How to bias a MOSFET amp?

[SioL]

"Paul Burridge" <pb@notthisbit.osiris1.co.uk> wrote in message
news:1rgvi0hffk555p4nl1403enjkgcd6rtvql@4ax.com...

On Fri, 27 Aug 2004 13:00:56 -0700, John Larkin
jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

That means, to get a linear amp, the input signal has to be converted
to PWM gate drive. That's hard to do at high frequencies. At 300 MHz,
a power mosfet doesn't much look like a high-speed switch any more.

Certainly not at that kind of frequency! But for the lower HF bands,
it's *perfectly* feasible.

You should know.

SioL

 

[Tam/WB2TT]

"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:10ivmqkhu7e3mee@corp.supernews.com...

John Fields wrote:
On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
tim@wescottnospamdesign.com> wrote:

.............................................................................

...........

Presumably you could make a REALLY EFFICIENT setup with transistors by
operating the RF final in class E, but you get that pesky capacitance
problem back...

You are only scratching the surface. Check out the Harris DX series of high
power AM transmitters. It will blow your mind away. Basically, the
instantanous RF output power is synthesized by turning on 0 to 64 fairly low
power (~KW) modules. I don't know what the sampling frequency is, but
probably >20 KHz.. All modules are driven by a square wave signal at the
carrier frequency. There is no modulator.

Tam

* So I understand, I'm just a tube wannabe.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Tam/WB2TT]

"Tim Wescott" <tim@wescottnospamdesign.com> wrote in message
news:10ivmsu6030nd33@corp.supernews.com...

Paul Burridge wrote:

On Fri, 27 Aug 2004 15:40:15 -0700, Tim Wescott
tim@wescottnospamdesign.com> wrote:


Don't operate SSB much, do you?


Nope. I'm a CWer. But the use of MOSFETs at RF for Anything other than
SSB (FM & AM in particular are ideally-suited) is as Kosher as Jim
Thompson's Saturday afternoon lunch of salt beef sandwiches with extra
dill.

Motorola used to list RF MOSFETs for linear amplifier use. I don't know
if they went to On semi or FreeFall.

I believe Tyco. Yes, that same Tyco.

Tam

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Rich Grise]

John Fields wrote:

On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
tim@wescottnospamdesign.com> wrote:


Well, AM tube finals were often operated class C with the modulation
applied to the plate supply.

---
Funny, I never considered plate modulation to be class C; that is if
we're talking about the same thing. What I'm thinking about is when
you key the transmitter on and it starts putting out a carrier at some
level, then you modulate the plate supply with audio so that at the
low peaks of the audio waveform the output of the TX is zero, but at
the output of the high peaks it's twice (?) what it was with no
modulation. Is that class C?

No, that's plate modulation. Class C is when the active element

conducts for less than 180 degrees of the cycle. A lot of times
when they plate modulate, they'll also apply the modulation to the
screen grid or even the control grid or previous stage.

Cheers!
Rich

 

[Paul Burridge]

On Fri, 27 Aug 2004 19:01:37 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
tim@wescottnospamdesign.com> wrote:


Well, AM tube finals were often operated class C with the modulation
applied to the plate supply.

---
Funny, I never considered plate modulation to be class C; that is if
we're talking about the same thing. What I'm thinking about is when
you key the transmitter on and it starts putting out a carrier at some
level, then you modulate the plate supply with audio so that at the
low peaks of the audio waveform the output of the TX is zero, but at
the output of the high peaks it's twice (?) what it was with no
modulation. Is that class C?

As you presumably know, that's just 100% AM.

--

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

 

[Paul Burridge]

On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
<jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

Have you actually built a class C linear RF power amp? Tell us how it
works.

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. :-/
--

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

 

[Ian Jackson]

In message <nbk0j0tkn4frucfi9qa4m0223ifogaj6sb@4ax.com>, Paul Burridge
<pb@notthisbit.osiris1.co.uk> writes

On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

Have you actually built a class C linear RF power amp? Tell us how it
works.

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. :-/

Don't forget that you cannot really modulate a 'linear' amplifier by
varying the supply rail (which is what 'plate & screen' mod does). The
modulated stage has to be non-linear (eg Class C) where the power output
varies as the square of the supply volts. If the PA was biassed in Class
A, there wouldn't be any modulation.
Ian.
--

 

[Paul Burridge]

On Sat, 28 Aug 2004 10:56:03 +0100, Ian Jackson
<REMOVE-THIS-BIT@g3ohx.demon.co.uk> wrote:

Don't forget that you cannot really modulate a 'linear' amplifier by
varying the supply rail (which is what 'plate & screen' mod does). The
modulated stage has to be non-linear (eg Class C) where the power output
varies as the square of the supply volts. If the PA was biassed in Class
A, there wouldn't be any modulation.

