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All things being equal: AB vs C
- Thread starter ROC1
- Start date
Chuck,
After smoothing out a few of the 'bumps', what's happening here really is 'elmering'. May not be exactly what the most common answer might be, or even close to the 'common' thoughts about it, but it's still 'helping'.
The title of this thread has a sort of clue in it, it's that "all things being equal" part. They aren't 'equal' to start with, the types of amplification have specific purposes, better for specific modes, which is their whole reason for being. If one particular class of amplifier would work really well for all modes, there wouldn't be very many different classes to start with. Would there? If the question had been "Can a class 'C' amplifier work -almost- as well as a class 'AB', the answer would be, "Yes, almost.". Then the question would be just how much is that 'almost'? And that would depend on the person doing the listening. Not a very good answer huh? Nope, sort of 'wormy', lots of 'wiggle' room, no definite absolute answer. Oh well, wish it weren't true, sorry 'bout that.
It's the nature of everything "general", there are -always- exceptions...
- 'Doc
PS - I agree, making fun of people isn't gonna do much good... in most cases.
After smoothing out a few of the 'bumps', what's happening here really is 'elmering'. May not be exactly what the most common answer might be, or even close to the 'common' thoughts about it, but it's still 'helping'.
The title of this thread has a sort of clue in it, it's that "all things being equal" part. They aren't 'equal' to start with, the types of amplification have specific purposes, better for specific modes, which is their whole reason for being. If one particular class of amplifier would work really well for all modes, there wouldn't be very many different classes to start with. Would there? If the question had been "Can a class 'C' amplifier work -almost- as well as a class 'AB', the answer would be, "Yes, almost.". Then the question would be just how much is that 'almost'? And that would depend on the person doing the listening. Not a very good answer huh? Nope, sort of 'wormy', lots of 'wiggle' room, no definite absolute answer. Oh well, wish it weren't true, sorry 'bout that.
It's the nature of everything "general", there are -always- exceptions...
- 'Doc
PS - I agree, making fun of people isn't gonna do much good... in most cases.
Class AB (and class A) keeps the pills turned on during little or no RF input/drive.
The AM carrier applies RF drive at all times while transmitting, so the pills stay ON.
However, during the negative cycle of modulation the power can drop to zero for that short period. A class C amplifier can generate a small amount of distortion on the peak of negative modulation. But usually the vast majority of distortion of this kind is from the CB transmitter.
I think 74IN said it best. CB is not about being clean. class C is fine for AM CB.
Another way to think of it: NASCAR is not about reducing global warming.
The "ultimate solution" for an AM power transmitter is collector modulation, or plate modulation as they called it in tube days. In common radios like the Cobra-29, the collector voltage on the finals are modulated by a audio power amplifier. The "ultimate solution" would be to scale up the audio power amplifier to power the (big) amp, and just feed the RF input with a dead carrier.
The AM carrier applies RF drive at all times while transmitting, so the pills stay ON.
However, during the negative cycle of modulation the power can drop to zero for that short period. A class C amplifier can generate a small amount of distortion on the peak of negative modulation. But usually the vast majority of distortion of this kind is from the CB transmitter.
I think 74IN said it best. CB is not about being clean. class C is fine for AM CB.
Another way to think of it: NASCAR is not about reducing global warming.
The "ultimate solution" for an AM power transmitter is collector modulation, or plate modulation as they called it in tube days. In common radios like the Cobra-29, the collector voltage on the finals are modulated by a audio power amplifier. The "ultimate solution" would be to scale up the audio power amplifier to power the (big) amp, and just feed the RF input with a dead carrier.
"But usually the vast majority of distortion of this kind is from the CB transmitter."
more bull.
Class C is defined as an anode conduction angle of less than 180 degrees. In Class C, the amplifying device is DELIBERATELY NOT OPERATED LINEARLY. Instead, IT IS OPERATED AS A SWITCH in order to reduce resistance loss. The anode conduction angle in Class C operation is usually made as short as is possible. In effect, the tank circuit makes the RF output sine wave--like a bell that is struck at a constant rate by a hammer. This is similar to the principal behind SPARK TRANSMITTERS which were NOTORIOUS FOR PRODUCING HIGH LEVELS OF HARMONIC RADIATION. that's what switches do.
The efficiency of a typical Class C amplifier is high. When compared to a Class AB1 or Class AB2 amplifier operating at the same power input, a Class C amplifier will deliver a received signal increase of about 1db--in other words, 1/6 of 1 S-unit. However, SIGNIFICANT TRADE-OFFS are required to achieve that 1/6 of 1 S-unit. As is the case with Class B operation, the DISTORTION from Class C operation is so high that SSB operation is precluded. Only CW, FM or FSK operation is practical. THE HARMONIC OUTPUT LEVEL from a Class C amplifier IS SUBSTANTIAL. Extra filtering is usually needed to control harmonic radiation.
more bull.
