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Old 5th November 2006, 11:47 PM   #11
ilimzn is offline ilimzn  Croatia
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Join Date: Feb 2005
Location: Zagreb
I think I have wrote about this many times before...

Regarding the use of HEXFETs as output transistors, there is really no reason to suppose they are inferior than the 'made for audio' MOSFETs. As with all components, the trick to making good use of them, is how they are used.

Some things to remember:
'Made for audio' MOSFETs are primairly optimized to give good complementary performance. It is known in advance what the complementary pairs are, they were designed that way. The key thing here is that to get good complementarity of the P and N part, the construction of the respective parts has to be quite different. In other words, finding HEXFETs with good complementarity is NOT a matter of just using IRFxxx and IRF9xxx, xxx being the same number in both the P and N part! You have to do a bit more work there.
Lateral MOSFETs are often quoted as having lower Ciss. However, using them in an amplifier only on that merit, is irresponsible. The reason is, how much of Ciss actually figures as a load for the previous stage, depends on how it is divided between Cgs and Cgd, and, much more so, on the MOSFET's gm. It should be noted that lateral MOSFETs have fairly low gm compared to HEXFETs and even PIMOS (Toshiba audio MOSFETs) - in fact, what is important here is to look at Ciss/gm. This is a relatively constant number for a given MOSFET technology, and it is in fact HIGHER for laterals, which is not desirable. On the other hand, laterals have slightly less pronounced Ciss nonlinearity, and, which is usually more important when it comes to parts substitutions, their thermal coefficient changes from positive to negative at usably low currents (about 100mA) so for bias currents close to or above 100mA, no thermal compensation is necessary. The parts cost savings due to this are usually negative, because the laterals cost much more than HEXFETs with the necessary thermal compensation components.
'Made for audio' MOSFETs are few in type and much more difficult to get than HEXFETs, which, in turn, are easily available, although the best choices may require a bit more work to get. With the former, the choice of maximum current is limited to 4 basic parts, it is much better with HEXFET and similar parts.
To sum things up: the choice of the tool is no guarantee the work gets done, the skill with which it is used plays a major part
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