Analog Electronics
The input stage of an op amp is usually a

  CE amplifier.
  differential amp.
  swamped amplifier.
  class B push-pull amplifier.

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Analog Electronics
Optocouplers combine

 Power transistor and silicon transistor.
 SITs and BJTs.
 IGBTs and MOSFETs.
  Infrared light emitting diode and a silicon photo transistor.

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