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Linear Power Supplies

50Hz transformer: to step-up or step-down 50Hz AC voltage and to be used for electrical isolation
BJT: operates in the active region and acts as an adjustable resistor in response to its base current, iB
Feedback control: The error amplifier is used to compare Vo with a reference voltage Vo,ref and incorporated with the base control circuit (which adjusts iB) to keep Vo = vd(t) - vCE(t) = Vo,ref = constant.
Possible sources of changes:
  1. vd ripple voltage
  2. AC input rms voltage: This changes the vd average value.
  3. load: This changes the output current io and consequently the vd average value due to the transformer load regulation
Linear Power Supplies Disadvantages:
  1. The low-frequency (50Hz) transformer is larger in size and weight compared to a high-frequency transformer which has the same ratings.
  2. The power loss in the series BJT (vCEiC) is significant. This loss can be minimized by proper selection of the transformer turns ratio. The overall efficiencies of linear power supplies are usually in a range of 30% to 60%.
  1. Linear power supply circuitry is simple. Therefore, the construction cost is less for small power ratings (<25W).
  2. Linear power supplies do not produce large EMI (electromagnetic interference) with other equipment.
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