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How to bias a MOSFET amp?
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 |
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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 |
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. |
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 |
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 |
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 |
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 |
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 |
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 |
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. |
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. |
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 |
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 |
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 |
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 |
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 |
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 |
"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 |
"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 |
"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 |
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 |
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. |
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. |
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. -- |
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. |
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. |
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 |
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 |
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. |
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 |
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 m 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 |
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 |
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. |
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 |
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. |
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. |
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. |
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 |
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