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A texts mentions that: "It is possible to have coupling circuits whose gain stays constant down to zero frequency - such circuits are called direct current coupled or 'DC coupled'"

Does that mean all the opamps are that sort of direct-coupled amplifiers?

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    \$\begingroup\$ You could certainly build an opamp that is ac coupled if you wanted to... \$\endgroup\$
    – PlasmaHH
    Sep 27 '18 at 8:59
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    \$\begingroup\$ A circuit is generally direct-coupled unless you put decoupling in (often in the form of a big capacitor, but it could also be a transformer). \$\endgroup\$ Sep 27 '18 at 9:35
  • \$\begingroup\$ The standard opamp needs to control the DC operating point of its internal transistors. That requires DC coupling of some nature in the external feedback. There are wonderful combinations of feedback, with combined DC and AC in separate paths. \$\endgroup\$ Sep 27 '18 at 17:44
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AC coupled amplifier is an amplifier with capacitors between it is stages. but DC coupled amplifier has no capacitors between stages. Operational amplifiers IC have no place for capacitors due to size and other issues.But you can build it yourself .

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