Switching current source for Class A?

[h_a]

Cool idea, very impressive!

Of course some hash will endup in the output, but certainly a creative solution!

Single ended ClassA - ClassD style

Have fun, Hannes

[darkfenriz]

Originally posted by AndrewT

2A (100% duty cycle) from the supply, but 3.8A (100% duty cycle) through the Follower.

How?

That's what I've been pointing, there is no voltage drop on current source, so it effectively works with half the voltage and double the current.

[QED047]

Thanks Hannes!

In doing a "cost-benefit analysis" I have to try and weigh the "hash" you refer to. At the moment It seems to be an entirely subjective cost as I'm finding that the ripple present at the sense resistor is immeasurable at the loudspeaker. On the other hand, the objective reduction in dissipation is huge.

With Class D the signal has to be accurately converted into a PWM train, and requires signal feedback to make it work in practice (I've not seen any designs that don't use signal feedback). For this reason alone I find Class D to be undesirable - no feedback is one of my key objectives.

[darkfenriz]

P.S. You can reduce the dissipation further by supplying the output mosfet follower from a tracking step-down-regulator diven by input signal. That would become some incredible ultra-efficient single-ended class A design!!!

[AndrewT]

It's become obvious to me at least that I don't understand this.

In the ultimate implementation we could have a single ended follower that dissipates Vce * Ic and the other dissipations approach zero.
That would be twice as efficient as a CCS loaded follower and 4times as efficient as a resistor loaded follower.

[Nrik]

QED047 - this is really a cool thing!
You should patent it.

[QED047]

Originally posted by darkfenriz
P.S. You can reduce the dissipation further by supplying the output mosfet follower from a tracking step-down-regulator diven by input signal. That would become some incredible ultra-efficient single-ended class A design!!!

Maybe, but a "tracking step-down-regulator driven by input signal" sounds a bit like a complete Class D amp Although I suppose you might mean for the supply to change outside the audio band (very slowly?) so as not to interfere with the signal. Perhaps simply slave the supply voltage to the preamp volume control?

But this would necessitate a switch-mode power supply design - which is a reasonably proposition but not one that is altogether appealing. Having said that, a buck regulator topology fed from a conventional transformer defining the maximum DC supply might be more palatable than a complete off-line design using HF transformers.

I often wonder if anyone has thought about using a digital delay between two DACs to "prepare" in advance a suitable supply voltage for the output stage. Without such "premonition" a significant loss of response to low to high signal level seems inevitable.

[QED047]

Originally posted by Nrik
QED047 - this is really a cool thing!
You should patent it.

I really haven't invented anything here. Besides, I think patents suck big-time. They're only worth the money you have to defend them with - and I deplore the practice of companies patenting stuff that's not inventive but merely common sense to everyone (e.g. Color Kinetics patenting the use of RGB LED lighting when the properties of additive light have been exploited ever since Sir Isaac Newton played around with a prism).

But you're right - half an amp based in this would be cool (to the touch)

EDIT: If not already established, this thread may serve as evidence of PRIOR ART dating from July 16 2009 (just in case some idiot does try to patent it)

[Nrik]

Originally posted by AndrewT
It's become obvious to me at least that I don't understand this.

I think - correct me if I'm wrong - that it goes down like this:
The CCS shifts between shortcircuit, and double the wanted pull down impedance with a 50% duty cycle smoothed out by the coil = result: continous current in the follower mosfet = class A sound!
Yeah?

But where is the oscilator?

[QED047]

Originally posted by Nrik


I think - correct me if I'm wrong - that it goes down like this:
The CCS shifts between shortcircuit, and double the wanted pull down impedance with a 50% duty cycle smoothed out by the coil = result: continous current in the follower mosfet = class A sound!
Yeah?

But where is the oscilator?

I don't think you have uinderstood it completely. I'll have another go at explaining how it works: The inductor prevents current flowing into a dead-short when the bottom transistor switches on. Instead, due to the reactive nature of the magnetic field opposing the current, the current rises linearly in time - in proportion to the value of inductor and the potential across it. The differential amplifier senses the current build-up and trips the comparator (switching off the transistor) when it reaches a set point. The current then decays (linearly) and when the hysteresis of the comparator circuit is exceeded, the transistor switches back on. Hence it is self-governing and oscillates at a variable rate depending on the potential of the follower stage. The frequency is determined by a number of factors, but they are all controlled (do not depend on load).