Irf9640

[AndrewT]

Q is it normal for IRF640 to have much better current and Rds(on) than the complimentary IRF9640?
From the retailers data table they are not good compliments!
I have seen a reference to amp design using more Pchannel fets than Nchannel.
Q is this an attempt to balance the current capacity?
regards Andrew T.

[quasi]

The only real match is the power rating at 125W. So this could be the reason they are nominated as complimentary. I.e to allow designers to build amps (and similar ccts) knowing the power rating is the same for both rails.

The input capacitance is similar for both devices. This is important for a symmetrical load on the driver stages.

The Rds-on for P channel devices from IRF is always higher for some reason, but I think this would only be an issue if the FETS were saturated (clipping). The amp you see with more P channel devices would probably be an attempt to match the total Rds-on with the N device side. This will cause however a mismatch in the input capacitance (can be fixed with an external capacitor).


Cheers

[richie00boy]

It's harder to make p-channel MOSFETs so these devices are never as good as their n-ch counterparts. Hence why SMPS almost always use just n-ch.

[AndrewT]

Hi,
sounds like it's normal for N & P channel to be a poor match.
Q Is this why Nchannel power amps are popular? (I have been following a couple of long threads on this recently).
Q Does the poor matching affect the quality of the sound? or is this more down to the quality of the design feeding the FETs?
thanks so far Andrew T.

[djQUAN]

Q Is this why Nchannel power amps are popular? (I have been following a couple of long threads on this recently).

based on my experience, N channel mosfets are cheaper, easier to obtain, more power handling ability in comparison to P channel devices.

[richie00boy]

Originally posted by AndrewT
Hi,
sounds like it's normal for N & P channel to be a poor match.
Q Is this why Nchannel power amps are popular? (I have been following a couple of long threads on this recently).
Q Does the poor matching affect the quality of the sound? or is this more down to the quality of the design feeding the FETs?
thanks so far Andrew T.

The preceeding circuitry (actually all the clever bit that does the error correction) will always have an effect on the sound, as will the output stage. It's logical to suppose that both these things could have an equal effect upon the sound.

Generally vertical MOSFETs (IRF and the like) are not all that complementary between n-ch and p-ch. Proper audio lateral MOSFETs (Exicon etc.) are much better in that respect. Bipolars are better still. That's not to say though that you can't make a great amp with IRFs, because you can.

As well as the equality in a complementary pair for output stages, you need to consider a whole load of other factors such as transconductance and overall linearity.