Pavel Macura's MOSFET power follower circa 2002 and updated 2005 showed an IRFP350 in the circuit. In the text Pavel talks about input capacitance as a limiting factor in the MOSFET. The IRFP350 has input capacitance above 1000 pf
I know that in the past some of the device makers have marketed MOSFETs specifically for audio with low input capacitance. Now it looks like the Class D craze has finally gotten chip makers serious about audio MOSFETs.
In looking through the International Rectifier site I cam across a couple of devices that look like the might be just the ticket for getting the last ounce of performance out of a Class A power follower. Take a look at a couple of these devices. Any of these look particularly tasty for Class A operation?
IRFB4019
http://www.irf.com/product-info/datasheets/data/irfb4019pbf.pdf
IRF6665
http://www.irf.com/product-info/datasheets/data/irf6665.pdf
Any other new MOSFETs out there that look really good for Class A?
I know that in the past some of the device makers have marketed MOSFETs specifically for audio with low input capacitance. Now it looks like the Class D craze has finally gotten chip makers serious about audio MOSFETs.
In looking through the International Rectifier site I cam across a couple of devices that look like the might be just the ticket for getting the last ounce of performance out of a Class A power follower. Take a look at a couple of these devices. Any of these look particularly tasty for Class A operation?
IRFB4019
http://www.irf.com/product-info/datasheets/data/irfb4019pbf.pdf
IRF6665
http://www.irf.com/product-info/datasheets/data/irf6665.pdf
Any other new MOSFETs out there that look really good for Class A?
Input capacitance is not the only limit for a follower - rather, it's a combination fo input capacitance and gm.
This is because in a follower, the Cgs part of the input capacitance appears reduced 1+(gm*Rload) times to the signal source for the follower. The Cgd component remains constant (here I am disregarding capacitance nonlinearity which is Vds / Vgd dependant).
So, in essence, you need a MOSFET with minimum Cgs/gm + Cgd. The thing is, higher gm MOSFETs within the same basic technology, tend to have a higher maximum current, and proportionally higher C, because they are internally just more of the same kind of cell in parallel. Within a certain technology, C/gm remains roughly constant. Still, this 'constant' varies somewhat, mostly with maximum Vds, and this is where you optimize, unless you want to use different cell technology (for instance, IRFP240, IRFP240B, IRFP240N all have different cell technologies and the c/gm constant is different).
Another way to minimize the C/gm is to use a small MOSFET nearer to it's current limit, as gm increases with current, while Cin remains roughly the same. Of course, this has other repercussions, mostly to reliability, but is also one of the reasons why it is often said that MOSFETs sound best in class A or at least high bias AB.
This is because in a follower, the Cgs part of the input capacitance appears reduced 1+(gm*Rload) times to the signal source for the follower. The Cgd component remains constant (here I am disregarding capacitance nonlinearity which is Vds / Vgd dependant).
So, in essence, you need a MOSFET with minimum Cgs/gm + Cgd. The thing is, higher gm MOSFETs within the same basic technology, tend to have a higher maximum current, and proportionally higher C, because they are internally just more of the same kind of cell in parallel. Within a certain technology, C/gm remains roughly constant. Still, this 'constant' varies somewhat, mostly with maximum Vds, and this is where you optimize, unless you want to use different cell technology (for instance, IRFP240, IRFP240B, IRFP240N all have different cell technologies and the c/gm constant is different).
Another way to minimize the C/gm is to use a small MOSFET nearer to it's current limit, as gm increases with current, while Cin remains roughly the same. Of course, this has other repercussions, mostly to reliability, but is also one of the reasons why it is often said that MOSFETs sound best in class A or at least high bias AB.
Ilimizn has illuminated the matter. And I understand. Still, a couple of these Mosfets specifically designed for class D amps seem to have much better specs in areas where it counts for source followers. The point about power handling is important... These might be limited to 10 watt head phone amps. The 6669 doesn't dissipate enough power to be useful for much more than driving a pair of 500 ohmish things.
As long as the think was kept within power ratings, the IRF6665 outperforms anything else I've seen on the source follower related capacitance. But it is a relatively low power device..
Only 42 watts.
As long as the think was kept within power ratings, the IRF6665 outperforms anything else I've seen on the source follower related capacitance. But it is a relatively low power device..
Only 42 watts.
Not being half as familiar with Class D design as I should (I used to test and repair switchmode power supplies), I assume the specs are pretty good >for< Class D operation where the devices shouldn't be operated in any other mode, else they'll cook.
I sort of suspect that any attempt to operate in Class A or even AB won't find them at their best, no matter how much heatsinking is applied. Alas, more suitable devices with higher dissipation will have to be chosen.
Don't have any Class D projects on my to-build list, though I'm interested in the subject.
I sort of suspect that any attempt to operate in Class A or even AB won't find them at their best, no matter how much heatsinking is applied. Alas, more suitable devices with higher dissipation will have to be chosen.
Don't have any Class D projects on my to-build list, though I'm interested in the subject.
I agree with ilimzn. I used IRF520 for power follower. It sounds great, better than any cheap vertical MOSFETs I ever heard. But it has very limited power ratings, so You need heavy heatsinks, or fan cooling. I killed many of them, due the very bad reliability.
Another experience: This MOSFETs sounds good only with high bias (4A at least). The realisation can be the bridged mode, with lower rails, and higher current
sajti
Another experience: This MOSFETs sounds good only with high bias (4A at least). The realisation can be the bridged mode, with lower rails, and higher current
sajti
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