A bit of a diversion, but are you all sure that device matching is needed for OPS? I had once asked Randy (Slone) about this, and he said that he's never matched OPS devices. He said that the differences in hfe and other parameters from device to device would be taken care of by the negative feedback. Of course, it's possible that amps designed by others may not have enough NFB to achieve this.
What are your views on this? I don't know enough about amp design to have views of my own. All I know is that when I build a Randy Slone amp, I won't bother to match the OPS devices.
What are your views on this? I don't know enough about amp design to have views of my own. All I know is that when I build a Randy Slone amp, I won't bother to match the OPS devices.
As I said, I don't know enough about amp design to have my own opinions. All I know is that unless he's lying, he's getting phenomenal measured performance, and amp builders on this forum who have built his OptiMOS (see "sam9" and "5th element" postings elsewhere) are raving about the lovely sound, and all this without matching OPS devices. I guess Randy Slone may not know how to build amps, but whatever he knows will probably be good enough for me.jackinnj said:
Obviously his amps work, so that's not the issue. But if the output stage FETs are not matched, then they are not all doing their "fair" share of the work. So maybe some are doing 50% of their share and some are doing 150%. Or maybe worse than that.
I suppose that the amps would even work if you just used a single pair of output devices. But there is a difference between just "working" and offering truly high performance.
To my way of thinking, it is false economy to save on the extra "work" or "cost" of matching the output devices. Think about how much time you spend on your project. Is it really worth it to save $10 or $20 plus an hour or so of time to not match the parts?
Nelson Pass says this on the PassDIY website:
"In the case of a power output stage, we often want to
parallel power devices so as to share the current and heat
dissipation, as in Figure 3a. Here it is desirable that the
devices be identical so that the current is shared equally.
If the Vgs of each device is different, some device will
find itself doing the disproportionate share of the work
while some other device might not be turned on at all."
I'll put my money with Nelson any day of the week.
I suppose that the amps would even work if you just used a single pair of output devices. But there is a difference between just "working" and offering truly high performance.
To my way of thinking, it is false economy to save on the extra "work" or "cost" of matching the output devices. Think about how much time you spend on your project. Is it really worth it to save $10 or $20 plus an hour or so of time to not match the parts?
Nelson Pass says this on the PassDIY website:
"In the case of a power output stage, we often want to
parallel power devices so as to share the current and heat
dissipation, as in Figure 3a. Here it is desirable that the
devices be identical so that the current is shared equally.
If the Vgs of each device is different, some device will
find itself doing the disproportionate share of the work
while some other device might not be turned on at all."
I'll put my money with Nelson any day of the week.
Charles Hansen said:
maybe it is indeed worse than that. But can you hear it?
I thought the best test is for some of us to intentionally mis-match output transistors and see if we can hear that inferior sound from mis-matched transistors.
If we cannot, what benefits do you get from matching transistors?
for the record, I haven't matched my transistors for a long time.
tcpip said:
It depends on how much you want to equalize current between devices. One school of thought might be to use large emitter resistors and not select parts, because the emitter resistors will encourage current sharing. This is practical with bipolars, not with mosfets. MOSFETs vary so widely in Vgs(on) and transconductance that matching is probably wise, since no practical source resistor will equalize currents. The good news is that once you have laterals matched, they share well and you don't need source resistors at all (but I use 0.1 ohm source resistors to simplify measuring bias currents, lazy me).
I do wonder if matching of MOSFETs might not be needed in one case. In the CF configuration, if you use multiple dedicated drivers, one per output device, and tie the driver emitter to the drain and drain resistor, the driver will compensate for wide variations in MOSFET parameters. It costs money to match the MOSFETs, and individual drivers are cheap, so would be a win. Wherever possible I try to design out the need for fussy adjustments and matching.
I have not been able to do a stable MOSFET CF output stage (Grr.... I'll get it if it kills me!!!), so I have not tried this but I want to. 100% local negative feedback is a wonderful thing, if it can be made stable.
