What are they ? How do they work ?
Well I don't know or I didn't know.
But I do know someone who does.

His name is : Archie A Anderson and this is what he said.



" The first thing you need to know about are the different parts of the sine wave. When you have an AC signal moving in a wire, to begin with the electrons are stationary than they begin to move in one direction they build to their peak than slow to a stop and begin to move in the opposite direction built to a peak then they slow to a stop. This describes one complete cycle. If you were to attach a pen to the bottom of a pendulum and move paper at a constant rate under it ( like a strip recorder ) the pendulum at rest would be 0 volts. As it swung it would draw a sine wave. 0 volts would be the beginning or 0 degrees. the first or positive peak would be 90 degrees as it crossed 0 volts again that be 180 degrees and the second or negative peak would be 270 degrees. As the sine wave drops back to 0 volts it ends at 360 which is also 0 degrees for the next cycle. If we pick different points on the sine wave and describe in terms of degrees, we are in fact taking about different points in time. For example if we look at 45 Degrees we are discribing the point that is halfway between 0 and 90 degrees. If the voltage is +10 volts at 90 degrees than the voltage at 30 degrees would be 5 volts and the voltage at 45 degrees is 7.07 volts. The sine wave in fact is based on the voltage induced in a coil rotating in a stationary magnetic field. The reason for all this explanation is that the class of operation or an amplifier has everything to do with how many degrees of a sine wave, do the devices in question turn on and conduct.

There are four main classes of amplfiers. There are other classes out there but thats a different subject, for a different day. The four main classes of amplifier are Class A, Class AB, Class B, and Class C.

Class A : this means that the device in question for 360 degrees of the sine wave. It never turns off. It has to be biased so that its idling current is half way between its maximum out and its minimum out. As such you can imagine that these amps run rather hot.

Class AB: This means that the amplifier conduct more than 180 degrees but less than 360 degrees. In this class of amplifier, it might for example, conduct at 0 degrees though 225 degrees. at 225 degrees it would enter cutoff and stop conducting. it would turn back on at 315 degrees and stay on to 360 degrees, the end of the cycle. so the amplifier would conduct for 270 of the 360 degrees of a sine wave.

Class B: This means that the amp conducts for exactly 180 degrees. For example a class b amp would conduct only the positive half of a sine wave and remain off during the negative half. or vise versa.

Class C: This means that the amp conducts for less than 180 degrees. For example, a class C amp might begin conducting at 45 degrees and shut off again at 135 degrees meaning that it was on for only 90 degrees.

Class A gives the best linearity and class C gives the worst. Where things get a little strange is when you type or the topology of the amplifier combined with the class. Most of the pills amps discussed in this forum are push pull type amps, which means the the amp has 2 halves that are mirror images of each other. One half handles the positive (0 to 180 degrees) half of the sine wave and the other half covers the negative (180to 360 degrees) Half of the sine wave. You can have the each half of the amp running class B and have the whole circuit behaving as though it was a class A amp. Basically you add the angle of conduction for each half of the amp to get the class for the whole. This is a very simple view. There is more to it than that but, you get the idea. In the case of pill amps often each half is running class c, maybe conducting 150 degrees. but when you add the halves together, the amp conducts for around 300 degrees. that puts you well into class AB which gives pretty good linearity. at least for voice.
If you see a number following the class that goes back to the days of tubes. That refers to if, by design you allow the grid to conduct current or not. For example Class AB-1 does not allow grid current and is much more linear than class AB-2 which does allow grid current and can produce a lot more power although at much higher distortion. "

The sine of an angle in a right triangle is defined as the ratio between the opposite side and hypotenuse.  In the table below you see the values of sine for all even angles from 0 to 90 degrees.  Most of the time the sine of an angle ends up being an irrational number, so the values in the table are actually just rounded values to six decimal places.  Sometimes, though, the sine of an angle is not an irrational number but a rational number.  Can you spot a couple of special angles where the sine of the angle is a rational number? Angle Sine of angle Angle Sine of angle Angle Sine of angle 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0 0.034899 0.069756 0.104528 0.139173 0.173648 0.207912 0.241922 0.275637 0.309017 0.34202 0.374607 0.406737 0.438371 0.469472 0.5 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 0.529919 0.559193 0.587785 0.615661 0.642788 0.669131 0.694658 0.71934 0.743145 0.766044 0.788011 0.809017 0.829038 0.848048 0.866025 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 0.882948 0.898794 0.913545 0.927184 0.939693 0.951057 0.961262 0.970296 0.978148 0.984808 0.990268 0.994522 0.997564 0.999391 1 This data gives us 46 pairs of numbers.  Each number pair corresponds to a single point on the coordinate plane.  In each case, the x-value is the angle, and the y-value is the sine of that angle.  This is how it looks like when those 46 dots are plotted.  It's the start of the familiar sine wave form! If you plotted more points, the graph would look more continuous.  Or you could connect the existing points with lines.  That's exactly what a scientific calculator does: it calculates a certain number of points, and connects those with lines. In this table are the same values for the sine of angle, and the same angles, but the angles are measured in radians, which is usually the default way of measuring angles in calculators.  The graph produced looks the same. Angle (radians) Sine of angle Angle (radians) Sine of angle Angle (radians) Sine of angle 0.000 0.035 0.070 0.105 0.140 0.175 0.209 0.244 0.279 0.314 0.349 0.384 0.419 0.454 0.489 0.524 0 0.034899 0.069756 0.104528 0.139173 0.173648 0.207912 0.241922 0.275637 0.309017 0.34202 0.374607 0.406737 0.438371 0.469472 0.5 0.559 0.593 0.628 0.663 0.698 0.733 0.768 0.803 0.838 0.873 0.908 0.942 0.977 1.012 1.047 0.529919 0.559193 0.587785 0.615661 0.642788 0.669131 0.694658 0.71934 0.743145 0.766044 0.788011 0.809017 0.829038 0.848048 0.866025 1.082 1.117 1.152 1.187 1.222 1.257 1.292 1.326 1.361 1.396 1.431 1.466 1.501 1.536 1.571 0.882948 0.898794 0.913545 0.927184 0.939693 0.951057 0.961262 0.970296 0.978148 0.984808 0.990268 0.994522 0.997564 0.999391 1 Now you might ask, "Doesn't the graph continue?  What about angles that are greater than or equal to 90 degrees?  I can't even draw a right triangle that would have a 91 degree angle (not even one with 90 degrees)." You are right.  You can't draw a right triangle with a 90 or 91 or 200 degree angle.  That's where the right-triangle definition of sine comes to its limit, and that's where the unit circle and radians that you see in math books come into play.  With the unit circle, one can define sine and other trigonometric functions for all kinds of angles, and using that definition, one can then draw the graph of sine function further and get the familiar wave: