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 posted: Well, AM tube finals were often operated class C with the modulation
--- 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?
Yes, that's class C. AM, CW, and FM (and NBFM) almost invariably used Class C Final due to the high efficiency (about 80%).
For 100% modulation the audio reqired from the modulator (for plate modulation, aka high level modulation) is 50% of the RF input power. For 100 Watts RF input, 50 Watts of audio will modulate 100% and result in 25 Watt USB and a 25 Watt LSB.
Don
You should know.
SioL
Define "linear"? You must be joking.
I'll take that as a "no" to my question. Not surprised.
John
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
--- 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
I am not aware of any such things. So I take it you have not designed any class C linear RF power amplifiers.
So, let's not have any definitions at all. Then nobody would ever be wrong.
John
As you presumably know, that's just 100% AM.
-- "What is now proved was once only imagin'd." - William Blake, 1793.
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.
In message , Paul Burridge writes
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.
--- Nonlinear? Yes. Class "C"? No.
From
" 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. "
--- 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
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.
-- Intentionally???
--- 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
-- 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.
....
It's cool that you mention that - I saw a construction article for an HF-VHF upconverter, with a couple of those VHF tubes similar to a 6146 or something like that - about 150 watts output. The interesting thing was, the plate supply for the converter was the RF from the "exciter", an ordinary 150 watt or so transmitter. (actually, it was a transceiver, but they used a different converter for the receiver channel. ;-) ) The LO went to the grids, and of course, the plate tank was tuned to the output freq. Pretty easy to filter, too, when the images are 28 MHz apart. ;-)
Cheers! Rich
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
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
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.
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
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