Analog Electronics
Voltage controlled oscillators are used commonly in

 Frequency Modulators
 All the above
 Pulse Modulators Frequency Modulators Phase Clocked loops
 Phase Clocked loops

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Analog Electronics
Optocouplers combine

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

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