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I am reading the book Operational Amplifiers by Steve Winder. On page 14, it explains input impedance of real op amps: "... there is a large differential input impedance ... ". How exactly is "differential input impedance" defined? Is it the small signale impedance? And then, the book mentions "common mode input resistance". Is this the large signal impedance? How is "common mode input resistance" defined? More generally, if input impedance or output impedance of op amps are mentioned, do they refere to z=dV/dI or Z=V/I?

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    \$\begingroup\$ Short: Differential impedance is between + and - terminal, common mode between inputs and supplies. \$\endgroup\$
    – Stefan Wyss
    Commented Aug 10 at 15:53
  • \$\begingroup\$ Oh ok, thanks. I thought "differential" refers to differential resistance in contrast to static resistance \$\endgroup\$
    – Jo123
    Commented Aug 10 at 16:06

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