Audio amplifiers, class-T, class-W, class-I, class-TD and class-BS

The last few years several companies have been inventing amplifier classes, not as a legitimate architecture class, but as a marketing trick. The first I heard about was TriPath who called their class-D amplifier a class-T. Interesting to note they are out of business now. Last year a nice marketing guy from Wolfson told me about their new “class-W” amplifier in their new codec. He kept asking me what I thought of the class-W moniker that I assumed he dreamed up. He was a nice guy and Wolfson is a really excellent company, so I didn’t have the heart to tell him that I thought inventing amplifier classes as a marketing ploy only degrades the message. At least they put scare quotes around the name.  I was at Fairchild last week and they talked about a class-F amplifier. I sure hope they are referring to the real class-F amp as described in the literature and not some marketing gimmick that uses first letter in their company name. The excellent amplifier class overview in Wikipedia says that Crown dreamed up a Class-I and Lab.gruppen has fabricated class-TD. Well, I consider all these to be class-BS. It does no service to engineers to invent a plethora of amplifier classes when pretty much every possible method already has a class. You can imagine my relief when Maxim called me up last year about their new audio chips and they properly called it a class-G rather then saying it was class-M.

Marketing people love to throw around adverbs and adjectives. Part of an editors job here at EDN is to strip out all the hyperbole and just give you the bare facts so you can make up your own mind. We all remember those op-amp datasheets with “low-power” in the title and they use 12 mA. That may have been low power in 1985, but we do a lot better today. If you want your company name to be in the initials of the amplifier class go buy Allen Bradley and introduce a new class-AB amplifier. ‘Til then, the engineering community is best served by the standard classes described in the Wikipedia article. Those are:

• Class-A            Used by audio nuts to waste power.
• Class-B            Used by the LM324 op amp, but you can pull up the output to force it into class-A. Cheap but it has crossover distortion.
• Class-AB            Used by most op-amps, it is a class-B with both transistors turned on a little to eliminate the crossover distortion.
• Class-C            For RF amps, a pulse drives a resonant output stage. A church bell is a class-C amplifier, the clapper supplies a pulse excitation and the resonant system makes a sine wave come out.
• Class-D            A switching power supply modulated at audio frequencies. Seeing a lot of use in audio amps. Texas Instruments, Maxim, Analog Devices and National Semi do great in this as well as fellow biker Skip Taylor over at D2Audio/Intersil. Getting a lot of use in LCD televisions to reduce power supply cost and conform with green power regulations. Inverters and motor drivers are essentially a class-D amp. Class-D has really crappy power supply rejection ratio (PSRR) unless it has feedback like Freescale did with its Symphony series meant for cars, where the battery voltage goes all over the place.
• Glass-G         This is what Maxim came out with last year. To save power you feed a class-AB output stage with stepped power rail voltages. For small outputs you feed the output transistors a lower voltage. For more power and larger output waveforms you switch in one or more higher power supply voltages. This is great to save battery power in cell phones, also to drive piezoelectric speakers along with a charge-pump, one of Maxim’s cool technologies.

These are the main classes that are in use; there is also a class-I that sounds like a power converter architecture (not sure if this is the Crown invention) and class-J that combines a class-D with a class-B. The class-S is now called a class-D, see the reference website in the Wiki article. Others are:

• Glass-H            Takes the class-G one step further and supplies a continuously varying power supply to the output transistors so the transistors never clip, but the peak voltages are just a volt or two under the instantaneous power supply. This saves power but has no harmonics and switching noise like a class-D. Class-D amps have gotten so good this class-H may never see much use in audio.

• Class-E            A single transistor resonant circuit that is fed by an inductor like a class-C and has an output filter like a class-D. I never hear much about this class.
• Class-F            Like class E, a resonant driver, else it would be a class C otherwise.

These classes might be used more in RF, I described the Doherty amplifier in my article about RF power amps. Now if Jim Williams or Bob Pease or Dave Freeman or Barry Harvey or Barry Gilbert calls me up all excited at four in the morning and tells me that they have invented a new amplifier class, I will be the first to tell you about it, but if it is a marketing person, well I have a very big grain of salt on my nightstand to take along with any claim of a new amplifier architecture class.