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How to bias a MOSFET amp?

A

Active8

Jan 1, 1970
0
On Sat, 28 Aug 2004 19:20:06 +0100, John Woodgate wrote:

With a **tuned load**, the output power also depends more or less
linearly on the supply voltage, so amplitude modulation can be achieved
by varying the supply voltage.

Just an obsevation... when you enclose a word or words in double
asterisks, my reader no longer renders them in bold, thus removing
the intended emphasis. IIRC it's the double underscore enclosure
that causes my reader to underline the text.

We now return you to our regularly scheduled terminology debate.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Active8 <[email protected]>
Just an obsevation... when you enclose a word or words in double
asterisks, my reader no longer renders them in bold, thus removing the
intended emphasis. IIRC it's the double underscore enclosure that causes
my reader to underline the text.

You should use a plain text reader, as Manitou intended.(;-)
 
D

Dave VanHorn

Jan 1, 1970
0
Dad just bought a 2kW FET amplifier. It's ex-industrial use, but has MRF
line transistors, and we expect no trouble putting it online for 160-10
meter use.
It's a dozen MRF-150s

Look up the specs (and the prices (OUCH)) on the MRF-154.
 
K

Ken Smith

Jan 1, 1970
0
John Woodgate said:
With a **tuned load**, the output power also depends more or less
linearly on the supply voltage, so amplitude modulation can be achieved
by varying the supply voltage.


Small quibble:

you need to add "for a large enough drive" to the above. For small drive
levels the drive and the device gm control the output power.
 
K

Ken Smith

Jan 1, 1970
0
John Woodgate said:
But with a **tuned load**, 'linearity' can be achieved even with Class C
biasing. This is why linearity in this case is defined as output power
being proportional to input power.

I think this is right:

If this isn't the slow modulation case, for a single device RF output
stage to work as a linear you need to bias it to about the point where gm
is 1/2 the "mid current" gm. This makes the stage class AB.

If you want to go towards class B or C there are things you can do in the
slow modulation case. Basically as the RF input increases, you move into
class C and as it decreases you move back towards AB. Moving to class C
lowers the output power for large signals a bit so the AB class's standing
current can be reduced by some amount.
 
K

Ken Smith

Jan 1, 1970
0
Ralph Mowery said:
Any class ( A, B , C ) of amp can be plate modulated for AM. It is then
not really an amplifier.

I disagree with this. If the stage puts out more RF than it takes in, it
is an amplifier even if the purpose of the stage is to be a modulator.
 
T

Tim Wescott

Jan 1, 1970
0
Paul said:
It depends on how you define "linear" basically. But the term is a
total misnomer in RF amp terminology and very misleading. I can't
understand how it got there. :-/

From basic systems theory:

A system S is linear if and only if for any two input signals x1 and x2
that generate the output signals y1 = S(x1) and y2 = S(x2), and for any
two real constants A1 and A2, the output signal y = S(A1*x1 + A2*x2) is
equal to A1*y1 + A2*y2.

This condition is approached with a properly adjusted RF linear
amplifier, even one operated class AB or B. It is _not_ approached with
a class C amplifier.

So if you define "linear" the way electronics engineering professionals
define "linear" a class A, AB or B amplifier can be made to act
linearly, more or less, and a class C amplifier cannot. So the term
isn't a misnomer, and its use is obvious.
 
T

Tam/WB2TT

Jan 1, 1970
0
Zak said:
There is also a fractional stage - a 64 stage AM modulation would sound
quite nasty, so an analogue signal is added to make up.

There is also a 'spare stage' dthat can be switched in if one of the
stages fails.

Nice stuff, but the only really interesting thing IMO is the output
combiner. The rest is just 'how do we make this digital'.


Thomas

Yeah, exactly what I was wondering about, how do they combine the outputs?
They imply that they are combining power, but I suspect they are adding
voltages. BTW, the modules are not all the same. As I recall fom the WLW web
site, 60 are 1-2 KW, 1 is 500W, 2 are 200W, and 1 is 100W. They claim
something like 90% efficiency from 3 phase 60~ in to RF out. Also, there
must be some huge bandpass filter on the output to suppress the sampling
frequency.

Tam
 
D

Dbowey

Jan 1, 1970
0
kensmith posted:
Ralph Mowery said:
Any class ( A, B , C ) of amp can be plate modulated for AM. It is then
not really an amplifier.

I disagree with this. If the stage puts out more RF than it takes in, it is an
amplifier even if the purpose of the stage is to be a modulator.
I believe that should say "...it is an amplifier even if the purpose of the
stage is to be modulated. The modulator is considered to be the Audio stages.

