How to bias a MOSFET amp?

[Bill N2CQR MOHBR]

I'm trying to improve my limited understanding of how RF amps operate.
I've been studying "Solid State Design for the Radio Amateur" and
Experimental Methods in RF Design"

SSDRA has a very helpful section that asks the reader to calculate max
output power for a Class A RF amp (common emitter). They ask the
reader to consider maximum voltage and current swings in the collector
circuit that will keep the output linear. Basically, my understanding
is that (with an RF choke in the Vcc line) max peak signal voltage on
collector is Vc-Ve. Max peak current is the standing or quiescent
current. In this way when the collector voltage is hitting its peak
collector current is dropping almost to zero.

While the SSDRA example uses BJTs, I'm guessing that essentially the
same restrictions would apply to the drain circuit of a MOSFET amp.

Here's my problem: When I look at MOSFET amp circuits in the
literature, they often have linear amps with 12 volts on the drain,
but with standing currents of only around 40 milliamps. And they
claim 7 watts out. How can that be? Using the analysis outlined
above, I'd think that you'd get max output of 12*.040 = .48 watts.

Here's an example: I've been looking at Farhan's very FB SSB
Transceiver
http://farhan.net.co.nr/xcvr1.html

I'm trying to understand the biasing on his IRF510 final, and the RF
output he's getting.

He says he measures 20-24 volts peak RF across a 50 ohm load at the
output. That's about 8 watts peak output.

He's using 12 volt supply, and recommends setting the idle current
through the MOSFET at 80 ma. Can that be right? According to my
reading of Solid State Design for the Radio Amateur (SSDRA)(page 23)
with a 12 volt supply we can expect peak signal voltage at the Drain
of around 12 volts (with an RF choke in Vcc line). Peak current could
be max 80 ma.(maintaining Class A). Under these biasing conditions,
assuming Class A operation, max output power of .96 watts would be
provided by a load of 150 ohms.

Even if he were to be running this amp Class B (or close to it), I
can't see how he'd get 8 watts out with only 80 milliamps of standing
current

I'm very new to this kind of analysis, and strongly suspect that I'm
misreading either SSDRA or Farhan's excellent article.

Can someone please let me know where this apparent discrepency is
coming from.

Thanks and 73

Bill N2CQR M0HBR CU2JL
http://planeta.clix.pt/n2cqr

[legg]

On 26 Aug 2004 22:40:36 -0700, (Bill N2CQR
MOHBR) wrote:

>I'm trying to improve my limited understanding of how RF amps operate.
>I've been studying "Solid State Design for the Radio Amateur" and
>Experimental Methods in RF Design"

Keep studying.

The static bias is the zero-signal drain current and your calculation
is for static DC loss. Signals applied to the gate can add to the
drain current, as well as subtract.

RF final amps are unlikely to be biased classA. Class B and C will
have the final acting more like a switch. Current is imited by the
load impedance - which in this case is a complex impedance that looks
like 50 ohms at the output only.

As to the answer for your posting - the fet in this case is biased by
varying gate static voltage using the variable resistor in the
schematic, as instructed.

RL

[John Walton]

It's a bias voltage, rather than a bias current. The MOSFET enters its
linear conducting area after a certain threshold voltage is attained.