Charles Hansen said:
I suppose you are now talking only about matching parallelled
N devices with each other and parallelled P devices with each
other, since previously in this thread you recommended
against matching complementary LMOSFETs? While I see your
previous point about matching complementary devices I have
a suspicion that the mismatching between the complementaries
may still be a bigger problem than mismatching between
parallelled devices. Some people, including Borbely and certain
people on this forum, do acutally sell matched complementary
pairs of Hitachi L-MOSFETs. Why would Borbely do that unless
he thinks it matters?
Of course, this does not in any way provide an argument
against matching parallell devices, which seems a sensible
thing to do.
Edit: The above was mainly considering distorsion etc. Of course,
matching parallell devices or otherwise guaranteeing a fair
current sharing is necessary to meet the intended power and
current handling spec.
What is the Vgs of this K1058 pairs? Is the Vgs for Nch and Pch differs alot like mosfets or similiar like bipolars?
I'm looking for device for VAS. Bipolars have too much gain, Mosfets and fets have lower gain, but their Vgs is not predictable as bipolars. Has anyone try this lateral mosfet for VAS?
I'm looking for device for VAS. Bipolars have too much gain, Mosfets and fets have lower gain, but their Vgs is not predictable as bipolars. Has anyone try this lateral mosfet for VAS?
Christer said:
You would have to ask Borbely why he does that, because it makes no sense to me.
In my experience, the transconductance of the MOSFETs is very consistent from part-to-part. What varies is the Vgs (threshold). If the transconductance is mismatched it will create distortion (essentially only a small amount of 2nd harmonic). But that is not an issue because the parts don't vary in this regard. If Vgs (threshold) is mismatched it will create a small DC offset. That is trivially compensated for elsewhere.
Re: Hello,Charles Hansen.
Just use the previous schematic I posted. You will want to reduce the operating current. If you are using a current source, try 10 mA or so. If you are using a resistor, try increasing the value to 1000 ohms or so.
(Normally there is no need to match these smaller parts. If you are planning to parallel them, you would be better off to use the bigger parts instead.)
audionline said:
Just use the previous schematic I posted. You will want to reduce the operating current. If you are using a current source, try 10 mA or so. If you are using a resistor, try increasing the value to 1000 ohms or so.
(Normally there is no need to match these smaller parts. If you are planning to parallel them, you would be better off to use the bigger parts instead.)
First a generalized observation about buying BJTs that seems to apply to both small ignal devices and output devices: When I buy a large quantity, 20 for example, the odds are that the difference between the highest and lowest measurement is less than when I buy just two. Some devices have lot numbers printed on them; getting out my magnifying lens and squinting at them confirms the following scenario.
This is counterintuitive at first glance but I think there is a reason that I can infer from being around stockroom operations from time to time. When a large quantity order is being pulled from stock, pre-packaged batched are just grabbed off the shelf and these are batches more likely to come from a single production run - maybe even the same wafer if you are lucky. When you order one or two, the pieces are pulled from a "left-overs" or "odds and ends" bin which have an inherently lower uniformity. I've seen a similar practice with a variety of products --that it is independant of the nature of the product. It applies equally to nuts&bolts, donuts, window frames and semi-conductors.
I see no reason why the phenomonon should be limited to BJTs. Slone probably buys his L-MOSFETS in larger batches than most DIYer's and is more likely to get devices made in the same run. Add to this that he seems to be using Magnatec devices. Since they make a "double die" package where two dies are package in parrallel without separate gate stoppers, drain or source resistors or any way to add such, it is likely that by necessity they have achieved a manufacturing uniformity greater than "average". Note they have a "quad-die" product under development which places even greater demands on their process. It is not likely to be just be post process sorting because that gets way too expensive, especially if you are trying to do a four way match. Thus I suspect that there is a high probability that Slone is correct with regard to the stock of components HE buys but it mostly likely does not apply to the small number of units you or I might buy.
[An slightly analogous aside: D. Self claims to have parralleled BJTs without RE resistors. He probably did. I tried it and could have use the result to roast marshmallows!! Possibly similar to the above situation]
Gratuitous plug: You can get matched sets (2 each) of 2SK1058 and 2SJ162 fom www.tech-diy.com. The price asked looks quite reasonable.
