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Old 13th November 2011, 01:28 PM   #1
gk7 is offline gk7
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Default AD797 Fet Inputs ?

In a review of Resonessence Labs Invicta on 6mmons ( 6moons audio reviews: Resonessence Invicta ) is stated:

"In the user manual we do mention that the analog output stage uses jfets. We do however not use discrete jfets but instead the AD797 op-amp as a jfet input amplifier with a DC-bootstrapped compensation capacitor and additional AC-coupled feedback path. While ADI's circuit diagram indicates bipolar input devices, those are not conventional bipolars as can be inferred from the data sheet where collector current and base current are mentioned in a ratio higher than any any reasonable bipolar device can do. We know this through a personal relationship with the AD797's designer. Hence we refer to jfets in the input stage of the AD797."

Why would the datasheet have wrong information on the input stage ?
Confusing...
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Old 13th November 2011, 02:08 PM   #2
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Default Wrong

They are simply wrong. Look at the input bias current in the data sheet and you will see that it's not a JFET input stage.
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Old 13th November 2011, 02:17 PM   #3
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by QSerraTico_Tico View Post
They are simply wrong. Look at the input bias current in the data sheet and you will see that it's not a JFET input stage.
And theres more

the data sheet mentions a 25ma max input current... so again not FET.
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Old 13th November 2011, 04:33 PM   #4
gk7 is offline gk7
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Yes thatīs exactly what I thought, Iīm pretty sure itīs bipolar. But because they say "We know this through a personal relationship with the AD797's designer" I hoped for a comment by Scott Wurcer.
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