"Bill N2CQR MOHBR" > wrote in message
om...
> I'm trying to improve my limited understanding of how RF amps operate.
> I've been studying "Solid State Design for the Radio Amateur" and
> Experimental Methods in RF Design"
>
> SSDRA has a very helpful section that asks the reader to calculate max
> output power for a Class A RF amp (common emitter). They ask the
> reader to consider maximum voltage and current swings in the collector
> circuit that will keep the output linear. Basically, my understanding
> is that (with an RF choke in the Vcc line) max peak signal voltage on
> collector is Vc-Ve. Max peak current is the standing or quiescent
> current. In this way when the collector voltage is hitting its peak
> collector current is dropping almost to zero.
>
> While the SSDRA example uses BJTs, I'm guessing that essentially the
> same restrictions would apply to the drain circuit of a MOSFET amp.
>
> Here's my problem: When I look at MOSFET amp circuits in the
> literature, they often have linear amps with 12 volts on the drain,
> but with standing currents of only around 40 milliamps. And they
> claim 7 watts out. How can that be? Using the analysis outlined
> above, I'd think that you'd get max output of 12*.040 = .48 watts.
>
> Here's an example: I've been looking at Farhan's very FB SSB
> Transceiver
> http://farhan.net.co.nr/xcvr1.html
>
> I'm trying to understand the biasing on his IRF510 final, and the RF
> output he's getting.
>
> He says he measures 20-24 volts peak RF across a 50 ohm load at the
> output. That's about 8 watts peak output.
>
> He's using 12 volt supply, and recommends setting the idle current
> through the MOSFET at 80 ma. Can that be right? According to my
> reading of Solid State Design for the Radio Amateur (SSDRA)(page 23)
> with a 12 volt supply we can expect peak signal voltage at the Drain
> of around 12 volts (with an RF choke in Vcc line). Peak current could
> be max 80 ma.(maintaining Class A). Under these biasing conditions,
> assuming Class A operation, max output power of .96 watts would be
> provided by a load of 150 ohms.
>
> Even if he were to be running this amp Class B (or close to it), I
> can't see how he'd get 8 watts out with only 80 milliamps of standing
> current
>
> I'm very new to this kind of analysis, and strongly suspect that I'm
> misreading either SSDRA or Farhan's excellent article.
>
> Can someone please let me know where this apparent discrepency is
> coming from.
>
> Thanks and 73
>
> Bill N2CQR M0HBR CU2JL
> http://planeta.clix.pt/n2cqr

[John Larkin]

On 26 Aug 2004 22:40:36 -0700, (Bill N2CQR
MOHBR) wrote:


>
>Even if he were to be running this amp Class B (or close to it), I
>can't see how he'd get 8 watts out with only 80 milliamps of standing
>current
>
>I'm very new to this kind of analysis, and strongly suspect that I'm
>misreading either SSDRA or Farhan's excellent article.
>
>Can someone please let me know where this apparent discrepency is
>coming from.
>
>Thanks and 73
>
>Bill N2CQR M0HBR CU2JL
>http://planeta.clix.pt/n2cqr


I think the RF guys (I'm not one!) call an amplifier "linear" if the
RF output amplitude follows the input drive amplitude. You can do this
with a transistor that has very low quiescent bias. So "linear" does
not mean "class A" to them. The key here is that an RF amp has a tuned
output, whereas an audio amp doesn't. So the lopsided bias would
normally produce intolerable distortion in something like audio, but
the tuned output circuit changes the pulsey-looking collector/drain
current back into a nice sine wave. So you don't need a lot of idle
current, and the transistor really amplifies half of the incoming sine
cycle.

Most mosfets are pretty nicely linear (ie, straight-line Ic/Vd curve)
beyond the initial knee. You could get gobs of watts at zero standing
current, but then you'd have some zero-clipping (no output) for the
smallest drive levels, so a little idle current helps.

John

[Paul Burridge]

On Fri, 27 Aug 2004 08:07:02 -0700, John Larkin
> wrote:

>I think the RF guys (I'm not one!) call an amplifier "linear" if the
>RF output amplitude follows the input drive amplitude. You can do this
>with a transistor that has very low quiescent bias. So "linear" does
>not mean "class A" to them. The key here is that an RF amp has a tuned
>output, whereas an audio amp doesn't. So the lopsided bias would
>normally produce intolerable distortion in something like audio, but
>the tuned output circuit changes the pulsey-looking collector/drain
>current back into a nice sine wave. So you don't need a lot of idle
>current, and the transistor really amplifies half of the incoming sine
>cycle.
>
>Most mosfets are pretty nicely linear (ie, straight-line Ic/Vd curve)
>beyond the initial knee. You could get gobs of watts at zero standing
>current, but then you'd have some zero-clipping (no output) for the
>smallest drive levels, so a little idle current helps.

The only sensible way to do it AFAICS is to operate the MOSFET in
class C as a high speed switch and reconstruct the pulsed output into
a sine wave carrier by means of a suitable tuned circuit. I wouldn't
consider driving a MOSFET for RF use in any other way. The efficiency
should be pretty darned good, too.
--

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

[John Fields]

On Fri, 27 Aug 2004 19:28:39 +0100, Paul Burridge
> wrote:

