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AD797 schematic
AD797 schematic
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Old 16th April 2018, 03:04 AM   #11
jcarr is offline jcarr  United States
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I believe that Q18 and Q19 are a bootstrap to keep the VCe of Q17 constant, regardless of signal level.
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Old 16th April 2018, 01:27 PM   #12
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by jcarr View Post
I believe that Q18 and Q19 are a bootstrap to keep the VCe of Q17 constant, regardless of signal level.
They bootstrap all the pairs of devices in the stage, in an ordinary circuit they would be the Vas devices and the voltage gain would be at best a few 100's of thousand's. This way what would be the Vas error voltage becomes a common mode error and a single gain stage achieves >5-10Meg. The input still sees the current burden of the output buffer as a differential error so it had to be a triple darlington with cascode from the output voltage.
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Old 16th April 2018, 02:53 PM   #13
tomchr is offline tomchr  Canada
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Ah. Very cool. Thanks.

Tom
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Old 16th April 2018, 05:20 PM   #14
myhrrhleine is offline myhrrhleine  Belize
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i had this crazy idea of cascading two chips for more gain but i do not need the current gain of both chips. so i was looking at where i might access just the voltage gain stage.
thanks
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Old 16th April 2018, 06:01 PM   #15
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by myhrrhleine View Post
i had this crazy idea of cascading two chips for more gain but i do not need the current gain of both chips. so i was looking at where i might access just the voltage gain stage.
thanks
The other side of the gain stage, (at R3, the opposite sense of where C2 is connected) is pin 8 on the IC. This would be the output unbuffered. I have heard folks put a resistor to ground here and use the part open-loop. Stuff like this needs some care and I have not tried any of it. If you can I would use John's (jcx) LTSPICE files and play around. For this the schematic here gives you all the info you need, some decent discrete models with around 600 MHz ft should be OK.

Your best bet in general would be to research the various composite amplifier schemes, there are lots of ways to skin that cat.
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Old 23rd May 2018, 02:09 PM   #16
FritzS is offline FritzS  Austria
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another discussion about AD797
Headphone Amp - AD797 compensation, distortion cancellation caps
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