This is counterintuitive at first glance but I think there is a reason that I can infer from being around stockroom operations from time to time. When a large quantity order is being pulled from stock, pre-packaged batched are just grabbed off the shelf and these are batches more likely to come from a single production run - maybe even the same wafer if you are lucky. When you order one or two, the pieces are pulled from a "left-overs" or "odds and ends" bin which have an inherently lower uniformity. I've seen a similar practice with a variety of products --that it is independant of the nature of the product. It applies equally to nuts&bolts, donuts, window frames and semi-conductors.
I see no reason why the phenomonon should be limited to BJTs. Slone probably buys his L-MOSFETS in larger batches than most DIYer's and is more likely to get devices made in the same run. Add to this that he seems to be using Magnatec devices. Since they make a "double die" package where two dies are package in parrallel without separate gate stoppers, drain or source resistors or any way to add such, it is likely that by necessity they have achieved a manufacturing uniformity greater than "average". Note they have a "quad-die" product under development which places even greater demands on their process. It is not likely to be just be post process sorting because that gets way too expensive, especially if you are trying to do a four way match. Thus I suspect that there is a high probability that Slone is correct with regard to the stock of components HE buys but it mostly likely does not apply to the small number of units you or I might buy.
[An slightly analogous aside: D. Self claims to have parralleled BJTs without RE resistors. He probably did. I tried it and could have use the result to roast marshmallows!! Possibly similar to the above situation]
Gratuitous plug: You can get matched sets (2 each) of 2SK1058 and 2SJ162 fom www.tech-diy.com. The price asked looks quite reasonable.
Charles Hansen said:
From my experienc the distribution of devices is pretty "normal". You do get some real outliers where the Vgs(t) is 1.4 or 1.5, but very few. It doesn't matter whether you're matching devices from Hitachi, International Rectifier, On-Semi or ST -- I want to be careful that the definition of "random" actually means statistically random, and exhibitiing the probabilities one would expect in a sample chosen at random. from a properly defined popoulation.
As the device warms up (as any semiconductor warms up) things begin to change. Moreover, the ambient temperature will have an effect.
But how does it sound?
I've been searching the forum and read both articles about matching on the passdiy site, but I'm still somewhat unsure of a couple things. I set up the circuit on the "How to: Matching Devices" page using an R1 of 62 ohms. I'm testing IRFP044's: the datasheet says that its a Hexfet--whether that classifies as "lateral" or not, I'm not sure. I'm mainly wondering if I need to measure the Vgs immediately upon connecting the circuit or if I need to wait until the temperature and Vgs stabilizes?
Many thanks!
Many thanks!
First off an IRFP044 is a vertical Mosfet.
Second, you want to test the Mosfet under the conditions
of linear usage, so you set the current (i.e. the resistor in
series with the supply) to the appropriate value, considering
the 3-4 volt drop of the Vgs. For example a 4 volt typical
Vgs means that a 12 volt supply will want an 8 ohm resistor
for a 1 amp test.
None of this is critical, but it's nice to be in the ball park. We
are looking to match devices, and so the important thing is
consistency in testing more than anything else. The Mosfets
should start out at the same temperature, and you should
take readings after a consistent number of seconds (1 - 2- 3)
unless you are heat sinking the devices, in which case you are
playing a more sophisticated game.
Second, you want to test the Mosfet under the conditions
of linear usage, so you set the current (i.e. the resistor in
series with the supply) to the appropriate value, considering
the 3-4 volt drop of the Vgs. For example a 4 volt typical
Vgs means that a 12 volt supply will want an 8 ohm resistor
for a 1 amp test.
None of this is critical, but it's nice to be in the ball park. We
are looking to match devices, and so the important thing is
consistency in testing more than anything else. The Mosfets
should start out at the same temperature, and you should
take readings after a consistent number of seconds (1 - 2- 3)
unless you are heat sinking the devices, in which case you are
playing a more sophisticated game.
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