>On Fri, 27 Aug 2004 08:07:02 -0700, John Larkin
> wrote:
>
>>I think the RF guys (I'm not one!) call an amplifier "linear" if the
>>RF output amplitude follows the input drive amplitude. You can do this
>>with a transistor that has very low quiescent bias. So "linear" does
>>not mean "class A" to them. The key here is that an RF amp has a tuned
>>output, whereas an audio amp doesn't. So the lopsided bias would
>>normally produce intolerable distortion in something like audio, but
>>the tuned output circuit changes the pulsey-looking collector/drain
>>current back into a nice sine wave. So you don't need a lot of idle
>>current, and the transistor really amplifies half of the incoming sine
>>cycle.
>>
>>Most mosfets are pretty nicely linear (ie, straight-line Ic/Vd curve)
>>beyond the initial knee. You could get gobs of watts at zero standing
>>current, but then you'd have some zero-clipping (no output) for the
>>smallest drive levels, so a little idle current helps.
>
>The only sensible way to do it AFAICS is to operate the MOSFET in
>class C as a high speed switch and reconstruct the pulsed output into
>a sine wave carrier by means of a suitable tuned circuit. I wouldn't
>consider driving a MOSFET for RF use in any other way. The efficiency
>should be pretty darned good, too.

---
That doesn't make any sense to me.

Unless things have changed pretty drastically from how they were when
I was doing RF, class "C" was pretty much relegated to FM, so that
when you hit PTT, you banged the hell out of the final and filtered
the hell out of the carrier, which went to maximum amplitude and
stayed there, and the information was put on the constant amplitude
carrier by varying its frequency (or phase).

AM and SSB finals were _always_ linear amps and, like John said, the
_amplitude_ of the carrier/sideband(s) followed the amplitude of the
modulating audio precisely.

Whether you use a MOSFET as a switch or as a resistive element
yielding a linearly varying output depends on how you tailor the
characteristics of the MOSFET to fit the application. After all,
there are lots of linear audio amps out there with MOSFET class A and
class B finals, aren't there? So why shouldn't there be linear MOSFET
RF amps as well?

--
John Fields

[John Larkin]

On Fri, 27 Aug 2004 19:28:39 +0100, Paul Burridge
> wrote:

>On Fri, 27 Aug 2004 08:07:02 -0700, John Larkin
> wrote:
>
>>I think the RF guys (I'm not one!) call an amplifier "linear" if the
>>RF output amplitude follows the input drive amplitude. You can do this
>>with a transistor that has very low quiescent bias. So "linear" does
>>not mean "class A" to them. The key here is that an RF amp has a tuned
>>output, whereas an audio amp doesn't. So the lopsided bias would
>>normally produce intolerable distortion in something like audio, but
>>the tuned output circuit changes the pulsey-looking collector/drain
>>current back into a nice sine wave. So you don't need a lot of idle
>>current, and the transistor really amplifies half of the incoming sine
>>cycle.
>>
>>Most mosfets are pretty nicely linear (ie, straight-line Ic/Vd curve)
>>beyond the initial knee. You could get gobs of watts at zero standing
>>current, but then you'd have some zero-clipping (no output) for the
>>smallest drive levels, so a little idle current helps.
>
>The only sensible way to do it AFAICS is to operate the MOSFET in
>class C as a high speed switch and reconstruct the pulsed output into
>a sine wave carrier by means of a suitable tuned circuit. I wouldn't
>consider driving a MOSFET for RF use in any other way. The efficiency
>should be pretty darned good, too.

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.

John

[Tim Wescott]

John Larkin wrote:
> On 26 Aug 2004 22:40:36 -0700, (Bill N2CQR
> MOHBR) wrote:
>
>
>
>>Even if he were to be running this amp Class B (or close to it), I
>>can't see how he'd get 8 watts out with only 80 milliamps of standing
>>current
>>
>>I'm very new to this kind of analysis, and strongly suspect that I'm
>>misreading either SSDRA or Farhan's excellent article.
>>
>>Can someone please let me know where this apparent discrepency is
>>coming from.
>>
>>Thanks and 73
>>
>>Bill N2CQR M0HBR CU2JL
>>http://planeta.clix.pt/n2cqr
>
>
>
> I think the RF guys (I'm not one!) call an amplifier "linear" if the
> RF output amplitude follows the input drive amplitude. You can do this
> with a transistor that has very low quiescent bias. So "linear" does
> not mean "class A" to them. The key here is that an RF amp has a tuned
> output, whereas an audio amp doesn't. So the lopsided bias would
> normally produce intolerable distortion in something like audio, but
> the tuned output circuit changes the pulsey-looking collector/drain
> current back into a nice sine wave. So you don't need a lot of idle
> current, and the transistor really amplifies half of the incoming sine
> cycle.
>
> Most mosfets are pretty nicely linear (ie, straight-line Ic/Vd curve)
> beyond the initial knee. You could get gobs of watts at zero standing
> current, but then you'd have some zero-clipping (no output) for the
> smallest drive levels, so a little idle current helps.
>
> John
>
RF guys call the amplifier "linear" if the output, after filtering,
looks like a bigger version of the input -- basically the same criterion
as any other amplifier. The reason that you can get away with half as
many active elements as with an audio amplifier is because if the
modulation is narrow compared to the carrier each half of the waveform
looks the same, so amplifying half of it then filtering reconstructs the
half you didn't play with.

