The final say/ Question on Output transistor matching.

[diydidi]

I know a lot has been written on the topic of parallel output devices.
But, after repairing over 100 power amps of varying makes in the past two years, i have started once again questioning the matching issue again.
I have been matching power BJT 's on a home made matcher and the process has been really cumbersome, not to mention expensive.
I was reading a post on FB where a guy fixed some hifi amp without matching, because he reasoned that current sharing between parallel devices was insured by the emitter resistors.
This makes sense to me and its something I want to try out. Thoughts?
I however still think mosfets used as output devices need to be matched because of the really large Vgs spread on these devices.
Can someone please concur my thoughts on this topic.

[J M Fahey]

It all depends on actual level of mismatching and whether you use emitter resistors or not.

A few amps did not use them, and just a 1 or 2 ohm resistor in series with the Base, such as old Acoustic amps.

Those *absolutely* require matching .

Now a sensible emitter resistor value will do lots of "forced" current sharing equalization.

5A into 0.33 ohm emitter resistors, which is a very usual set of parameters, means 1500mV between Base and Rail voltage needed to drive that transistor, which must be added to Vbe around 700mV, for a total driving voltage of 2200mV.

Compared to that, a **GROSS** 200mV mismatch (which is never ever seen) amounts to 10% difference in current sharing.
I can live with that

You will find an important difference when biasing, obviously under idle conditions, then in a bunch of parallel transistors one might show, say, 5mA, another 10mA, another 20mA , a 4:1 spread which drives many nuts .

But then even if dissipation spread is also 4:1 ... we are talking very low values, both current and power ... once "all horses are pulling the loaded cart" differences even out for good.

I have almost always replaced just damaged transistors, leaving the old but good ones, and never noticed a problem.

As of MosFets, it again boils to idle bias differences; I would never pair a 200mV class Lateral Mosfet to a 3.8V class Vertical onbe, thatīs **gross**, or to a 700mV class Bipolar, which besides takes Base current, but have no problem in replacing with same type as others staying in their places.

That said, power MosFets nowadays are incredibly consistent if from same batch, so much so that I stopped testing and labelling new purchase lots because it was a waste of time.

I use lots of IRFP250, usually Fairchild but now and then another "good" brand, think ST or IR ... I can impunely replace any from same batch without even measuring anything , go figure.

Maybe because I buy straight from the official Importer, and in bulk, not sure about getting 10 here, 6 there and so on, and from small vendors.

FWIW I do not see large manufacturers (from Ampeg to car amp manufacturers) measuring their MosFets (of which they use Tons) and matching one by one, not in *production* anyway.