WorldwideDX Amateur Radio Forums

I've posted this same answer to this same question many times.

http://www.tpub.com/neets/book7/25e.htm

Where the article states that Class C amplifiers are used for broadcast transmitters, what it fails to point out is that these are FM transmitters, not AM (and certainly not SSB). In order for an amplifier to be "linear", the output has to look like the input, just larger. No portions cut off, no distortion because of overdriving. Class B and C aren't linear, period.

Nor is Class AB, but if a Class AB amplifier is operated intelligently, it will be almost as linear as Class A, but a bit more efficient.

__________________
If you're like everybody else,
you're about average.
73 de K7KBN
CWO4 USNR RET.

the portion of the input signal for which there is an output produced (measured in degrees) determines the class of operation of the amplifier. the larger the portion of the input signal is that produces an output the less distortion and the higher the linearity when the devices are operated along the flat portion of the transistor gain curve.

Class A: output which is a replica of the input. (minimum distortion)
collector output current flows for the complete (360 degree) duration of the input. the dc operating point is between cutoff and saturation

Class AB: crossover distortion attenuated, linearity improved.
collector output current will flow for more than 180 degrees but less than 360 degrees of the input signal. the dc operating point is such that collector current is zero for a portion of one alternation of the input signal.



Class B: crossover distortion is present.
collector current will flow for approximately 180 degrees (half) of the input signal. the dc operating point for this class of amplifier is set up so that base current is zero with no input signal.

Class C: output not a replica of the input. (maximum distortion)
collector output current flows for less than one half cycle (<180 degrees) of the input signal. the dc operating point is below cutoff and allows only the portion of the input signal that overcomes the reverse bias to cause collector current flow.

OK , very good. For me , there's alot more to it then just reading it , it's like math of any kind , you have to understand it to get it, other then just reading it. Beings that IM a simple minded cb radio / amp user , if AM is all that I use ? class c will fill the bill ? . If IM more into sideband ? AB will be a cleaner situation then C although C will work on SSB to a degree of cleanliness.

(You should be familiar with two terms used in conjunction with amplifiers - FIDELITY and EFFICIENCY. Fidelity is the faithful reproduction of a signal. In other words, if the output of an amplifier is just like the input except in amplitude, the amplifier has a high degree of fidelity. The opposite of fidelity is a term we mentioned earlier - distortion. Therefore, a circuit that has high fidelity has low distortion. In conclusion, a class A amplifier has a high degree of fidelity. A class AB amplifier has less fidelity, and class B and class C amplifiers have low or "poor" fidelity. )

That part I do understand. It's the "why" part that tends to avail me , but does somebody like me really need to understand it in the first place ? I don't think so , but I also don't want to be a poupous ass either. (although it can happen from time to time) It's obvious I didn't get it when Freecell tried to explain it (thanks Freecell for trying) So I was wrong to say that a amplifier with a more so high dead key from a lower drive in with less swing but with a higher power reading was more Linear , I stand corrected.

It just goes back to the beginning for me , and I know Freecell mentioned the MRF455s were why this little amp keyed so high with such a low DK into it. The only other amps I've seen simlar to this one over the years were the older original Palomar mobile transistored amps that usually only needed about 1 watt input on the AM sides of those. Freecell said it was all in the spec's of the transistors. Like all that has been said up to this point , there's a lot more that I would need to understand concerning those spec's.

I truly appreciate the time taken here by those that do understand these formulas and "why". IM just a simple minded guy when it comes to all of this and I often look for simple answers to what I don't understand. IM either teachable or not and the flip side of the coin of that is .....will it benifit me ? I try and keep it as simple minded as I can , I still appreciate the truth and know how on all of this , weather it computes in my mind or not. I tend to get by on the basics. Every now and then , I tend to wonder why and how things work.(weather I understand it or not) Thank you gentleman.

Exactly right. And the most important factor in establishing this is the amplifier's bias.

Neither the amount of "dead key" nor the amount of "swing" has anything to do with the class of amplifier. If the amplifier is used for any amplitude modulation scheme (including SSB in any of its forms), Class C will distort the signal that goes on the air. "Loud"? Probably. Clean? No.

__________________
If you're like everybody else,
you're about average.
73 de K7KBN
CWO4 USNR RET.

Well Beetle , I think I've understood what Class C has meant over the years , LOUD ? sure why not , Dirty ? You bet'cha ! this part about Class C reminds me of Pyscho's analogy which others tend to think he's crazy about when it comes to class C amplifiers on the 11 meter band. I personally think his analogy hasn't been that far off. CB radio is what it is , it's almost as if your DAMNED if you do or your DAMNED if you don't know matter what class amplifier you use . Although , it is nice to have a decent amplifier ,especially if SSB is your thing ? AM ? it's almost why have a cleaner box if it's DIRT IN AND DIRT OUT anyways ? I have a nice Cobra XL 450 , I seldom use SSB until DX picks up and becomes strong again ,so AM has always more so been my way of life on the 11 meter band. I still use a cap resistor mod for this perpous into my amp , IM just back to dirt in and dirt out , but I seldom get any complaints from anybody nor do I want any. Is this a "truly" clean amp ? would a (qoute on quote) perfectly tuned radio into this thing "truly" going to make that much differance on the other end ? If class C is dirty from the get go ? What does it truly matter ? in other words , can class C ever be clean ? Thanks Beetle , you have always been a great help around here and else where. Peace

Class C can never be LINEAR. If you're feeding an FM signal to the input, the Class C amplifier will work just fine; this is what it is born to do. The amplitude of the input is virtually constant so there are no modulation transients to cause problems.

If you're feeding an AM signal (full carrier, no sideband suppression) to the input, and you're careful to have only a minimum of "swing", you probably won't have technical problems with a Class C amplifier. Again, you're keeping the mod transients to a minimum.

For SSB, however, the input signal is ALL "mod transients". The signal will splatter all over, and sound ungood.

__________________
If you're like everybody else,
you're about average.
73 de K7KBN
CWO4 USNR RET.

I thought I heard that mentioned about Class C as far as FM is concerned , again , whom uses FM on the 11 meter band in America ?

( If you're feeding an AM signal (full carrier, no sideband suppression) to the input, and you're careful to have only a minimum of "swing", you probably won't have technical problems with a Class C amplifier. Again, you're keeping the mod transients to a minimum. )

You probably won't ? in other words you will , but your just making it as clean as clean can be concerning class C as far as AM is concerned ? Still far from the best ? or does this mean you can utilize more power from the class C amp with "less" distortion ? Would this mean the more so cleanly tuned radio into the class C
application and this would all show up clean on the testing machines ?

.

__________________
If you're like everybody else,
you're about average.
73 de K7KBN
CWO4 USNR RET.