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Old 5th May 2004, 02:40 PM   #11
azira is offline azira  United States
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Default Re: Re: Re: Biasing for different classes

Quote:
Originally posted by slowhands

Class A means simply that current always flows through an output device all the time during the output waveform. Class B implies a push pull amp of some sort, and the output devices each are on for either the positive or negative output polarity only; thus with a sine wave without offset, each output device would be on 50% of the time. Class AB is a hybrid, with the devices simultaneously on for significantly greater than 50% of that same sine wave. These definitions are true regardless of the amplification device -- transistor, mosfet, tube or whatever.
However, you have not explaind two critical behaviors.
Firstly, with NO bias circuitry, a push-pull BJT configuration has 2*Vbe of dead band. As I showed in my 2nd post, this is a significant portion of the wave (and therefor conduction for much less than 50%) even up to 3-watts output power. This topology does not fall into your definition of a class B ever.
Secondly, doesn't your class AB definition fall under the catagory of class A for power developed for current up to 2*bias? Where do you draw the line? At 300mA bias this is almost 3-watts of power. What if you raise the bias a bit more? Is this not a push-pull but also a class A? So what is it?

Quote:
Originally posted by slowhands

Self recommends class B highly over class AB because it has the lowest measured distortion. He considers there is a precisely optimal class B bias for any output BJT stage, whether Emitter Follower EF or Complementary Feedback Pair CF, which yields minimum measured distortion.
But refering back to sreten's and my discussion, Self's class B is not the same as another authorities class B. He considers class B what you are calling class AB.

Quote:
Originally posted by slowhands

[Yes, strictly speaking this is not class B since there is some simultaneous conduction of the outputs -- but clearly, not much, and this is called class B by most authorities.]
Which authorities? These definitions of class A, AB, B are not what Sedra/Smith defines nor it seems what Self defines.

I agree there is a need to have clear definitions because these are tools we all use for discussion but since there are disparities between the authoritative sources, the only thing to do is to state your definition before your conclusion and let the readers translate to their belief system.
--
Danny
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Old 5th May 2004, 05:57 PM   #12
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"Secondly, doesn't your class AB definition fall under the catagory of class A for power developed for current up to 2*bias? Where do you draw the line? At 300mA bias this is almost 3-watts of power. What if you raise the bias a bit more? Is this not a push-pull but also a class A? So what is it?"

It's Class A up to that point, "Class B" beyond. Taken as a whole,
it's Class AB. If an amp runs Class A up to its rated power into
a particular load, it's conventionally referred to as a Class A
amp, with the understanding that beyond that output current
it's likely "B", if anything at all.

I refer to it as "B" with quotes, as someone here will argue
that it's Class AB. Subtle distinction not worth arguing about
as long as we know what we're talking about.
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