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#2
August 27th 04, 07:28 PM
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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. |
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#3
August 27th 04, 08:31 PM
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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 |
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#4
August 27th 04, 11:42 PM
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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 |
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#5
August 28th 04, 01:01 AM
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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 |
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#6
August 28th 04, 02:07 AM
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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 |
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#7
August 28th 04, 02:09 AM
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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 |
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#8
August 28th 04, 02:23 AM
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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 |
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#9
August 28th 04, 04:12 AM
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"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 |
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#10
August 28th 04, 10:23 AM
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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 |
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