Certainly there would be no *amplitude* modulation, but that doesn't
preclude FM and various other schemes.
--

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

 

[John Fields]

On Sat, 28 Aug 2004 10:36:21 +0100, Paul Burridge
<pb@notthisbit.osiris1.co.uk> wrote:

On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

Have you actually built a class C linear RF power amp? Tell us how it
works.

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. :-/

---
Let's say that you have an audio amp with an input resistance of 1000
ohms and that, with a 1V input, it puts 10 volts across an 8 ohm load.

That's a voltage gain of

Vout 10V
Av = 20 log ------ dB = 20log ---- dB = 20dB
V in 1V

and a power gain of

Pout 12.5W
Aw = 10 log ------- dB = 10log -------- dB ~ 41dB
Pin 0.001W



Now let's say that you up the input voltage to 2V and that the output
voltage goes to 20V. That's still a voltage gain of 20dB and a power
gain 41dB.

Finally, let's say that no matter what voltage you apply to the
input,(up to some reasonable limit) the output voltage is always 10
times higher. That's a linear amplifier.


It's no different with RF.

Let's say, for example, that we have an RF amp with a 50 ohm input and
output impedance and that with a 1 watt input it puts out 10 watts
That is, it has 10 dB of power gain. If it's a linear amplifier and
we exercise its input according to the following table, the
relationships given in the table will be true. If it isn't, they
won't be.

Pin Pout Aw Ein Eout Av
W->50R W->50R dB V->50R V->50R dB
------------------------------------------
1 10 10 7.07 22.4 10
2 20 10 10.0 31.6 10
3 30 10 12.3 31.6 10
4 40 10 14.1 44.7 10
5 50 10 15.8 50.0 10
6 60 10 17.3 54.8 10
7 70 10 18.7 59.2 10
8 80 10 20.0 63.2 10
9 90 10 21.2 67.1 10
10 100 10 22.4 70.7 10

So, that should take the mystery out of why it's called a
"linear amplifier"


--
John Fields

 

[Dbowey]

jfields posted:
<< On Sat, 28 Aug 2004 10:36:21 +0100, Paul Burridge
<pb@notthisbit.osiris1.co.uk> wrote:

On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

Have you actually built a class C linear RF power amp? Tell us how it
works.

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. :-/

---
Let's say that you have an audio amp with an input resistance of 1000
ohms and that, with a 1V input, it puts 10 volts across an 8 ohm load.

That's a voltage gain of

Vout 10V
Av = 20 log ------ dB = 20log ---- dB = 20dB
V in 1V

and a power gain of

Pout 12.5W
Aw = 10 log ------- dB = 10log -------- dB ~ 41dB
Pin 0.001W



Now let's say that you up the input voltage to 2V and that the output
voltage goes to 20V. That's still a voltage gain of 20dB and a power
gain 41dB.

Finally, let's say that no matter what voltage you apply to the
input,(up to some reasonable limit) the output voltage is always 10
times higher. That's a linear amplifier.


It's no different with RF.

Let's say, for example, that we have an RF amp with a 50 ohm input and
output impedance and that with a 1 watt input it puts out 10 watts
That is, it has 10 dB of power gain. If it's a linear amplifier and
we exercise its input according to the following table, the
relationships given in the table will be true. If it isn't, they
won't be.

Pin Pout Aw Ein Eout Av
W->50R W->50R dB V->50R V->50R dB
------------------------------------------
1 10 10 7.07 22.4 10
2 20 10 10.0 31.6 10
3 30 10 12.3 31.6 10
4 40 10 14.1 44.7 10
5 50 10 15.8 50.0 10
6 60 10 17.3 54.8 10
7 70 10 18.7 59.2 10
8 80 10 20.0 63.2 10
9 90 10 21.2 67.1 10
10 100 10 22.4 70.7 10

So, that should take the mystery out of why it's called a
"linear amplifier"
A class C amp is not a "linear amp" and has never been called that within the
big circle of those who work with RF. Its very non-linearity is what permits
plate modulation; it is a mixer/multiplier - pick your favorite term.