Class A amplifiers (and push-pull class AB or B amplifiers) are used in
RF work, but mostly because they cut down on the harmonics that must be
filtered out.

--

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

[Tim Wescott]

Paul Burridge wrote:

> On Fri, 27 Aug 2004 08:07:02 -0700, John Larkin
> > wrote:
>
>
>>I think the RF guys (I'm not one!) call an amplifier "linear" if the
>>RF output amplitude follows the input drive amplitude. You can do this
>>with a transistor that has very low quiescent bias. So "linear" does
>>not mean "class A" to them. The key here is that an RF amp has a tuned
>>output, whereas an audio amp doesn't. So the lopsided bias would
>>normally produce intolerable distortion in something like audio, but
>>the tuned output circuit changes the pulsey-looking collector/drain
>>current back into a nice sine wave. So you don't need a lot of idle
>>current, and the transistor really amplifies half of the incoming sine
>>cycle.
>>
>>Most mosfets are pretty nicely linear (ie, straight-line Ic/Vd curve)
>>beyond the initial knee. You could get gobs of watts at zero standing
>>current, but then you'd have some zero-clipping (no output) for the
>>smallest drive levels, so a little idle current helps.
>
>
> The only sensible way to do it AFAICS is to operate the MOSFET in
> class C as a high speed switch and reconstruct the pulsed output into
> a sine wave carrier by means of a suitable tuned circuit. I wouldn't
> consider driving a MOSFET for RF use in any other way. The efficiency
> should be pretty darned good, too.

Don't operate SSB much, do you?

--

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

[Tim Wescott]

John Fields wrote:

> On Fri, 27 Aug 2004 19:28:39 +0100, Paul Burridge
> > wrote:
>
>
-- snip --
>>The only sensible way to do it AFAICS is to operate the MOSFET in
>>class C as a high speed switch and reconstruct the pulsed output into
>>a sine wave carrier by means of a suitable tuned circuit. I wouldn't
>>consider driving a MOSFET for RF use in any other way. The efficiency
>>should be pretty darned good, too.
>
>
> ---
> That doesn't make any sense to me.
>
> Unless things have changed pretty drastically from how they were when
> I was doing RF, class "C" was pretty much relegated to FM, so that
> when you hit PTT, you banged the hell out of the final and filtered
> the hell out of the carrier, which went to maximum amplitude and
> stayed there, and the information was put on the constant amplitude
> carrier by varying its frequency (or phase).
>
> AM and SSB finals were _always_ linear amps and, like John said, the
> _amplitude_ of the carrier/sideband(s) followed the amplitude of the
> modulating audio precisely.
>
> Whether you use a MOSFET as a switch or as a resistive element
> yielding a linearly varying output depends on how you tailor the
> characteristics of the MOSFET to fit the application. After all,
> there are lots of linear audio amps out there with MOSFET class A and
> class B finals, aren't there? So why shouldn't there be linear MOSFET
> RF amps as well?
>
Well, AM tube finals were often operated class C with the modulation
applied to the plate supply. This is harder to do with silicon because
the varying collector voltage modulates the collector-base capacitance
and causes weird phase shifts.

And there are linear MOSFET RF amps; they're necessary for single-sideband.

--

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

[John Larkin]

On Fri, 27 Aug 2004 08:07:02 -0700, John Larkin
> wrote:


>Most mosfets are pretty nicely linear (ie, straight-line Ic/Vd curve)
^^^^^

Oops, I meant Id/Vg. But you all knew that.

John

[Paul Burridge]

On Fri, 27 Aug 2004 13:00:56 -0700, John Larkin
> 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.
--

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

[Paul Burridge]

On Fri, 27 Aug 2004 15:40:15 -0700, Tim Wescott
> 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.

--

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

[John Fields]

On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
> 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?

