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Why does replacing a transistor sometimes change a stage's gain substantially?

  • A.The saturation voltage varies from device to device
  • B.The breakdown ratings differ between production batches
  • C.Beta differs widely between nominally identical devices
  • D.The transition frequency is not controlled in manufacture

Answer: C

Beta can vary three to one within one part number, so any stage whose gain or bias depends on beta changes when the device does. The other parameters vary too but affect different aspects of behaviour. A circuit that survives device replacement without adjustment is one whose performance rests on resistor ratios, and getting there is the practical purpose of negative feedback.

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