amp efficiencies

[xplod1236]

What makes class d amps more efficient than class a?
How do class a amps work that makes them so inefficient?
What's the difference between class a and b amps?
I know that class d amps switch the output transistors at high frequencies, but I don't see how that will make them more efficient.

[Sch3mat1c]

Well first of all let's go over the classes:

Class A = always conducting
Class B = push-pull only, one side drives the positive half of the wave, the other side drives only the other. Hence each is on for half the waveform
Class AB = a cross between A and B where, within a certain (small) signal range, both devices are conducting simultaneously as a class A amplifier
Class C = both are off for less than 1/2 the wave. For small signals, rather than being treated well (as with class AB), they are cut out entirely because neither side (positive or negative) is conducting, hence unable to amplify it.
Class D = always in two states: ON or OFF.

Class A must burn a distinct amount of power constantly, because it has to be "ready" to accept a signal. This alone makes it atrocious because in this condition it is 100% inefficient, consuming power for no output whatsoever. At maximum output it is maybe 33% efficient(?), even with perfect components (which transistors approach pretty well).

Class B gains a significant bit of efficiency due to the devices being off at rest. Sorta. Pure B is a mathematical curiosity, unattainable in practice because devices always conduct a little bit, even in the cutoff region. So technically it's either class AB or C, never exactly B. For this reason of course, there is a practical definition, but I don't know what it is. Efficiency is maybe 65% max.

Class C produces tremendous amounts of distortion (known as crossover distortion) because the devices are off for a certain percentage of the waveform. No good for audio. It does, however, find use in radio because the distortion products are filtered out by the tuned circuits. Efficiency is up around 75% for some transmitting tubes.

Now, class D would seem utterly useless for audio because you don't listen to ones and zeroes! (If you have a CD player that's old enough to play data CD's as audio, you'll know how horrible this sound is.) However, there's this magical technique known as pulse width modulation where you take a square wave and control how much time it spends ON vs. OFF, thereby causing an average shift in the output, after integrating out the carrier wave that is. Transistors make awesome switches and as such, efficiency is easily above 90%.

To sum up, class A amplifiers are inefficient because the varying current, which produces the very signal itself that you hear, must always be flowing, and controlled by the device. Class D has this current flowing from a filter instead, which is then supplied by pulses of HF current from the PWM amplifier.

Tim

[bigparsnip]

Hi, I think one thing that you missed out from your point above here is why you will burn more power in class A or A/B amps as opposed to class D.

Basicaly, when a transistor is condicting curent, and has some voltage dropped accross it it will dissipate some power. I in the case of class A, at zero input signal, there will still be all of the bias curent flowing through the transistor, but it will also have the full supply voltage droped accross it as well, meaning that all of the power is being dissipated in the transistor and none in the output load.

For class A/B you will have a much smaller bias curent when no signal is present so that for a given rail voltage the dissipation at zero signal input will be much lower. This also continues as the output power increases, as the trasistors are only drawing as mcuh curent as needed rather than the maximum output current all of the time, so when take into account with the similar voltage drps the wasted power will still be lower as the lower curent gives a lower value of IV for the transistors.

Class D however works a bit differently to either of the above, as Sch3mat1c said, the transistors are either ON or OFF (except when they are switching, but that makes things rather complicated if you start to get into that area) so the transistors can be in either of tow states:

Fully on, with high curent flowing through them, but almost no voltage drop across them, hence low IV and low power dissipation

Fully off, with the full supply voltage dropped accross the trasistor, but no current flowing, so again zero or at least very low IV and almost no power dissipated.

Anyway, that is a kind of brief description of how they burn up their power and may hopefuly help show why they are of varying efficiencies (and hopefully I haven't made too many mistakes either).

Andrew.

[xplod1236]

What class are chip amps? (lm3886)

[Sch3mat1c]

Whatever they are. I think most are B or AB?

Some are class H, where charged capacitors are switched in series with the output stage supply, thus momentarily increasing peak output capability! Amazing stuff...

Tim

[xplod1236]

can you post a schematic or a sketch of a class h?
I can't figure out where the switched caps fit in.