--
John Fields

[John Larkin]

On Sat, 28 Aug 2004 00:28:21 +0100, Paul Burridge
> wrote:

>On Fri, 27 Aug 2004 13:00:56 -0700, John Larkin
> 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.

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

John

[Active8]

On Fri, 27 Aug 2004 19:01:37 -0500, John Fields wrote:

> On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
> > 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?

It can be. Depends on the biasing. It *is* AM, however.

Class C just means that the conduction angle is less than 180 deg. I
recall seeing it specified as a certain amount less than 180 once,
but I don't recall how much. The idea is to give the output tank
enough kick to get it to swing.

So you amplify the carrier with class C and modulate the plate. I've
seen projects which do this with bipolars. One that comes to mind is
an old Radio Electronics article for a uWave ATV commo system using
gunplexers, but that can't be right since gunnplexers are modulated
at the gunnplexer. I think it was another ATV project. If I ever
find that old article I'll letcha know.
--
Best Regards,
Mike

[Tim Wescott]

John Fields wrote:
> On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
> > 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?
>
In a tube setup the RF amplifier should be operating in class C and the
power audio amplifier should provide nice linear modulation to the RF
amp's plate supply. In fact* one needs to provide sufficient excitation
to the RF final, too, lest the thing go into a current limited mode on
the modulation peaks.

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...

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

--

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

[Tim Wescott]

Paul Burridge wrote:

> On Fri, 27 Aug 2004 15:40:15 -0700, Tim Wescott
> > 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.

--

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

[Active8]

On Fri, 27 Aug 2004 18:09:43 -0700, Tim Wescott wrote:

> John Fields wrote:
>> On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
>> > 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?
>>
> In a tube setup the RF amplifier should be operating in class C and the
> power audio amplifier should provide nice linear modulation to the RF
> amp's plate supply. In fact* one needs to provide sufficient excitation
> to the RF final, too, lest the thing go into a current limited mode on
> the modulation peaks.
>
> 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...
>
> * So I understand, I'm just a tube wannabe.

I don't know how well it works, but I saw a class E schem using a
section of line to take out the odd harmonics.
--
Best Regards,
Mike

[SioL]

"Paul Burridge" > wrote in message
...
> On Fri, 27 Aug 2004 13:00:56 -0700, John Larkin
> > 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" > wrote in message
...
> John Fields wrote:
> > On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
> > > 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" > wrote in message
...
> Paul Burridge wrote:
>
> > On Fri, 27 Aug 2004 15:40:15 -0700, Tim Wescott
> > > 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
> > 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
> wrote:

>On Fri, 27 Aug 2004 15:42:53 -0700, Tim Wescott
> 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
> 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 >, Paul Burridge
> writes
>On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
> 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 09:23:40 GMT, Rich Grise > wrote:

>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. :-)

Can't blame 'em! Given that the audio power level needs to be
commensurate with the transmitter's RF power output level, using final
plate modulation with a 500KW TX might be a more than a little
inefficient. :-)


--

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

[Paul Burridge]

On Sat, 28 Aug 2004 10:56:03 +0100, Ian Jackson
> 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
> wrote:

>On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
> 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

[John Larkin]

On Sat, 28 Aug 2004 10:36:21 +0100, Paul Burridge
> wrote:

>On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
> 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
> wrote:

>On Sat, 28 Aug 2004 10:36:21 +0100, Paul Burridge
> wrote:
>
>>On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
> 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
> wrote:

>In message >, Paul Burridge
> writes
>>On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
> 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
> wrote:

>On Sat, 28 Aug 2004 08:43:21 -0700, John Larkin
> wrote:
>
>>On Sat, 28 Aug 2004 10:36:21 +0100, Paul Burridge
> wrote:
>>
>>>On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
> 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
> 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
> 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
> wrote (in <ooe1j0hk07082mnusi6nr5vqq0sntl0
>) 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
> wrote:

>On Sat, 28 Aug 2004 18:07:06 +0100, Paul Burridge
> 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
> 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.

[Zak]

Tam/WB2TT wrote:

> 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]

On Sat, 28 Aug 2004 19:28:03 +0100, Paul Burridge
> wrote:

>On Sat, 28 Aug 2004 12:39:34 -0500, John Fields
> wrote:
>
>>On Sat, 28 Aug 2004 18:07:06 +0100, Paul Burridge
> 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.

---
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'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...

---
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.

