Explanation of Class A

LiquidMids

Banned
2013-05-30 12:55 am
Most of the places I look and the definitions I have seen all echo the same explanation that is seen everywhere..

I am left handed as well, so this may help..

Here is what you won't see said:

Class A operation retains control of the voltage as it falls towards zero, even from the peaks of either side.
- there are times that the voltage needs to drop faster than it would naturally fall.

Class A operation is inside the bias zone. - meaning no noise from transistors turning on off..


- This means that at all times, there is a positive and a negative
transistor right there, ready to push or pull the speaker, anywhere in the
voltage scale.

Class A amps sound like a well biased AB amp at low volume. As soon as you hear any raspyness, you have entered Class AB.

You may see Class AB amps delivering a nice, perfect sine wave on a scope with a test tone and load resistor, but this is not even close to what you see with a music signal and inductive/capacitive loading. The resistor and frequency is such that the voltage is just naturally falling as it's transistor is turning off, tracking it perfectly. Keep in mind that the opposite polarity transistor cannot pull the voltage towards zero until it's bias is reached, which is usually not far from zero volts.

- in the above example, if the frequency was raised, you would see the trailing edge of each peak smearing, or overhanging until
it can be grabbed and corrected by the opposite polarity transistor.


Hopefully this will help clear up some of the excessive hype and misunderstanding
that there is with Class A operation..
 
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LiquidMids

Banned
2013-05-30 12:55 am
I was thinking of that, but I wanted to give kind of a basic explanation..

To be honest, I don't see how a single ended can push and pull equally
well, maybe they don't?

I understand the capacitor output though, and how it is biased half way.

How can a single ended transistor fall as hard as it rises, to maintain control?

Put another way, how can a transistor turn off with the same force that it turns on?

I guess that is why we are not seeing many SE amps with much power..
 
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LiquidMids said:
Hopefully this will help clear up some of the excessive hype and misunderstanding
that there is with Class A operation..
You don't help clear up alleged myths about Class A by spreading misunderstanding about Class AB.

Class A operation retains control of the voltage as it falls towards zero, even from the peaks of either side.
- there are times that the voltage needs to drop faster than it would naturally fall.
Not true. Class A can do current drive (although voltage drive is more common); current drive has no control over voltage. Other classes can do voltage drive too.

Class A operation is inside the bias zone. - meaning no noise from transistors turning on off.
Transistors generate noise when they are on. They can't generate noise when they are off. They may generate transients (not noise) as they switch.

You may see Class AB amps delivering a nice, perfect sine wave on a scope with a test tone and load resistor, but this is not even close to what you see with a music signal and inductive/capacitive loading. The resistor and frequency is such that the voltage is just naturally falling as it's transistor is turning off, tracking it perfectly. Keep in mind that the opposite polarity transistor cannot pull the voltage towards zero until it's bias is reached, which is usually not far from zero volts.
It is possible to design both good and bad Class A amps and Class AB amps. You won't make much progress in understanding while you compare a bad example of one against supposed ideal behaviour of the other.

I was thinking of that, but I wanted to give kind of a basic explanation.
Why not give correct explanations instead?
 
Put another way, how can a transistor turn off with the same force that it turns on?

For class A operation there is no "turn off-turn on", it only conducts more or less current. As for your idea of "force", I guess you're talking about electrical current, which as already mentioned, in class A operation fluctuates as the input signal dictates, that's all.

Mike

Mike
 

LiquidMids

Banned
2013-05-30 12:55 am
You guys are not friendly, and not right either.

I was describing a push pull amplifier, the most common type..

Voltage drives the speakers, current is there to back it up.

Remember, voltage is like water pressure, current is gallons per minute

Before you guys get too critical, consider my explanation, and tell me exactly

where I'm wrong here. I described it like what is going on, not what you learned in

basic electronics.

Voltage drives the speakers, current backs it up, when current runs out, the voltage drops.

You guys need to back off, unless you can point out where I'm wrong here.
 

LiquidMids

Banned
2013-05-30 12:55 am
For class A operation there is no "turn off-turn on", it only conducts more or less current. As for your idea of "force", I guess you're talking about electrical current, which as already mentioned, in class A operation fluctuates as the input signal dictates, that's all.

Mike

Mike

To put it another way, how can the device reduce as fast, strong, or forceful as it can increase?

I mean reduce hard enough to drop the voltage instantly, forcing it towards zero just as fast as it could rise?

and I mean at the speaker terminal.

real world, folks, not a test resistor. This would be like dampening factor..
 
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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Voltages don't rise instantly and neither do they fall instantly. Such descriptions belong to classroom questions 'at the instant the switch is closed....... calculate xyz'.

Audio signals are positively pedestrian in their rate of rise and fall.

Actively helping pull down a transistors base to facilitate it switching off more quickly is a technique that has been used for decades, whether in audio or in more demanding applications like SMPS.

Remember nothing happens instantaneously.
 
Hopefully this will help clear up some of the excessive hype and misunderstanding
that there is with Class A operation..
The definition of Class-A is, that quiescent power is at least twice as high as maximum output power. (I dunno the derivation of this definition, but i remember, that there is one.) Hence Class-A cannot exceed an efficiency of one half.
 

LiquidMids

Banned
2013-05-30 12:55 am
Yes, Class A operation would control the voltage moving toward zero, being an intermediate stage, or final output.

