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In the circuit description they claim that one stage would have 12dB gain. From what I learned is that in negative feedback operation the gain is 1+Rf/Rg=1+20/47=1.42=~1.5dB Where is my mistake?

Schematic

From: http://publications.lib.chalmers.se/records/fulltext/218195/218195.pdf

Amplifier schematic is on page 60, description on page 30.

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    \$\begingroup\$ There are 4 opamp stages in that complete circuit, and each stage has 3dB of voltage gain (20log(1.42), not 10log(1.42)). So the total gain is 12dB. \$\endgroup\$
    – brhans
    Aug 24 '16 at 13:44
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    \$\begingroup\$ I wouldn't trust the text. They say unity gain is a gain of 1dB, so meh. \$\endgroup\$
    – JRE
    Aug 24 '16 at 13:53
  • \$\begingroup\$ Well yes, their amp didn't worked in the end, they said it was too slow. But I wonder at what gain they were operating finally. With 12dB per stage it would have been too slow of course. \$\endgroup\$
    – kernash
    Aug 24 '16 at 14:02
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The voltage gain is 3 dB per stage not 1.5 db - you probably multiplied by ten instead of twenty thinking you were calculating power gain.

As for what it says on page 30 don't let words get in the way of a schematic. Bottom line is that the document appears contradictory.

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    \$\begingroup\$ The schematic has 4 stages of 3dB each. \$\endgroup\$
    – brhans
    Aug 24 '16 at 13:47
  • \$\begingroup\$ @brhans I here what you say but page 30 also says they need about 50 dB of amplification and 4 stages of 12 dB = 48 dB. \$\endgroup\$
    – Andy aka
    Aug 24 '16 at 13:50
  • \$\begingroup\$ Aah - ok - I only looked at the schematic - didn't read page 30. \$\endgroup\$
    – brhans
    Aug 24 '16 at 13:53

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