Analog Electronics
The voltage follower has a

  closed - loop bandwidth of zero.
  small open - loop voltage gain.
  large closed - loop output impedance.
  closed - loop voltage gain of unity.

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Analog Electronics
Optocouplers combine

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

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