The opposing polarity device is biased on hard enough to still be turned on when the
signal is at it's peak, allowing for the signal to be "pulled down" if need be, with force if needed, to track the signal, and not the natural voltage drop. The device may be turning off, but the capacitors and moving coils in the speakers are still holding a voltage. It does not just perfectly track without it.

Single ended amplifiers seem to have a big output resistor along with the speaker, to
help the voltage reduce as fast as necessary.

Like a muscle can only pull one way, so can a device. It can release, but not with
the dynamic control it has contracting.

Once again, I HOPE THIS HELPS CLEAR UP CLASS A OPERATION.

If your mind works differently, or you don't see it this way, re-read and think carefully before blasting my comments. To me, it makes it more clear than "180 degrees of operation" - yeah the 90 degrees on both sides where the voltage is returning to zero.

Thank You.
 
LiquidMids said:
You guys are not friendly, and not right either.
This is us being friendly (and right).

You guys need to back off, unless you can point out where I'm wrong here.
Posting confused ramblings, in the guise of correcting others, then demanding that detailed corrections be offered is not a good way of achieving understanding of a new subject area. Why not read a good book instead? Then, when you have specific questions about things which puzzle you, come here and ask them and we will be very happy to help you.

Once again, I HOPE THIS HELPS CLEAR UP CLASS A OPERATION.
No, it doesn't but fortunately Class A operation does not need to be cleared up.

If your mind works differently, or you don't see it this way, re-read and think carefully before blasting my comments.
Class A operation, like the rest of engineering and science, is not a matter of how my mind works or your mind works; it is a matter of how certain circuits work. If you choose to interpret criticism as "blasting" then maybe internet forums are not a suitable form of pasttime for you.
 
I have pointed out why I do not intend to point out where your 'explanations' are wrong. I admit that I find it difficult to attach meaning to some of what you write. I am not confused.

I repeat my advice to go away and read a good book, then come back with questions. We like questions; we respond positively to questions. Bring your questions, not your 'explanations'.
 

TNT

Member
Paid Member
2003-04-26 10:25 pm
Sweden
This is what wiki says:

Amplifying devices operating in class A conduct over the entire range of the input cycle. A class-A amplifier is distinguished by the output stage devices being biased for class A operation.

This is the definition. All other properties that you are claiming belongs to class A is false and could very well be exhibited by other bias strategies. You don't seem quite ready to lecture the forum just yet.

I advice you not to battle solid state technology with DF96, as you will lose. Ask him well founded question I you will have answers.

//
 

LiquidMids

Banned
2013-05-30 12:55 am
Like I said, I'm left handed. - one of the few in their right mind!

My explanation agrees with the definition of Class A - 360 degrees of duration,
meaning that the transistor(s) is always on.

I'm sure this will simplify the understanding, as if you were looking at a scope, imagine the higher peaks not having control when the voltage drops towards zero.

- This was the moment I really understood the difference, when I thought of it not having control of the voltage when it drops towards zero.

There are lots of people on here that can be helped by this explanation.

I am an experienced tech, and you guys don't know everything.

..so quit making people feel ignorant.
 
liquid, I think you are confused between "class A" and "push-pull", and maybe confusing slew rate in there as well. They are unrelared types. There is both single ended class A and push pull class A, though only push-pull calss AB (at least in linear amplification).

When an amp has to switch directions of the current flowing out of it(whether driving a resistor, speaker, capacitor or a bowl of cereal) in PP class A, one device conducts more while the other conducts less. It controls current, the effect of an output voltage target is enforced by way of feedback - feedback applies more ore less current, within the limits of device capabilities. The feedback could be in the final stage onto (a "follower" configuration or space-charge feedback) but usually includes gain of preceding stages, all directing current to force instantaneous target voltages.

In single ended class A, the one device type conducts more or less along with signal , but can usually conduct much more than it can reduce its conduction (depending on bias level and saturation characteristics). But it is not infinite, and a device can be certainly be limited in how hard it can conduct as well as how much it can reduce its conduction. Tubes in particular have limits that way, but all devicesn do at some point. That is also true for each device in a push-pull class A amp.

In class A, most of that current being conducted isnt into the load, but into the other polarity device (push-pull or SE with active current source) or into passive circuitry (resistors, transformer gap, or inductor).

In a class AB amp (push pull implied), the currently conducting device can increase or decrease its conduction, with most of it going into or out of the load rather than through other amp circuitry. The non-conducting device switches in when its opposite can no longer decrease current conduction. It has no trouble doing that at peaks or non-resistive loads, though its transconductance is lower as it turns on (but he same feedback that makes its output target a voltage level also helps correct for that). In class AB, both devices can conduct much more than they can reduce conduction from their idle level, since the idle level is very low. So they can dishcharge or charge loads very quickly... just like (most) claas A push-pull amps. Clas A has no advantage at all there, otrher than not having as much nonlinearity for the fedback to correct at a conduction"handover").

You statement of tracking waveforms into non-resistors being "not even close" is easily disproved with an oscilloscope, or by subtracting input signal from scaled output signal and seeing how very little difference there is. In fact, I think you will usually see more difference between input and output with commercial class A amplifiers because their target markets often ask for lower overall feedback as well! Whether the types of differences are less likely to be audible than with higher feedback class AB is a subject of much discussion here.