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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. |
In article ,
Paul Burridge wrote: On Sun, 29 Aug 2004 00:48:51 +0000 (UTC), (Ken Smith) wrote: In article . net, 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 |
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 |
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 |
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. |
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 |
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 |
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 m 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 |
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