Amplification questions....

From your comments I had assumed you were biased against class C (Pun intended, I reckon.). So class C is OK in certain situations?

Here's a link to the D8 deadkey mod I mentioned in the post above. This should be cleaner than messin' with the modulation limiter?

Ted

 

I completely understand this, but what I am asking is why are they selling amplifiers labeled as class C if in fact a class C amp can not be used on AM? Are they really AB biased and just trying to pull one over on CB user's? I have pictures of the boards from some of the various amps I have had over the years if you would like to look at them and tell me your thoughts on the biasing.

 

I have an old SuperStar 200. What class is it?

 

THE ONLY REASON I CAN THINK OF MIGHT BE....

CBers might think CLASS C is somehow better .....but a TRUE Class C amp is only turn on for a short part of the duty cycle and would chop up a am signal and produce a bunch of trash .....

Since i run FM it dosn't matter what class to many of my radios.

 

MAT, This is a post from eagle1911 on another forum, I found it informative. Hopefully WA4GCH wil be along to add to it.




First off let me state that when an amplifier adds parts to or removes parts of the input signal, that is distortion. Since the transistors in class C amps conduct less than 180 deg. of the full waveform, the clipping of the waveform (missing parts) is a big part of what causes the distortion. If you take a sine wave, as an example, and clip better than half of it, you only get part of the original waveform back, (and a small part at that) but the points at which the wave is clipped form sub-waveforms that are at multiples of the original frequency. This is harmonic distortion. When you increase the bias current and/or voltage, the transistor begins to reproduce more and more of the waveform until, with class A amps, the full input waveform is reproduced exactly. This is the best possible design, since there are no missing parts of the waveform, but requires the most current.

Typically, what a conscientious operator wants is full power, with the least amount of trash (spurious emissions like harmonics and splatter). This is easily obtained with properly driven class A circuitry. Even with class A though, if you overdrive the amp it will cause distortion just like class C amps. Class B amps are dirtier and more efficient than class A by a large margin, but they are cleaner than class C. Class AB is a compromise between class A and class B, resulting in a good balance between cleanliness and efficiency.

Now my understanding of IMD is a bit limited, but as I understand it when a transistor passes the threshold between it's "off" state and it's "on" state (the point at which it starts conducting), at that instant the transistor's output is not always perfectly faithful to the input waveform. I think of it like what happens when you flip a switch. Often there is a spike of power at the instant the switch engages. This is more pronounced when there is a lot of current involved. When there is a large signal entering a transistor, every time it switches on (which happens every cycle) a tiny spike is generated. This train of spikes becomes it's own signal superimposed on the main signal, which is distortion. IMD is much less of a problem at low drive levels, and becomes a BIG problem when an amp is overdriven.

eagle1911

 

IN SHORT ....

Class "A" lowest distorion only about 30% efficent GREAT if you don't mind
a lot of heat since 70% of the power is lost .... The final is ON 100% of the time

Class "B" more efficent about 50% higher distortion but good enough for any mode AM/FM/SSB/CW . The final os ON 50% of the time

AB is in between them .....

Class "C" high distortion up to 70% efficent Good for FM/CW NOT good for AM. The final is ON LESS than 50% of the time.

When the final is ON it is burning up power ....

 

This is great stuff guys. How do you determine the biasing of an amplifier? I don't feel content accepting the marketing hype of the sellers description of an amp. I guess what I'm asking is how to tell a class C biasing circuit vs. the other three?

Ted

 

Honestly Mayhem I wouldn't worry about trying to find a certain bias. If you get an amplifier from Davemade, Fatboy, Joker, Noname, etc, etc... they say they are class C but they work great on AM. So who know's what the actual biasing is since supposedly Class C doesn't work on AM. They are cheaper than texastars and put out a lot more power with less strain on your electrical system.