Class C is defined as an anode conduction angle of less than 180 degrees. In Class C, the amplifying device is DELIBERATELY NOT OPERATED LINEARLY. Instead, IT IS OPERATED AS A SWITCH in order to reduce resistance loss. The anode conduction angle in Class C operation is usually made as short as is possible. In effect, the tank circuit makes the RF output sine wave--like a bell that is struck at a constant rate by a hammer. This is similar to the principal behind SPARK TRANSMITTERS which were NOTORIOUS FOR PRODUCING HIGH LEVELS OF HARMONIC RADIATION. that's what switches do.
The efficiency of a typical Class C amplifier is high. When compared to a Class AB1 or Class AB2 amplifier operating at the same power input, a Class C amplifier will deliver a received signal increase of about 1db--in other words, 1/6 of 1 S-unit. However, SIGNIFICANT TRADE-OFFS are required to achieve that 1/6 of 1 S-unit. As is the case with Class B operation, the DISTORTION from Class C operation is so high that SSB operation is precluded. Only CW, FM or FSK operation is practical. THE HARMONIC OUTPUT LEVEL from a Class C amplifier IS SUBSTANTIAL. Extra filtering is usually needed to control harmonic radiation.
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Doc maybe I didn't make the original post clear; "all things being equal" up too the amp. I get that the amps are inherently different, although I can't tell you what those differences are... All I was proposing was as close as we could get to an apples to apples comparison up to the point where the amp received the signal, and then an honest comparison of what would happen with that signal once delivered into the amp and processed...
I'm actually taking ALOT from this and appreciate all the input.
Well I guess I was one of the few that picked up on that??
I mean when I first read it that was the question that poped in my head? Or at least that is what I was hopeing to get an answer to.
I much like ROC don't have all the answers but I thirst for them.
Just because other people do drugs don't mean I am going to? If ALL of the CB opps. were as careless as some think, NOONE would be able to use a radio. I know there are some who talk on the WHOLE band everytime they keyup but that dosn't mean the rest of us HAVE to ??? I like the fact that I have the option to run clean if I chose to
Chuck
Well I guess I was one of the few that picked up on that??
I mean when I first read it that was the question that poped in my head? Or at least that is what I was hopeing to get an answer to.
I much like ROC don't have all the answers but I thirst for them.
Just because other people do drugs don't mean I am going to? If ALL of the CB opps. were as careless as some think, NOONE would be able to use a radio. I know there are some who talk on the WHOLE band everytime they keyup but that dosn't mean the rest of us HAVE to ??? I like the fact that I have the option to run clean if I chose to
Chuck
I have a friend who is very much a Dave Made, Drive it like it was NASCAR, CB'er... when listening to my system he has more than once commented "Clean is Mean"... Even those guys can appreaciate good clean audio... then they chop your lips off! LOL
It's funny, I don't know how, but there are actually a couple of guys in town that actually know it's me when I just dead key...
"....Pills ran at class C have their bases tied to ground........"
yeah and the base-emitter junction is reverse-biased as a result, so what?
that's what makes them so efficient when they're used with the proper
modulation schemes. they're switches. no collector current flows until
the reverse bias is overcome by sufficient levels of rf input. when that
input level is reached the base-emitter is now forward biased and (rf output
is produced) collector current flows. that why by the time you get 1 watt
of drive into it you've got 100W of output as in the case of a palomar
elite 250 mentioned in another forum here earlier. that's an indication that
the amplifier is biased class c.
simply by virtue of the manner in which a class c amplifier operates there's
no headroom to accomodate the proper peak-to-carrier levels required
to produce adequate modulation levels once sufficient input drive has
been applied to create enough forward bias to "turn on" the collector
outputs.
on the other hand, in a class ab amplifier the output is always in proportion
to the input. consonant sounds in human speech have less energy than vowel
sounds. class ab is required to faithfully reproduce those nuances in energy
levels that attend human speech. that's why class ab amplifiers are forward biased
from the get-go.
that's why ab is used with signals that are modulated with asymmetrical human
speech and we save the class c switches for modulation modes where the
asymmetry of human speech is not involved.
class c switches cannot faithfully reproduce the variations in human speech
without excessive levels of distortion because they're either ON or OFF.
that's why class c amplifiers are used in these services:
and FM transmitter is either ON or OFF, there's no in between.
a CW transmitter is either ON or OFF, there's no in between.
an RTTY transmitter is either ON or OFF, there's no in between.
that's why class ab amplifiers are used in these services:
an AM transmitter produces a signal that constantly varies in amplitude.
an SSB transmitter produces a signal that constantly varies in amplitude.
a DSB transmitter produces a signal that constantly varies in amplitude.
and just so there's no confusion, ab can be used in all these services
while c doesn't enjoy the same all-round operating flexibility.