I believe confusion comes from some books describing the process as linear, and
explaining that as being because the sidebands are an "exact" replica of the
modulating voltage.

Don

 

[John Larkin]

On Sat, 28 Aug 2004 10:36:21 +0100, Paul Burridge
<pb@notthisbit.osiris1.co.uk> wrote:

On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

Have you actually built a class C linear RF power amp? Tell us how it
works.

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. :-/

Define "linear"? You must be joking.

I'll take that as a "no" to my question. Not surprised.

John

 

[Paul Burridge]

On Sat, 28 Aug 2004 08:43:21 -0700, John Larkin
<jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

On Sat, 28 Aug 2004 10:36:21 +0100, Paul Burridge
pb@notthisbit.osiris1.co.uk> wrote:

On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

Have you actually built a class C linear RF power amp? Tell us how it
works.

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. :-/

Define "linear"? You must be joking.

I'll take that as a "no" to my question. Not surprised.

Actually I've built *several* class C RF amps, John. However, I
wouldn't call any of them linear. You will be aware than linearity
starts to go out of the window when Class A slides into Class AB and
beyond. Let's not have an argument over definitions. It's an open
invitation to John Woodgate. ;-)
--

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

 

[John Fields]

On Sat, 28 Aug 2004 10:56:03 +0100, Ian Jackson
<REMOVE-THIS-BIT@g3ohx.demon.co.uk> wrote:

In message <nbk0j0tkn4frucfi9qa4m0223ifogaj6sb@4ax.com>, Paul Burridge
pb@notthisbit.osiris1.co.uk> writes
On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

Have you actually built a class C linear RF power amp? Tell us how it
works.

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. :-/

Don't forget that you cannot really modulate a 'linear' amplifier by
varying the supply rail (which is what 'plate & screen' mod does). The
modulated stage has to be non-linear (eg Class C) where the power output
varies as the square of the supply volts.

---
Nonlinear? Yes. Class "C"? No.

From http://sound.westhost.com/class-a.htm :

"
Class-A Output device(s) conduct through 360 degrees of input cycle
(never switch off) - A single output device is possible. The device
conducts for the entire waveform in Figure 1

Class-B Output devices conduct for 180 degrees (1/2 of input cycle) -
for audio, two output devices in "push-pull" must be used (see
Class-AB)

Class-AB Halfway (or partway) between the above two examples (181 to
200 degrees typical) - also requires push-pull operation for audio.
The conduction for each output device is shown in Figure 1.

Class-C Output device(s) conduct for less than 180 degrees (100 to 150
degrees typical) - Radio Frequencies only - cannot be used for audio!
** This is the sound heard when one of the output devices goes open
circuit in an audio amp! See Figure 1, showing the time the output
device conducts (single-ended operation is assumed, and yes this does
work for RF)

Class-D Quasi-digital amplification. Uses pulse-width-modulation of a
high frequency (square wave) carrier to reproduce the audio signal -
because of frequency limitations (and the fact that they nearly all
seem to sound disgusting), many are only suitable for industrial
control of motors and loud but crappy sub-woofers (this may change if
transistors with an infinite bandwidth become available soon - yeah,
right!) All Class-D amps have a major limitation in the output filter,
whose response is highly dependent on the load impedance.
"

If the PA was biassed in Class A, there wouldn't be any modulation.

---
There could be; all that would be required would be for the gain of
the stage to vary with the modulating input. I don't believe there's
a constraint on class A biasing which inherently precludes a class A
stage from being modulated.




--
John Fields

 

[John Larkin]

On Sat, 28 Aug 2004 18:07:06 +0100, Paul Burridge
<pb@notthisbit.osiris1.co.uk> wrote:

On Sat, 28 Aug 2004 08:43:21 -0700, John Larkin
jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

On Sat, 28 Aug 2004 10:36:21 +0100, Paul Burridge
pb@notthisbit.osiris1.co.uk> wrote:

On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

Have you actually built a class C linear RF power amp? Tell us how it
works.

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. :-/

Define "linear"? You must be joking.

I'll take that as a "no" to my question. Not surprised.

Actually I've built *several* class C RF amps, John. However, I
wouldn't call any of them linear. You will be aware than linearity
starts to go out of the window when Class A slides into Class AB and
beyond.

I am not aware of any such things. So I take it you have not designed
any class C linear RF power amplifiers.

Let's not have an argument over definitions. It's an open
invitation to John Woodgate. ;-)

So, let's not have any definitions at all. Then nobody would ever be
wrong.

John

 

[John Fields]

On Sat, 28 Aug 2004 18:07:06 +0100, Paul Burridge
<pb@notthisbit.osiris1.co.uk> wrote:

Actually I've built *several* class C RF amps, John.

---
Intentionally???
---

However, I
wouldn't call any of them linear. You will be aware than linearity
starts to go out of the window when Class A slides into Class AB and
beyond.

---
Really? I'd _love_ to hear your explanation for why that "happens".

I've heard a lot of amps that sounded pretty good at both low and high
volumes, and in between, and they've almost all had class AB outputs.
---

Let's not have an argument over definitions. It's an open
invitation to John Woodgate. ;-)

---
And you don't like getting your ears pinned back?^)

--
John Fields

 

[Paul Burridge]

On Sat, 28 Aug 2004 10:27:42 -0700, John Larkin
<jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

I am not aware of any such things. So I take it you have not designed
any class C linear RF power amplifiers.

It depends on what you call "power" (here we go again). Certainly not
beyond 500mW, no, if that answers your question.

So, let's not have any definitions at all. Then nobody would ever be
wrong.

I imagine Kevin would be the major beneficiary of that measure.
--

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

 

[John Woodgate]

I read in sci.electronics.design that Paul Burridge
<pb@notthisbit.osiris1.co.uk> wrote (in <ooe1j0hk07082mnusi6nr5vqq0sntl0
40l@4ax.com&gt about 'How to bias a MOSFET amp?', on Sat, 28 Aug 2004:

It's an open invitation to John
Woodgate. ;-)

Oh, thank you, Paul. Remind me to invite you to explain something one
day.

People are using 'linear' in two different senses. For audio, in fact
for any amplifying stage with an **untuned load**, linearity requires
linearity of output current with respect to input voltage, (Class A
single ended or push-pull, Class B push-pull).

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.

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.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Paul Burridge]

On Sat, 28 Aug 2004 12:39:34 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Sat, 28 Aug 2004 18:07:06 +0100, Paul Burridge
pb@notthisbit.osiris1.co.uk> wrote:


Actually I've built *several* class C RF amps, John.

---
Intentionally???

Aha! very amusing. Most of them have been intentional, yes, but who
here can say they haven't ended up at some point with something they
hadn't bargained for?

However, I
wouldn't call any of them linear. You will be aware than linearity
starts to go out of the window when Class A slides into Class AB and
beyond.

---
Really? I'd _love_ to hear your explanation for why that "happens".

I won't bore you with explanations you're already well acquainted
with. But I'm still reeling from the revelation that you confused AM
with Class C. :-/

I've heard a lot of amps that sounded pretty good at both low and high
volumes, and in between, and they've almost all had class AB outputs.

I'm sure you have. But even class A isn't perfect. The pitfalls of
large-signal handling and all that. Do you know of an active device
with a *perfectly* linear transconductance between say 0 and 20V? No?
I thought not...
--

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

 

[Paul Burridge]

On Sat, 28 Aug 2004 19:20:06 +0100, John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

Oh, thank you, Paul. Remind me to invite you to explain something one
day.

People are using 'linear' in two different senses.

No kidding? Only two? ;-)

For audio, in fact
for any amplifying stage with an **untuned load**, linearity requires
linearity of output current with respect to input voltage, (Class A
single ended or push-pull, Class B push-pull).

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.

Okay. I'm quite happy with that. Any not?
--

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

 

[Ralph Mowery]

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.

Okay. I'm quite happy with that. Any not?
--

I am not. For a normal ham amp to be linear it can not be biased class C.
Class C will not reproduce a SSB or AM signal. It only works with constant
signal levesl such as FM or CW. The tunes circuit "rings" and reproduces
the missing portion of the sine wave of a single frequency. It can not do
this for signasl where the amplitude is constantly changing such as SSB or
AM.
As a circuit is baised from A to B to C portions of the waveform is clipped
out. Class B can be used for audio or rf if it is in a push pull circuit
so that as one device (tube or transistor) is cut off the other is
conducting on the other portion of the cycle.

The term linear is now being used incorrectly for almost any RF amp even if
the amp is biased class C. While it is not linear many use the term linear
when the word amplifier or class B or C ampifier should be used.

Any class ( A, B , C ) of amp can be plate modulated for AM. It is then
not really an amplifier.