How to bias a MOSFET amp?

[Tam/WB2TT]

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

 

[Tam/WB2TT]

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

 

[Rich Grise]

---
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]

---
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.

 

[Paul Burridge]

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. :-/

 

[Ian Jackson]

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]

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.

 

[John Fields]

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 Larkin]

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]

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. ;-)

 

[John Fields]

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.
"

 

[John Larkin]

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]

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.
---

 

[Paul Burridge]

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.

 

[John Woodgate]

I read in sci.electronics.design that Paul Burridge

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.

 

[Paul Burridge]

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?

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...

 

[Paul Burridge]

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?

 

[Ralph Mowery]

But with a **tuned load**, 'linearity' can be achieved even with Class C

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.

 

[Zak]

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.

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

 

[John Fields]

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?


I won't bore you with explanations you're already well acquainted
with.

---
On the contrary, I'd like to hear why you think class AB or B isn't
(or can't be) linear, input-to-output.
---

But I'm still reeling from the revelation that you confused AM
with Class C. :-/

---
Go back and read it again in the context of "is that a smart thing to
do?" with your tongue-in-cheek detector energized.
---

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...

---
We weren't talking about components with perfectly linear transfer
functions, _you_ were alluding to deterioration of input-to-output
linearity in systems using different driver biasing schemes. Or so I
thought, when you said:

"You will be aware than linearity starts to go out of the window when
Class A slides into Class AB and beyond."

And I'll repeat:

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


But never mind, now that Woodgate's cleared it up there's no need for
you to embarrass yourself further.