Any practical applications for class b amplifiers?

[KaffiMann]

Just wondering, in this day and age it might not be applicable to anything at all.
But to this day and age, I have only seen and used various amplifiers of these different topologies:
Class A, AB, C, D and T.

I have never even seen a Class B amplifier, neither as a finished product or in use as a DIY solution for a particular use-case.

Can anyone point me towards a Class B amplifier in practical use, or as a finished "off the shelf" product?

 

[JonSnell Electronic]

Class B amplifiers have a much better efficiency than class A. That's about it.
Class-B is often used for radio frequency broadcast amplifiers, where high power and low spurious RF emissions (sorry, no class-D in radio!) make it a great choice. On its own though, class-B suffers from relatively high (crossover) distortion and is unsuitable for audio.
When a little quiescent current is applied though, the output transistors are biased to stay on past the zero-cross point, and you then have a class AB amplifier. This dramatically reduces distortion and is what most proper consumer-grade hi-fi amplifiers use these days.

 

[ejp]

Practixly every solid state amplifier you will find is class B. There is an unfortunate fashion in audio for describing them as AB but going by the original definitions using conduction angles that’s incorrect.

A ‘class B’ amplifier with ‘relatively high crossover distortion’ is really operating in class C.

 

[MarcelvdG]

There are different definitions of class B in use. Some call it class B when the quiescent current is chosen such that the amplifier is in a distortion minimum and class AB when the quiescent current is higher than that. No bias at all is then class C.

In the valve age, class B also meant a small quiescent current that's large enough to get rid of crossover distortion, and class AB a somewhat higher quiescent current.

 

[Jeffh01]

Douglas Self’s “Compact Blameless Amplifier” has excellent distortion figures, is class B, is available as a bare pcb and, is available here: http://www.signaltransfer.freeuk.com/compact.htm

I built one (more to follow) and I think it is the best amp I’ve built to date.

Comments above that class AB is really class B are correct according to Douglas Self. Bob Cordell prefers to call these Class AB and is generally happy with highly biased solutions.

The Chipamp LM3886 is a class B chip and is a very popular device. See Tom’s Neurochrome site to see how good these can be.

So class B is very much alive and well and available commercially.

 

[Mooly]

I have never even seen a Class B amplifier, neither as a finished product or in use as a DIY solution for a particular use-case.

I know of only one and that is the Creek 4040v1 Even more remarkable is the fact the output pair are complementary Darlingtons and so have a huge 'dead zone'. The v2 of this amp did add a couple of diodes to lessen 'that gap'.

 

[MarcelvdG]

The QUAD current dumping amplifiers are class-C&A: the dumpers have a dead zone, but it is filled up in a clever way with a class-A part.

 

[planet10]

Aren’t chips like LM1875/3875/3886 basically running Class B? Transistora close enuff to matched to not have to turn the bias up.

A Class B amplifier shouldn’t a\have GM droop?

dave

 

[alexcp]

Class B is subject to much misunderstanding. It is often said that a pair of output transistors operated without any bias are working in Class-B, and therefore generate severe crossover distortion. In fact, with no bias each output device is operating for slightly less than half the time, and the
question arises as to whether it would not be more accurate to call this Class-C and reserve Class-B for that condition of quiescent current which eliminates, or rather minimises, the crossover artefacts

So what's called Class B in audio is actually Class AB in terms of conduction angles.

 

[AjohnL]

There was some work in the cross over distortion area that stated in the 70's I'll try and upload a pdf but it may be too big. If so wireless world 71 Feb and Mar. There was also a later design. I've lost somehow or the other. 😉 Comment too good for HiFi and more suitable for instrumentation.

The idea has been mentioned in a post but it's still class B.

 

[Citizen124032]

Hearing aid equipment, like magnetic loops in auditoria. Freq range from 400 till 4000 will suffice. B is heavenly for those. Engage!

 

[Jeffh01]

@planet10 A class B amp amp doesn’t have “GM doubling” whereas class AB does. Beta droop at high currents affects both equally assuming the same topology.

 

[Jeffh01]

@Mooly , Doesn’t the “Dead Zone” make that class C?

 

[planet10]

@planet10 A class B amp amp doesn’t have “GM doubling” whereas class AB does.

Yes, that is what i understand, GM will be a straight line (with the slope the same as the ‘droopy” part of an AB curve?) in Class B.

dave

 

[planet10]

Doesn’t the “Dead Zone” make that class C?

That illustrates the problem with Class B. Trying to get the 2 pieces to line up with no dead zone and no overlap. dead zone = Class C. Overlap = Class AB.

dave

 

[WhiteDragon]

But basically the benefit is less idle current.
less power consumption.
More possibility to be thermally stable
are not runaway thermally. Such as high heat/high power
situations or locations/cases which have limited
ventilation.

Can take a AB amp and make up all kinds of names.
" push pull A" " class B"

you either over bias or under bias a push pull amp.
And would rather call it over or under biased B lol

 

[EdGr]

The "RCA Solid-State Power Circuits Designer's Handbook" (1971) described a class B amplifier. It was intended for less expensive equipment (read: not worth repairing). The RCA book also described a class AB amplifier for better equipment.

To be pedantic, I will point out that the connected NPN and PNP bases make this a true class B amplifier. The collector current is exactly zero when Vbe is zero (according to Ebers-Moll). For any non-zero Vbe, one transistor conducts, just not appreciably until Vbe reaches 0.6 volts. The exponential relationship between Ic and Vbe is applicable down to 0 volts.
Ed

 

[Mooly]

@Mooly , Doesn’t the “Dead Zone” make that class C?

I guess it does, yes. The v2 added diodes but technically it is still Class C as you say.

 

[MarcelvdG]

To be pedantic, I will point out that the connected NPN and PNP bases make this a true class B amplifier. The collector current is exactly zero when Vbe is zero (according to Ebers-Moll). For any non-zero Vbe, one transistor conducts, just not appreciably until Vbe reaches 0.6 volts. The exponential relationship between Ic and Vbe is applicable down to 0 volts.
Ed

At a nonzero collector-base voltage, there must still be some collector-base junction leakage current flowing. Anything is class A when you take the stuff about conduction angles too literally, as there is always some leakage.

 

[Jeffh01]

I would have to agree that Douglas Self’s “Optimally Biased” class B could be considered Class AB and IIRC, so does he. However, the amount of Class A power in an optimally biased cfp output stage can be measured in microwatts. I find that defining that “Goldilocks” zone for optimally biased amps in class B does make sense, since he demonstrates that in practical designs that definition of Class B produces a lower distortion amplifier than Class AB due to the sudden gain transition out of the class A region for the AB amps that is not present in the class B ones.

So that is why it makes sense to me to use the class B designation. The old fashioned zero bias class B is really Class C.

However, much as I take a great interest in this, I am far from being an authority so I think the ambiguity here will remain.

At the end of the day though I do think we answered the OP’s question! 😀