When you are looking to buy an amplifier, often a class is listed. They never explain what this means, and many people just ignore this spec. What does amplifier class mean? How does this affect the sound of an audio amplifier? How does it affect the efficiency?
There are four basic classes of amplifier(in truth, there are classes between these, but for clarity we'll focus on the main four). They are:
- Class A
- Class B
- Class C
- Class D
Class A Amplifiers
Class A amplifiers are "active" during the full cycle of the wave - this means they are amplifying all the time, no matter where the wave is in it's cycle. Sound appears primarily in the form of a sine wave. A sine wave is very much like a stretched out circle. If you were to take a graphing calculator, and graph sin(x), it would produce a sine wave. Our wave reaches a peak value, goes down to 0 volts, then to it's negative peak. In order to amplify a signal, we use a "switch" - a transistor - that is controlled by the input to turn a voltage supply on and off. This causes our voltage supply to have the same pattern as the input signal, but because it has a higher voltage, we get a larger amplitude - simply, it's louder. The gain of the amplifier is how much louder it makes the output. Imagine that we have a signal that is 5v peak to peak. That means that it goes up to +5v, down to 0v, then keeps going down to -5v and repeats the cycle. If we are going to amplify this to 18v peak to peak, we can't just have our transistors switch on and off(known as going from saturation to cut-off). We need to operate our transistors in what's called the linear region. That means that the output of the transistor is going to be proportional to the input. So if our input signal goes up by one volt, our output will go up by one volt multiplied by the gain. If we have a gain of 10, our output would go up by ten volts.
If our supply voltage is 18v, then where does this ten volts come from? An element in the circuit has to make it. That element is our transistor. It produces the ten volts by only partially turning on. It's at 55% of being fully turned on. This has a disadvantage. It's behaving like a resistor, and dissipating that remaining 45% of our voltage as heat! That's wasted energy! That's cooling! What happens when the input wave is only a tenth of a microvolt above zero? We're converting most of our energy from the power supply into heat. This is a very inefficient way to run. This is the drawback of a class A amplifier. You get the most faithful reproduction of the input waveform, but it's the least efficient and requires the most cooling.
Class B amplifiers have a pair of transistors driving the output, of opposite polarity. With this amplifier, when the input wave goes negative, the positive side transistor turns off, and we begin using the negative side. This offers greater output efficiency, while giving us decent linearity. There's a bit of distortion that is introduced in the portion of the wave where the transistors switch over.
The class C amplifier conducts for less than 180 degrees of the wave. This lowers the amount of energy we're dissipating, and improves efficiency greatly. This style of amplifier works well for RF applications, but for audio it introduces a great deal of distortion. It's not really appropriate for a stereo.
The class D is a newer type. Newer chips have made it possible to achieve decent linearity and S/N ratio out of this design. This amplifier takes the transistors straight from saturation to cutoff, and ignores the linear region altogether. Our transistors aren't dropping the voltage, so the energy they dissipate is drastically reduced. A common means of making the output match the input is to drive the transistors using PWM(Pulse-Width Modulation). We turn them on and off at a predetermined frequency, as the input voltage rises, we leave the transistors on longer. After we feed it through a low-pass filter, we are left with a decent approximation of the input waveform. This is by far the most efficient, and is what you will normally see used in automotive stereo amplifiers. Downsides are that the transistors can produce noise because of running at higher frequencies. The PWM output is also a squarewave. If you are familiar with Fourier's work you know where this is going. Square waves have an almost infinite number of harmonics, so the noise these transistors produce is very broadband and can show up all over the place depending on the power. This requires good shielding and power supply filtering.
There are other classes that are less common. There are a lot of interesting circuits to play with. Later I'll post a discussion of a technique for measuring the gain of your new amplifier! So, go build already!