Well I have been updated with new information, but my design is pretty much going back to a blank slate. I'm now required to replicate existing circuits, but modify them for a variable gain. The existing circuits used AD840 op amps, but they are now obsolete. From TI I've found OPA846 as the cross reference, but if anyone knows of another substitute for that op amp, please let me know. Also most circuitry use transformer coupling, which I've never been introduced to. Is there a certain type of transformer I need to use?