--
John Fields

[Active8]

On Sat, 28 Aug 2004 19:20:06 +0100, John Woodgate wrote:

<snip>
>
> 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.

--
Best Regards,
Mike

[John Woodgate]

I read in sci.electronics.design that Active8 >
wrote (in >) about 'How to bias a
MOSFET amp?', on Sat, 28 Aug 2004:

>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.(;-)
--
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

[Dave VanHorn]

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.

--
KC6ETE Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR

[Ken Smith]

In article >,
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.


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.

--
--
forging knowledge

[Ken Smith]

In article >,
John Woodgate > wrote:
[...]
>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.


--
--
forging knowledge

[Ken Smith]

In article et>,
Ralph Mowery > wrote:
[...]
>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.


--
--
forging knowledge

[Tim Wescott]

Paul Burridge wrote:

> On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
> > 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. :-/

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.

--

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

[legg]

On Sat, 28 Aug 2004 10:36:21 +0100, Paul Burridge
> wrote:

>On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
> 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. :-/

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

RL

[John Woodgate]

I read in sci.electronics.design that Tim Wescott
> wrote (in .
com>) about 'How to bias a MOSFET amp?', on Sat, 28 Aug 2004:

>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? (;-)
--
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 Sun, 29 Aug 2004 00:48:51 +0000 (UTC),
(Ken Smith) wrote:

>In article et>,
>Ralph Mowery > wrote:
>[...]
>>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

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

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

[Paul Burridge]

On Sun, 29 Aug 2004 00:46:11 +0000 (UTC),
(Ken Smith) wrote:

>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.
--

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

[Paul Burridge]

On Sun, 29 Aug 2004 02:10:52 GMT, legg > wrote:

>On Sat, 28 Aug 2004 10:36:21 +0100, Paul Burridge
> wrote:
>
>>On Fri, 27 Aug 2004 17:43:34 -0700, John Larkin
> 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. :-/
>
>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!
--

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

[Ken Smith]

In article >,
Paul Burridge > wrote:
>On Sun, 29 Aug 2004 00:48:51 +0000 (UTC),
>(Ken Smith) wrote:
>
>>In article et>,
>>Ralph Mowery > wrote:
>>[...]
>>>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
>
>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.


--
--
forging knowledge

[Ken Smith]

In article >,
Paul Burridge > wrote:
>On Sun, 29 Aug 2004 00:46:11 +0000 (UTC),
>(Ken Smith) wrote:
>
>>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.

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.

--
--
forging knowledge

[John Larkin]

On Sun, 29 Aug 2004 12:49:46 +0100, Paul Burridge
> wrote:

>
>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

[Paul Burridge]

On Sun, 29 Aug 2004 11:12:18 -0700, John Larkin
> wrote:

>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!
--

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

[John Larkin]

On Sun, 29 Aug 2004 19:34:57 +0100, Paul Burridge
> wrote:

>On Sun, 29 Aug 2004 11:12:18 -0700, John Larkin
> wrote:
>
>>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!


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

[John Fields]

On Sun, 29 Aug 2004 12:08:37 -0700, John Larkin
> wrote:

>On Sun, 29 Aug 2004 19:34:57 +0100, Paul Burridge
> wrote:
>
>>On Sun, 29 Aug 2004 11:12:18 -0700, John Larkin
> wrote:
>>
>>>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!
>
>
>What makes an amp Class C is a low conduction angle while operating.
>When it's not operating, is has no class at all.

---
I ran into a surgeon like that once...

--
John Fields

[keith]

On Sun, 29 Aug 2004 15:04:53 -0500, John Fields wrote:

> On Sun, 29 Aug 2004 12:08:37 -0700, John Larkin
> > wrote:
>
>>On Sun, 29 Aug 2004 19:34:57 +0100, Paul Burridge
> wrote:
>>
>>>On Sun, 29 Aug 2004 11:12:18 -0700, John Larkin
> wrote:
>>>
>>>>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!
>>
>>
>>What makes an amp Class C is a low conduction angle while operating.
>>When it's not operating, is has no class at all.
>
> ---
> I ran into a surgeon like that once...

