A while ago I was searching the net to find information about the theory and calculations of diff amps used for feedback in amps. Maybe these sites can answer your questions about feedback using diff amps:
http://www.st-and.ac.uk/~www_pa/Scots_Guide/audio/part1/page3.html
=> this site is really simple (ideal for beginners)
(main site: http://www.st-and.ac.uk/~www_pa/Scots_Guide/audio/Analog.html)
http://home.eunet.cz/rysanek/op_amp_int_en.html
http://www.dnp.fmph.uniba.sk/~kollar/je_w/op_amp4.htm#0b
http://home.no.net/andiha/articles/audio/dcamp.htm
best regards,
HB.
http://www.st-and.ac.uk/~www_pa/Scots_Guide/audio/part1/page3.html
=> this site is really simple (ideal for beginners)
(main site: http://www.st-and.ac.uk/~www_pa/Scots_Guide/audio/Analog.html)
http://home.eunet.cz/rysanek/op_amp_int_en.html
http://www.dnp.fmph.uniba.sk/~kollar/je_w/op_amp4.htm#0b
http://home.no.net/andiha/articles/audio/dcamp.htm
best regards,
HB.