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19th January 2005, 02:25 PM
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#1 |
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diyAudio Member
Join Date: Nov 2002
Location: Grenoble, FR
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Input stage with MOSFets?
I was just wondering, why are amps with MOSFets in the input stage less popular than amps with BJTs of JFets?
Except in the amps made of 100% MOSFets (a la Pass), input stages nearly always use BJTs or JFets? Is there a reason for that? JFets are sometimes used because of theys high input impedance, but MOSFets have an even higher input impedance!
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Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease |
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19th January 2005, 03:10 PM
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#2 |
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Did it Himself
diyAudio Member
Join Date: Nov 2003
Location: Gloucestershire, England, UK
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Vgs matching between MOSFETs is much more variable than with BJTs. Any variation here is amplified by the VAS. Also, the capacitance of MOSFETs is much higher.
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