Burridge? <shudder>

--
Keith

[John Larkin]

On Sun, 29 Aug 2004 21:57:39 -0400, keith > wrote:

>On Sun, 29 Aug 2004 15:04:53 -0500, John Fields wrote:
>
>> On Sun, 29 Aug 2004 12:08:37 -0700, John Larkin
>> > wrote:
>>
>>>On Sun, 29 Aug 2004 19:34:57 +0100, Paul Burridge
> wrote:
>>>
>>>>On Sun, 29 Aug 2004 11:12:18 -0700, John Larkin
> wrote:
>>>>
>>>>>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!
>>>
>>>
>>>What makes an amp Class C is a low conduction angle while operating.
>>>When it's not operating, is has no class at all.
>>
>> ---
>> I ran into a surgeon like that once...
>
>Burridge? <shudder>


He said surgeon, not sturgeon.

John

[John Woodgate]

I read in sci.electronics.design that John Larkin <jjlarkin@highSNIPland
THIStechPLEASEnology.com> wrote (in <7fo6j05rp857et2n7tbqlp12bavl48qrrv@
4ax.com>) about 'How to bias a MOSFET amp?', on Mon, 30 Aug 2004:
>He said surgeon, not sturgeon.


But, being male, PB is not even a potential source of caviar.
--
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 Mon, 30 Aug 2004 18:53:56 +0100, John Woodgate
> wrote:

>I read in sci.electronics.design that John Larkin <jjlarkin@highSNIPland
>THIStechPLEASEnology.com> wrote (in <7fo6j05rp857et2n7tbqlp12bavl48qrrv@
>4ax.com>) about 'How to bias a MOSFET amp?', on Mon, 30 Aug 2004:
>>He said surgeon, not sturgeon.
>
>
>But, being male, PB is not even a potential source of caviar.

Hey ladies, gimme a break will ya? Don't make your PMS my problem. ;-)
--

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

[John Fields]

On Mon, 30 Aug 2004 18:53:56 +0100, John Woodgate
> wrote:

>I read in sci.electronics.design that John Larkin <jjlarkin@highSNIPland
>THIStechPLEASEnology.com> wrote (in <7fo6j05rp857et2n7tbqlp12bavl48qrrv@
>4ax.com>) about 'How to bias a MOSFET amp?', on Mon, 30 Aug 2004:
>>He said surgeon, not sturgeon.
>
>
>But, being male, PB is not even a potential source of caviar.

---
But most of what he sources _is_ fishy.

--
John Fields

[keith]

On Mon, 30 Aug 2004 18:53:56 +0100, John Woodgate wrote:

> I read in sci.electronics.design that John Larkin <jjlarkin@highSNIPland
> THIStechPLEASEnology.com> wrote (in <7fo6j05rp857et2n7tbqlp12bavl48qrrv@
> 4ax.com>) about 'How to bias a MOSFET amp?', on Mon, 30 Aug 2004:
>>He said surgeon, not sturgeon.
>
>
> But, being male, PB is not even a potential source of caviar.

OTOH, he's a good source of other slime. If you want ot go there, be my
guest!

--
Keith

[keith]

On Mon, 30 Aug 2004 19:11:01 +0100, Paul Burridge wrote:

> On Mon, 30 Aug 2004 18:53:56 +0100, John Woodgate
> > wrote:
>
>>I read in sci.electronics.design that John Larkin <jjlarkin@highSNIPland
>>THIStechPLEASEnology.com> wrote (in <7fo6j05rp857et2n7tbqlp12bavl48qrrv@
>>4ax.com>) about 'How to bias a MOSFET amp?', on Mon, 30 Aug 2004:
>>>He said surgeon, not sturgeon.
>>
>>
>>But, being male, PB is not even a potential source of caviar.
>
> Hey ladies, gimme a break will ya? Don't make your PMS my problem. ;-)

Paul's Miserable ****?

--
Keith

[Rich Grise]

keith wrote:

> On Mon, 30 Aug 2004 19:11:01 +0100, Paul Burridge wrote:
>
>> On Mon, 30 Aug 2004 18:53:56 +0100, John Woodgate
>> > wrote:
>>
>>>I read in sci.electronics.design that John Larkin <jjlarkin@highSNIPland
>>>THIStechPLEASEnology.com> wrote (in <7fo6j05rp857et2n7tbqlp12bavl48qrrv@
>>>4ax.com>) about 'How to bias a MOSFET amp?', on Mon, 30 Aug 2004:
>>>>He said surgeon, not sturgeon.
>>>
>>>
>>>But, being male, PB is not even a potential source of caviar.
>>
>> Hey ladies, gimme a break will ya? Don't make your PMS my problem. ;-)
>
> Paul's Miserable ****?
>
Psychotic Male Syndrome.