Trying to explain amp classes is a little complicated, but I’ll try.
Transistors don’t just turn on as soon as you apply some sort of signal to them. It depends on the transistor, but a typical value just to turn them on is a .7V difference between the gate and source or base and collector pins. It can vary a good bit though.
That’s a problem. For the sake of discussion we’re going to talk about complimentary push-pull topology because all class AB amps are that topology. If it takes .7 volts (an arbitrary number for description sake), the trasistors aren’t going to produce any gain until that signal strength is reached. That produces what’s called crossover distortion. Crossover distortion is nonlinearity in gain as the signal swingings from positive to negative and is handed off from the transistor pushing to the transistor pulling.
The solution to crossover distortion is to bias the transistors. Biasing is putting some DC on the gate or base pins of the transistor to turn it on. This is where we get into the difference between class AB and class A. In a class AB amp the bias is just enough to turn the transistors on a little bit just to get the into their linear region and past that .7 volts where they do nothing. In a class a A amp you go much further than that. In a class A amp you bias the transistors deep into their linear region. This causes them to draw amps of current at idle instead of milliamps like in class AB. That’s why class A amps are so big and heavy. The cooling solutions need to be vastly more robust and the power supply needs to be much more muscular.
As a class AB amp amplifies a signal, one half of the amp amplifies the positive half of the signal while the other half amplifies the negative half. In a class A amp both halves of the amp are biased so high that (ideally) neither side of the push-pull ever shuts off and both sides carry the signal completely. The benefit of this is that the nonlinearity of the individual gain devices are minimized. The strengths of one transistor minimize the weaknesses of the other transistor. That’s the idea at least.
A good class AB amp with well matched parts can hold it’s own against a class A amp, but many, myself included, feel class A has superior sound.
Additionally, a push-pull class A amp can easily transition to class AB and belt out 2 or 3 times it’s rated power for headroom vastly higher than it’s nominal power rating suggests. It does so with a small distortion penalty. However, class AB has a similar distortion penalty as the signal surpasses the low bias. Typically that shows up as higher distortion at low power levels and in micro-details.
Class D is a radically different animal. In class D the devices are only turned on at super high frequencies related to the instantaneous demands of the signal. The output signal is nothing more than very high frequency pulses in proportion to the input signal. The output then needs to be extensively filtered to turn it back into a linear waveform without the high frequency noise produced by the constant switching of the gain devices. Opinions on class D are world’s apart. Some hate it. Some love it. Ultimately, the gold standard class D is being judged by is class A, and I don’t think anybody disagrees with class A being the ideal class of amplification.
If you have any other questions, feel free. To get better clarity on the subject of classes I suggest Nelson Pass’s article "leaving class A" and his Burning Amp lecture "selected topics on push-pull topologies"
Transistors don’t just turn on as soon as you apply some sort of signal to them. It depends on the transistor, but a typical value just to turn them on is a .7V difference between the gate and source or base and collector pins. It can vary a good bit though.
That’s a problem. For the sake of discussion we’re going to talk about complimentary push-pull topology because all class AB amps are that topology. If it takes .7 volts (an arbitrary number for description sake), the trasistors aren’t going to produce any gain until that signal strength is reached. That produces what’s called crossover distortion. Crossover distortion is nonlinearity in gain as the signal swingings from positive to negative and is handed off from the transistor pushing to the transistor pulling.
The solution to crossover distortion is to bias the transistors. Biasing is putting some DC on the gate or base pins of the transistor to turn it on. This is where we get into the difference between class AB and class A. In a class AB amp the bias is just enough to turn the transistors on a little bit just to get the into their linear region and past that .7 volts where they do nothing. In a class a A amp you go much further than that. In a class A amp you bias the transistors deep into their linear region. This causes them to draw amps of current at idle instead of milliamps like in class AB. That’s why class A amps are so big and heavy. The cooling solutions need to be vastly more robust and the power supply needs to be much more muscular.
As a class AB amp amplifies a signal, one half of the amp amplifies the positive half of the signal while the other half amplifies the negative half. In a class A amp both halves of the amp are biased so high that (ideally) neither side of the push-pull ever shuts off and both sides carry the signal completely. The benefit of this is that the nonlinearity of the individual gain devices are minimized. The strengths of one transistor minimize the weaknesses of the other transistor. That’s the idea at least.
A good class AB amp with well matched parts can hold it’s own against a class A amp, but many, myself included, feel class A has superior sound.
Additionally, a push-pull class A amp can easily transition to class AB and belt out 2 or 3 times it’s rated power for headroom vastly higher than it’s nominal power rating suggests. It does so with a small distortion penalty. However, class AB has a similar distortion penalty as the signal surpasses the low bias. Typically that shows up as higher distortion at low power levels and in micro-details.
Class D is a radically different animal. In class D the devices are only turned on at super high frequencies related to the instantaneous demands of the signal. The output signal is nothing more than very high frequency pulses in proportion to the input signal. The output then needs to be extensively filtered to turn it back into a linear waveform without the high frequency noise produced by the constant switching of the gain devices. Opinions on class D are world’s apart. Some hate it. Some love it. Ultimately, the gold standard class D is being judged by is class A, and I don’t think anybody disagrees with class A being the ideal class of amplification.
If you have any other questions, feel free. To get better clarity on the subject of classes I suggest Nelson Pass’s article "leaving class A" and his Burning Amp lecture "selected topics on push-pull topologies"