reactance is very
high. Xc=1/(2Pi*F*C), where F is the frequency in
cycles/sec.

Mea culpa. I thought you were referring to the signal impedance across
the B/E diode in a BJT.


Well, i kind of am, in the sense that the base-emmiter
diode has significant current across it past the threshold
voltage, whereas the FETs have more of a capacitor for the
gate-source where very little DC current flows.

That's why a BJT is more like a current-controlled-
current-source, and the FETs are more like voltage-controlled
-current-sources.
Click to expand...
At high frequencies, both look like charge controlled current sources.
The normal base current of a bipolar is a leak that takes the charge away.

--
--
kensmith@rahul.net forging knowledge
 
On 11 Nov 2004 11:58:44 -0800, radio913@aol.com (Dr. Slick) wrote:

Well, i kind of am, in the sense that the base-emmiter
diode has significant current across it past the threshold
voltage, whereas the FETs have more of a capacitor for the
gate-source where very little DC current flows.

That's why a BJT is more like a current-controlled-
current-source, and the FETs are more like voltage-controlled
-current-sources.
Click to expand...
I can see where you're comin' from; but you're gonna upset a lot of
folks here with that last para!
--

"What is now proved was once only imagin'd." - William Blake, 1793.
 
kensmith@green.rahul.net (Ken Smith) wrote in message news:<cn0udv$2tg$2@blue.rahul.net>...
yeah, the series parasitic inductance too, but
you have to get into UHF or microwaves before
they get bad.

Depends on what you call bad. With higher power MOSFETs (BLF276 IIRC) the
input impedance hits zero angle before 200MHz so at 100MHz, it will be
effecting the impedance.
Click to expand...
Parasitic inductance is usually less than about 2nH,
so it would be about XL=2*pi*f*L

So the inductive reactance would be about 1.25 Ohms
at 100 MHz. Certainly affecting the impedance, but
still pretty low.


S.
 
Dr. Slick wrote...
Why do you think that? CCCS for bipolars and VCCS for
FETs is standard in most texts.

That's why when you look at a I-V curves, for BJTs the
lines are denoted with base current, and for FETs the lines
are denoted with gate voltage.
Click to expand...
Nope.


--
Thanks,
- Win
 
On 12 Nov 2004 12:30:32 -0800, radio913@aol.com (Dr. Slick) wrote:

Paul Burridge <pb@notthisbit.osiris1.co.uk> wrote in message news:<br38p0hv4gv9h9rjr52f0f54t2aokg4ur6@4ax.com>...
On 11 Nov 2004 11:58:44 -0800, radio913@aol.com (Dr. Slick) wrote:

Well, i kind of am, in the sense that the base-emmiter
diode has significant current across it past the threshold
voltage, whereas the FETs have more of a capacitor for the
gate-source where very little DC current flows.

That's why a BJT is more like a current-controlled-
current-source, and the FETs are more like voltage-controlled
-current-sources.

I can see where you're comin' from; but you're gonna upset a lot of
folks here with that last para!


Why do you think that? CCCS for bipolars and VCCS for
FETs is standard in most texts.
Click to expand...
FETs are certainly accurately modelled as VCCS in a certain regions of
operation. BJTs are *inaccurately* modelled as CCCS. Vbe's what you
really want to look at with BJTs.
--

"What is now proved was once only imagin'd." - William Blake, 1793.
 
"Paul Burridge" <pb@notthisbit.osiris1.co.uk> a écrit dans le message de
news:16gap0138huajj8b3k0abf2h76opodmd39@4ax.com...
On 12 Nov 2004 12:30:32 -0800, radio913@aol.com (Dr. Slick) wrote:

Paul Burridge <pb@notthisbit.osiris1.co.uk> wrote in message
news:<br38p0hv4gv9h9rjr52f0f54t2aokg4ur6@4ax.com>...
On 11 Nov 2004 11:58:44 -0800, radio913@aol.com (Dr. Slick) wrote:

Well, i kind of am, in the sense that the base-emmiter
diode has significant current across it past the threshold
voltage, whereas the FETs have more of a capacitor for the
gate-source where very little DC current flows.

That's why a BJT is more like a current-controlled-
current-source, and the FETs are more like voltage-controlled
-current-sources.

I can see where you're comin' from; but you're gonna upset a lot of
folks here with that last para!


Why do you think that? CCCS for bipolars and VCCS for
FETs is standard in most texts.

FETs are certainly accurately modelled as VCCS in a certain regions of
operation. BJTs are *inaccurately* modelled as CCCS. Vbe's what you
really want to look at with BJTs.
Click to expand...
Please, tell us more :)


--
Thanks,
Fred.
 
kensmith@green.rahul.net (Ken Smith) wrote in message news:<cn2n2r$sun$2@blue.rahul.net>...

Parasitic inductance is usually less than about 2nH,
so it would be about XL=2*pi*f*L

So the inductive reactance would be about 1.25 Ohms
at 100 MHz. Certainly affecting the impedance, but
still pretty low.

Instead of counting on my memory. I dug out the BLF276's data sheet.

At 100MHz Zin = 1.3 -j1.7 (points taken from graph)

A 1.25Ohms Xl would be very important in this case since it is about as
big as R and Xc. Th package on this is a 6 pins as source configuration.
Click to expand...
mag[Z]=sqrt(1.3**2+1.7**2)

so it's still going to be low impedance.

S.
 
Winfield Hill <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote in message news:<cn38r009jo@drn.newsguy.com>...
Dr. Slick wrote...

Why do you think that? CCCS for bipolars and VCCS for
FETs is standard in most texts.

That's why when you look at a I-V curves, for BJTs the
lines are denoted with base current, and for FETs the lines
are denoted with gate voltage.

Nope.
Click to expand...
Not a very convincing argument!
 
Paul Burridge <pb@notthisbit.osiris1.co.uk> wrote in message news:<16gap0138huajj8b3k0abf2h76opodmd39@4ax.com>...
On 12 Nov 2004 12:30:32 -0800, radio913@aol.com (Dr. Slick) wrote:



Well, i kind of am, in the sense that the base-emmiter
diode has significant current across it past the threshold
voltage, whereas the FETs have more of a capacitor for the
gate-source where very little DC current flows.

That's why a BJT is more like a current-controlled-
current-source, and the FETs are more like voltage-controlled
-current-sources.


Why do you think that? CCCS for bipolars and VCCS for
FETs is standard in most texts.

FETs are certainly accurately modelled as VCCS in a certain regions of
operation. BJTs are *inaccurately* modelled as CCCS. Vbe's what you
really want to look at with BJTs.
Click to expand...

Well, it can be either base current or Vbe, obviously.

But you talk about the "Beta current gain factor" as
(delta Collector current)/(delta base current), and they
don't describe FETs like this.

However, you are correct that "gm" or "transconductance"
can be either dIdrain/dVgate or dIc/dVbe.

The point is that MOSFETs have far less gate current
than do BJTs have base current.


Slick
 
Dr. Slick wrote...
The point is that MOSFETs have far less gate current
than do BJTs have base current.
Click to expand...
At high frequencies you may find just the opposite is true.


--
Thanks,
- Win
 
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