if you want higher efficiency then you must sacrifice fidelity.
if you want more fidelity then you must sacrifice efficiency.
i just don't happen to think that a signal increase of .16db.
is worth the additional distortion and out of band products
and there are many here who would agree.
a LINEAR amplifier is required equipment when using asymmetrically
(amplitude) modulated signal sources for faithful reproduction of the
original signal as well as to produce output power in direct proportion
to the input signal. distortion in this case is defined as any signal or
signals not present in the original inputted signal.
as for your cb radio transmitter producing 100% negative modulation
levels for extended periods of time, you can eliminate that with
proper adjustment of the amc. they don't come from the factory
that way. i have yet to see a single cb radio that didn't produce
4W+ of carrier and anywhere from 12 - 18WPEP right out of the box.
now that you have helped make the case for the DIFFERENCE between
ab & c, where do you want to go next?
yeah and the base-emitter junction is reverse-biased as a result, so what?
that's what makes them so efficient when they're used with the proper
modulation schemes. they're switches. no collector current flows until
the reverse bias is overcome by sufficient levels of rf input. when that
input level is reached the base-emitter is now forward biased and (rf output
is produced) collector current flows. that why by the time you get 1 watt
of drive into it you've got 100W of output as in the case of a palomar
elite 250 mentioned in another forum here earlier. that's an indication that
the amplifier is biased class c.
simply by virtue of the manner in which a class c amplifier operates there's
no headroom to accomodate the proper peak-to-carrier levels required
to produce adequate modulation levels once sufficient input drive has
been applied to create enough forward bias to "turn on" the collector
outputs.
on the other hand, in a class ab amplifier the output is always in proportion
to the input. consonant sounds in human speech have less energy than vowel
sounds. class ab is required to faithfully reproduce those nuances in energy
levels that attend human speech. that's why class ab amplifiers are forward biased
from the get-go.
that's why ab is used with signals that are modulated with asymmetrical human
speech and we save the class c switches for modulation modes where the
asymmetry of human speech is not involved.
class c switches cannot faithfully reproduce the variations in human speech
without excessive levels of distortion because they're either ON or OFF.
that's why class c amplifiers are used in these services:
and FM transmitter is either ON or OFF, there's no in between.
a CW transmitter is either ON or OFF, there's no in between.
an RTTY transmitter is either ON or OFF, there's no in between.
that's why class ab amplifiers are used in these services:
an AM transmitter produces a signal that constantly varies in amplitude.
an SSB transmitter produces a signal that constantly varies in amplitude.
a DSB transmitter produces a signal that constantly varies in amplitude.
and just so there's no confusion, ab can be used in all these services
while c doesn't enjoy the same all-round operating flexibility.
if you want higher efficiency then you must sacrifice fidelity.
if you want more fidelity then you must sacrifice efficiency.
i just don't happen to think that a signal increase of .16db.
is worth the additional distortion and out of band products
and there are many here who would agree.
a LINEAR amplifier is required equipment when using asymmetrically
(amplitude) modulated signal sources for faithful reproduction of the
original signal as well as to produce output power in direct proportion
to the input signal. distortion in this case is defined as any signal or
signals not present in the original inputted signal.
as for your cb radio transmitter producing 100% negative modulation
levels for extended periods of time, you can eliminate that with
proper adjustment of the amc. they don't come from the factory
that way. i have yet to see a single cb radio that didn't produce
4W+ of carrier and anywhere from 12 - 18WPEP right out of the box.
now that you have helped make the case for the DIFFERENCE between
ab & c, where do you want to go next?
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I'm not sure if the question at the end is directed to me... but I'd rather have fidelity and a signal that isn't over-modulated and splattering on other channels (personally) than efficiency. So I'll be moving in that direction as much as you can on 11 meter...
Digging into this further... It appears to me that the 2879 creates the least amount of IMD at 2watts input (@ -38db). Am I reading the data sheet correctly? I want to make sure I understand the relationship...
http://www.datasheetcatalog.org/datasheet/toshiba/1468.pdf
http://www.datasheetcatalog.org/datasheet/toshiba/1468.pdf
you're reading it correctly.
2WIn = 60WOut @ -38db..
http://www.firecommunications.com/trdbase/2sc2879.pdf
2WIn = 60WOut @ -38db..
http://www.firecommunications.com/trdbase/2sc2879.pdf
"I think I understand a little bit now, how biasing would be important if you were trying to minimize IMD"
improper biasing and excessive drive input destroys linearity, increases distortion and out of band harmonic content and prematurely destroys both tube and transistor power devices.
Transmitter Power & Harmonic Line-Section OverSampling
improper biasing and excessive drive input destroys linearity, increases distortion and out of band harmonic content and prematurely destroys both tube and transistor power devices.
Transmitter Power & Harmonic Line-Section OverSampling
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