Anyone have any thoughts on this? Just a general question. Why or why not

On 10 Jan 2004 13:52:28 -0800, (Frank) wrote:

Anyone have any thoughts on this? Just a general question. Why or why not

Nope. They both have their strenghts and weaknesses in different
applications.
Certainly I'd say FETs are easier to understand in terms of how they
work; being more 'valve-like' than the BJT. But that's about it.
--

My opinion is worth what you've paid for it.

Frank ) writes:
Anyone have any thoughts on this? Just a general question. Why or why not

But what makes you ask the question?

When the transistor came along, almost right away they started to
be used in equipment. This despite the fact that those early bipolar
transistors were not all that suitable for some of the applications.
The FET came along in the early sixties, and it was seen as the salvation,
as the way of the future. But to some extent it was an illusion. The FET
was good for some things, and did have better strong signal handling
ability than the bipolars of the day. But, the bipolar and applying
it just hadn't moved far enough along the curve. By 1971, the ham magazines
(and obviously the professional literature) was talking in terms of
strong signal bipolars, using power transistors and biasing them
almost as if they were being used in transmitters despite the application
being for receive. And of course, better bipolars were coming along,
that gave good performance at the higher frequencies.

Somewhere in there, the schottky diode started appearing in practical
circuits, and obviously it made for a good mixer

The MOSFET came along, late sixties or early seventies, and it was seen
as an extension of the FET, in that they were both high input impedance
devices, and were seen as the future. They were seen as the future,
but that was thirty years ago.

Nowadays, MOSFETs for RF are relatively uncommon. You can do what
you need with bipolar transistors, or in the case of mixers (which
was seen as a prime use for dual-gate MOSFETs), passive diode mixers
are seen as the way to go.

If you'd asked this question thirty or even twenty years ago, it would
seem like an opportune question. MOSFETs were still relatively new,
and they seemed like a solution. But their use in RF circuits peaked
more or less. So thirty years later, the question doesn't seem to
be one to ask, since enough time has gone by that we know they will
not take over from bipolar transistors.

Michael VE2BVW

"Michael Black" wrote in message
...

Nowadays, MOSFETs for RF are relatively uncommon.

That would be news to manufacturers of broadcast transmitters.

I think that you'll find that the usual solid state AM transmitter is a huge
D-to-A converter consisting of lots of MOSFETS. The 50-kW Harris DX-50 uses
that technique and is just about the "standard" 50-kW rig. It is used by
many stations. The transistors are in modules that can be hot-swapped while
the rig is on the air. Distortion is within limits even with a few of the
modules missing.

MOSFETs are also used as the output amplifiers in high power FM broadcast
100-MHz transmitters - certainly up to the 10-kW level.

"Michael Black" wrote in message
...

Nowadays, MOSFETs for RF are relatively uncommon.

That would be news to manufacturers of broadcast transmitters.

I think that you'll find that the usual solid state AM transmitter is a huge
D-to-A converter consisting of lots of MOSFETS. The 50-kW Harris DX-50 uses
that technique and is just about the "standard" 50-kW rig. It is used by
many stations. The transistors are in modules that can be hot-swapped while
the rig is on the air. Distortion is within limits even with a few of the
modules missing.

MOSFETs are also used as the output amplifiers in high power FM broadcast
100-MHz transmitters - certainly up to the 10-kW level.

Besides the reasons already given for RF, in power electronics, the Ron of
a BJT is far lower than a MOSFET. There is a melding of the two
technologies, called the IGBT (Insulated Gate Bipolar Transistor), but
they are only good up to a few dozens of KHz where true power bipolars can
switch ealily into the tens of MHz.

For audio and instrument applications where noise levels need to be below
2nV/Hz, MOSFETS can't even go there (without being dunked in cryogenic
fluid).

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca

On 10 Jan 2004 13:52:28 -0800, (Frank) wrote:

Anyone have any thoughts on this? Just a general question. Why or why not

Nope. They both have their strenghts and weaknesses in different
applications.
Certainly I'd say FETs are easier to understand in terms of how they
work; being more 'valve-like' than the BJT. But that's about it.
--

My opinion is worth what you've paid for it.

Frank ) writes:
Anyone have any thoughts on this? Just a general question. Why or why not

But what makes you ask the question?

When the transistor came along, almost right away they started to
be used in equipment. This despite the fact that those early bipolar
transistors were not all that suitable for some of the applications.
The FET came along in the early sixties, and it was seen as the salvation,
as the way of the future. But to some extent it was an illusion. The FET
was good for some things, and did have better strong signal handling
ability than the bipolars of the day. But, the bipolar and applying
it just hadn't moved far enough along the curve. By 1971, the ham magazines
(and obviously the professional literature) was talking in terms of
strong signal bipolars, using power transistors and biasing them
almost as if they were being used in transmitters despite the application
being for receive. And of course, better bipolars were coming along,
that gave good performance at the higher frequencies.

Somewhere in there, the schottky diode started appearing in practical
circuits, and obviously it made for a good mixer

The MOSFET came along, late sixties or early seventies, and it was seen
as an extension of the FET, in that they were both high input impedance
devices, and were seen as the future. They were seen as the future,
but that was thirty years ago.

Nowadays, MOSFETs for RF are relatively uncommon. You can do what
you need with bipolar transistors, or in the case of mixers (which
was seen as a prime use for dual-gate MOSFETs), passive diode mixers
are seen as the way to go.

If you'd asked this question thirty or even twenty years ago, it would
seem like an opportune question. MOSFETs were still relatively new,
and they seemed like a solution. But their use in RF circuits peaked
more or less. So thirty years later, the question doesn't seem to
be one to ask, since enough time has gone by that we know they will
not take over from bipolar transistors.

Michael VE2BVW

Besides the reasons already given for RF, in power electronics, the Ron of
a BJT is far lower than a MOSFET. There is a melding of the two
technologies, called the IGBT (Insulated Gate Bipolar Transistor), but
they are only good up to a few dozens of KHz where true power bipolars can
switch ealily into the tens of MHz.

For audio and instrument applications where noise levels need to be below
2nV/Hz, MOSFETS can't even go there (without being dunked in cryogenic
fluid).

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca