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In Reply to: RE: I'm delighted that you acknowledge posted by E-Stat on February 08, 2025 at 07:03:39
"The Last Word on Class A
First of all, what is a class A amplifier?
A class A amplifier is defined as one which is biased to a point where plate current in all the output devices flows for the entire 360 degrees of an input cycle, at the full, unclipped output of the amplifier. Or, as stated in the RCA receiving tube manuals: "The classification depends primarily upon the fraction of input cycle during which plate current is expected to flow under rated full-load conditions". The key phrase being "under rated full-load condtions", which is a requirement for amplifier classifications to be meaningful.. This is typically done by biasing the output stage halfway between cutoff and saturation, with the plate load impedance to an appropriate value that gives maximum undistorted output power."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
I think it fine to classify an amplifier that is class A only for a fraction of its specified output power as Class AB even if it might end up spending most/all of its time operating in a class A regime. But, as pure solid state class A amplifiers are uncommon and class B is very rare (AFAIK) I also think classification with such fervor pretty pointless.
In your follow-up posts I see the inclusion of single-ended amplifier load lines. These are not relevant to solid state push-pull output stages, which can produce load current above their bias point as required (but only in one direction, hence the need for push-pull). And then I remebered having this discussion once before on AA and after a search it turned out to be with you!
As I told you more than 6 years ago, I'm not a SS guy."I see the inclusion of single-ended amplifier load lines. These are not relevant to solid state push-pull output stages, which can produce load current above their bias point as required (but only in one direction, hence the need for push-pull)"
Should I understand, from what you said, that a transistor can't idle at the mid point between cutoff and max current?
The single ended load line may not be relevant as the current moves up from the idle point but isn't it relevant as the current moves down from the idle point?
Or do transistors magically drop all the way to cutoff in a perfectly linear way?
If they don't then that's my point. In a true Class A tube amplifier (single ended or PP) none of the devices are driven into the non-linear regions, up or down. It transistors are linear all the way to cutoff then there is no need for Class AB, Class B with be notchless. (crossover distortion less)
As the dynamic operating curve chart (not a load line plotted on a set of plate curves but derived from that) shows, with a tube circuit Class A only operates the tube(s) in the "most linear part of the dynamic curve". I forget which book I'm quoting but that is a quote.
An AB tube amplifier doesn't do that and neither do SS circuits (unless transistors are linear all the way down to cutoff).
Side note, with a PP Class A tube amp, it you drop the idle current a little to get more power then it's not a Class A amplifier anymore and none of the power is Class A power. Even the part that is still operating in the "most linear part of the dynamic curve". The total rated power of the amplifier has to be power produced while all the tubes are operating most linear part of the dynamic curve. That is what all the old books discribe and show as examples of Class A.
Maybe with SS it's different. If a transistor stays linear all the way to cutoff. Tubes don't do that.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/10/25 02/11/25
And likewise I'm still not a tube guy :)
I've kind of lost the thread a bit but to answer your question nothing is linear! The further up and down the output current vs. Input voltage curve you go then the more distortion there is. The smaller the current excursion around a bias point then the less the distortion (with the limiting case being zero distortion for zero current change = small signal conditions). I don't know what the load line for an emitter/source follower would be but it would probably show the analogous effect that you are describing by tending to cut off the follower if it has to sink current from a load. Followers are great at supplying current to loads but that only works in one direction hence the need for push-pull unless you went with a single-ended follower biased so that it can sink the maximum load current before turning off (or getting too close to turning off) and that would be class A. But, IMHO, even if you did go class A, a push-pull would have the advantage of going into class AB for lower load impedances e.g. it could be 'pure' 50W class A into 8 ohms but give 100W into 4 ohms class AB, whereas a single ended 50W class A could only provide 25W into 4 ohms (current limited).
Transistors do not stay linear all the way to cutoff...otherwise, there would be no need for Class A to begin with as you would not have serious distortion at the handoff between transistors in a push/pull amp.
With push pull, there would be no need for Class AB because there would be no notch distortion in Class B.
I just thought of an analogy. It maybe not a good one but here goes.Class AB is a "fix" for Class B much like a suppressor grid in a pentode is a fix for the tetrode.
When they added the screen grid to a triode to block internal feedback path between the plate and the screen that lowered the gain and reduced the distortion, they created the problem of secondary emissions and had to add the suppressor grid to fix it.
When they went from Class A (push pull) to Class B (push pull), the non-linear behavior of amplifying devices in the cutoff region created cross over distortion and they had to put some of the idle current back to "fix" it.
A pentode is a fix for a tetrode but that doesn't make it a triode.
A triode is it's own thing.Class AB is a "fix" for Class B but that doesn't make it Class A.
Class A is it's own thing.I warned you that the analogy might not be very good. :-)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/11/25
!
The Mind has No Firewall~ U.S. Army War College.
Are really AB in that they operate with significant idle bias current thru the finals. Theory indicates that a unity coupled OPT could operate successfully with B biased finals but that's not what Mac did in practice, at least for their consumer stuff.
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