Don
 
L

legg

Jan 1, 1970
0
It depends on how you define "linear" basically. But the term is a
total misnomer in RF amp terminology and very misleading. I can't
understand how it got there. :-/

I'd always understood that it refered to an operating frequency range
with a wide bandwidth, therefore suited to FM or frequency hopping.

RL
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Tim Wescott
A system S is linear if and only if for any two input signals x1 and x2
that generate the output signals y1 = S(x1) and y2 = S(x2), and for any
two real constants A1 and A2, the output signal y = S(A1*x1 + A2*x2) is
equal to A1*y1 + A2*y2.

This condition is approached with a properly adjusted RF linear
amplifier, even one operated class AB or B. It is _not_ approached with
a class C amplifier.

The point that the discussion has reached is that the term 'linear' IS
used in a different sense that includes Class C, whether we like it or
not.
So if you define "linear" the way electronics engineering professionals
define "linear" a class A, AB or B amplifier can be made to act
linearly, more or less, and a class C amplifier cannot. So the term
isn't a misnomer, and its use is obvious.

These are the 'electronic engineering professionals' who have 49
different definitions of 'level' in the International Electrotechnical
Vocabulary? (;-)
 
P

Paul Burridge

Jan 1, 1970
0
I disagree with this. If the stage puts out more RF than it takes in, it
is an amplifier

By that definition, it could also be an oscillator!
 
P

Paul Burridge

Jan 1, 1970
0
If this isn't the slow modulation case, for a single device RF output
stage to work as a linear you need to bias it to about the point where gm
is 1/2 the "mid current" gm. This makes the stage class AB.

If you want to go towards class B or C there are things you can do in the
slow modulation case. Basically as the RF input increases, you move into
class C and as it decreases you move back towards AB. Moving to class C
lowers the output power for large signals a bit so the AB class's standing
current can be reduced by some amount.

Can't agree. If you're running any amount of standing current, then it
ain't class C.
 
P

Paul Burridge

Jan 1, 1970
0
I'd always understood that it refered to an operating frequency range
with a wide bandwidth, therefore suited to FM or frequency hopping.

Curious. I have a book (by an author whose name I can't recall off
hand) called "High-Linearity RF Amplifier Design" in which said author
claims that a linear RF amplifier is one in which the transmitted
signal, regardless of however non-linear it may be, is passed through
sufficient filtering to clean it up! Work that one out if you can!
 
K

Ken Smith

Jan 1, 1970
0
By that definition, it could also be an oscillator!

If you are putting RF in and getting RF out at the same frequency, in any
reasonable case the circuit is acting as an amplifier. The circuit may
well oscillate when no input is applied. This sort of amplifier was very
common in the past and still is somewhat common.

A super-regen receiver is the most obvious example. Many tube based FM
receiver designs had a FM detector that would oscillate with no input
signal. The "burst lock oscillator" in a TV is in fact a very narrow pass
filter and amplifier when there is a burst to lock to. With no burst it
oscillates.
 
K

Ken Smith

Jan 1, 1970
0
Can't agree. If you're running any amount of standing current, then it
ain't class C.

If the conduction angle is less than 180 degrees it is class C. Note that
I said "you move into class C" this means the amplifier isn't always
running as class C. It only runs in class C with large signals. Since
the large signal case is were the losses really matter, this sort of
design will work quite a bit better than a simple class AB circuit.
 
J

John Larkin

Jan 1, 1970
0
Can't agree. If you're running any amount of standing current, then it
ain't class C.

Tube class-C amps often self-bias (via grid rectification) when
operating normally. Without drive, they often have high, sometimes
dangerous, idle currents.

John
 
P

Paul Burridge

Jan 1, 1970
0
Tube class-C amps often self-bias (via grid rectification) when
operating normally. Without drive, they often have high, sometimes
dangerous, idle currents.

Toobz? I was talking of transistors. Many tranny designs for Class C
RF utilize self-bias, too, but there is still no quiescent current!
 
J

John Larkin

Jan 1, 1970
0
Toobz? I was talking of transistors. Many tranny designs for Class C
RF utilize self-bias, too, but there is still no quiescent current!


What makes an amp Class C is a low conduction angle while operating.
When it's not operating, is has no class at all.

John
 
J

John Fields

Jan 1, 1970
0
What makes an amp Class C is a low conduction angle while operating.
When it's not operating, is has no class at all.
 
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