Amplifier Classes - Differences?

[JJ]

I hear about Class A, Class A/B, Class B, Class D amplifiers - I think the difference is in regards to their power amplification methods - hence the difference in size between the As and A/Bs over the Ds? Could someone clarify?

[JasonR]

Power amplifier classes

[edit] Angle of flow or conduction angle
Power amplifier circuits (output stages) are classified as A, B, AB and C for analog designs, and class D and E for switching designs based upon the conduction angle or 'angle of flow' Θ of the input signal through the amplifying device, that is, the portion of the input signal cycle during which the amplifying device conducts. The image of the conduction angle is derived from amplifying a sinusoidal signal. (If the device is always on, Θ = 360°.) The angle of flow is closely related to the amplifier power efficiency. The various classes are introduced below, followed by more detailed discussion under individual headings later on.

Class A
100% of the input signal is used (conduction angle Θ = 360° or 2π, i.e. the active element works in its linear range all of the time). Where efficiency is not a consideration, most small signal linear amplifiers are designed as Class A, which means that the output devices are always in the conduction region. Class A amplifiers are typically more linear and less complex than other types, but are very inefficient. This type of amplifier is most commonly used in small-signal stages or for low-power applications (such as driving headphones).

Class B
50% of the input signal is used (Θ = 180° or π, i.e. the active element works in its linear range half of the time and is more or less turned off for the other half). In most Class B, there are two output devices (or sets of output devices), each of which conducts alternately (push–pull) for exactly 180 deg (or half cycle) of the input signal; selective RF amplifiers can also be implemented using a single active element.
These amplifiers are subject to crossover distortion if the handoff from one active element to the other is not perfect, as when two complementary transistors (i.e. one PNP, one NPN) are connected as two emitter followers with their base and emitter terminals in common, requiring the base voltage to slew across the region where both devices are turned off.[3]

Class AB
Here the two active elements conduct more than half of the time as a means to reduce the cross-over distortions of Class B amplifiers. In the example of the complementary emitter followers a bias network allows for more or less quiescent current thus providing an operating point somewhere between Class A and Class B. Sometimes a figure is added, e.g. AB1 or AB2, with higher figures implying a higher quiescent current and therefore more of the properties of Class A.

Class D
These use switching to achieve a very high power efficiency (more than 90% in modern designs). By allowing each output device to be either fully on or off, losses are minimized. The analog output is created by pulse-width modulation, i.e. the active element is switched on for shorter or longer intervals instead of modifying its resistor. There are more complicated switching schemes like sigma-delta modulation, to improve some performance aspects like lower distortions or better efficiency.

Other classes
There are several other amplifier classes, although they are mainly variations of the previous classes. For example, Class H and Class G amplifiers are marked by variation of the supply rails (in discrete steps or in a continuous fashion, respectively) following the input signal. Wasted heat on the output devices can be reduced as excess voltage is kept to a minimum. The amplifier that is fed with these rails itself can be of any class. These kinds of amplifiers are more complex, and are mainly used for specialized applications, such as very high-power units. Also, Class E and Class F amplifiers are commonly described in literature for radio frequencies applications where efficiency of the traditional classes deviate substantially from their ideal values. These classes use harmonic tuning of their output networks to achieve higher efficiency and can be considered a subset of Class C due to their conduction angle characteristics.

http://en.wikipedia.org/wiki/Electro...lifier_classes

[Big Daddy]

Read this article. You may not be able to understand everything, but the author does a good job.

http://www.audiodesignline.com/howto/177102531

[JJ]

Originally Posted by Big Daddy

Read this article. You may not be able to understand everything, but the author does a good job.

http://www.audiodesignline.com/howto/177102531

Bah, you don't give me enough credit - let's see what we have here...

[crazyBLUE]

Good post JJ ! Iread both & will do it again A couple more times just to make sure I understand !! THANK'S !

[Big Daddy]

Originally Posted by JJxiv1215

Bah, you don't give me enough credit - let's see what we have here...

There will be a quiz tomorrow. We expect you to explain the article in plain English (no Spanish please). You must start with these two formulas:

[gearyt]

Originally Posted by Big Daddy

There will be a quiz tomorrow. We expect you to explain the article in plain English (no Spanish please). You must start with these two formulas:

ok, let me get my slip stick out.... ( slide rule for you youngies )

[Purplegrasshopper]

I read the articles, and I'm still not clear...are class D amps better than class A/B amps? If so, I'm assuming they are more expensive?

[welwynnick]

Different, not better.

I've had a few; they can be very good indeed.

[FIDDYPOP]

I thought this was only applicable to car audio. From my understanding class A is for your highs, class D is for your lows. Class D pushes subs.

[Decepticon07]

Originally Posted by FIDDYPOP

I thought this was only applicable to car audio. From my understanding class A is for your highs, class D is for your lows. Class D pushes subs.

No, this is applicable to home audio as well. Class D's do exactly as stated earlier, less power loss and more efficient. They are usually more expensive and most run cooler than an A, B, or AB.

[FIDDYPOP]

So when would these be appropiate? For instance I have a class D amp in my car pushing my subs and a class A amp pushing my highes. That is where I have heard these terms. How does this come into play in home audio?

[welwynnick]

Getting a class D amplifier to work well for bass is easier than for full-band audio, but then they have only been developed for a fraction of the time that class A & B have had.

For this reason, class D was introduced for mobile applications where size, weight, consumption and dissippation are important. And for powerful active subwoofers.

However, companies like Sony, Integra, Bang & Olufsen (ICEPower), NuForce and Philips (ICD?) have been leading the way with high quality, full-band class D amplifiers. I won't tell anyone what to think - just have a look at the reviews on the NuForce website and see whether class D can compete at the highest level.

http://www.nuforce.com/

Nick