Aleph : Output Mosfet Matching

[vinay]

Perhaps I am missing something really obvious but.....

If the output gain mosfet(s) is fed by the constant current source(s), then regardless of the Vgs (of any mosfet, gain or current source), each gain device will get exactly the same current as determined by the source resistor of the current source feeding that device.

If this rationale holds (??), why is matching necessary?

Thx

Vinay

[GRollins]

Yes...and no...
There are several factors at play here, but amongst other things you want each device to respond (as nearly as possible) the same to the signal that comes from the front end differential. Taken to extremes you could have two devices, one not conducting at all and the other going full boat. Hmmm. Not a balanced picture, here. That's the distortion perspective.
Bear in mind that it takes two to tango. The current source and gain MOSFETs are in series. If you were to shut the gain device completely off, the current source(s) would try very hard--but ultimately fail--to force current through the gain device; it's just a lump of plastic and metal with no more life to it than a pencil eraser. Current sources are a powerful tool, but not omnipotent. If one gain device isn't receiving enough voltage at the Gate to "go live" then it is no longer part of the equation. It's not delivering current to the output and the others are having to do all the work. That ain't fair.

Grey

[Nelson Pass]

Parallel devices often share all pins, and so you
can't allocate the current via current sources for each.
Hence matching.

Matching is easy to do, though.

[vinay]

It becomes clearer now:

So even if there are three current source mosfets providing 1amp current each, with respect to the three gain devices it appears as though a single pool of 3amps is available and they have to share this evenly amongst themselves.

Thanks!!!

vinay

[GAK]

Something like that.
You have three currents from the current sources that add up at the common junction. This makes one big current, like it was one big current source. This current is the fed through the three gain mosfets . Each mosfet though doesn't use exactly one third from this current. Depending on it's characteristics it uses a different current. That why we measure the Vgs of each and match them for the same drive or bias. Same bias means same current and same current means all work with almost one third the current.

[Raka]

Hi,

Besides, it's not a equal-rights to share the current, but that the mosfets should have same bias point given for the junction voltage and current, that means keep in the same curve point i.e. same performance for a signal. I think this is the important point for distortion, isn't it?

[Nelson Pass]

Maybe, maybe not. Mostly we are concerned that
any given Mosfet not get much more than its share
of the current so that we don't run into reliability
problems.

Experimentally I have built Alephs with and without
matching, and have not seen a "lot" of difference either
in sound or distortion specs.

[fcel]

It's good to hear that there isn't much difference in sound and distortion.

On the subject of output matching, won't it be as important to match the source resistors since those resistors are allowing the certain amount of current to go to the output transistors?

Or have I got it backwards? ... current is flowing out of FET to the source resistors.

[Nelson Pass]

Checking what's out there, any quality resistor will
be matched a percent or so. I use lots of the 3 watt
Panasonics, and they're all pretty tight.