By theory, always without of any exception, op-amp amplifier is made by negative loop resistor between the output and negative input feet and then other resistor is going from the same negative feet to the signal source. Math is clear, works in zillion of cases without of any problem.
But then I met the femtoampere technics having typically say 10^-18 Ohm inner impedance, few femtoampere scale current and bit more nanovolts voltage what must be amplified becoming measured. For that reason exists a bunch of special operationals like AD7721, LMC662, ADA4530, LTC6268-10, AD549 and many more.
But as more I read the know-how things, more I see they many are switching op-amp in crazy way (but that are serious sci publications, thus the brutal typo error is near nil probability) - how sad they never explain WHY they do so - they switch source (for example Faraday Cup) via some 10 Meg resistor to the negative input, BUT the 1 Gig to10 Gig loop resistor they connect BEFORE the 10 Meg - straight on the contacts of F.Cup.
1) What is that the strange idea behind it?
2) How then I can calculate the K(u) and why it havent plainly diminishing value?
3) If they want to degrade the (for example AD549) toward prost transimpeance cascade with K(u)=1, then why to use ANY resistor at all if ever both are needed for nothing?
Circuit example here https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.496.7448&rep=rep1&type=pdf
But then I met the femtoampere technics having typically say 10^-18 Ohm inner impedance, few femtoampere scale current and bit more nanovolts voltage what must be amplified becoming measured. For that reason exists a bunch of special operationals like AD7721, LMC662, ADA4530, LTC6268-10, AD549 and many more.
But as more I read the know-how things, more I see they many are switching op-amp in crazy way (but that are serious sci publications, thus the brutal typo error is near nil probability) - how sad they never explain WHY they do so - they switch source (for example Faraday Cup) via some 10 Meg resistor to the negative input, BUT the 1 Gig to10 Gig loop resistor they connect BEFORE the 10 Meg - straight on the contacts of F.Cup.
1) What is that the strange idea behind it?
2) How then I can calculate the K(u) and why it havent plainly diminishing value?
3) If they want to degrade the (for example AD549) toward prost transimpeance cascade with K(u)=1, then why to use ANY resistor at all if ever both are needed for nothing?
Circuit example here https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.496.7448&rep=rep1&type=pdf