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, i_B

Feedback control: The error amplifier is used to compare V_o with a reference voltage V_o,ref and incorporated with the base control circuit (which adjusts i_B) to keep V_o = v_d(t) - v_CE(t) = V_o,ref = constant.

Possible sources of changes:

1. v_d 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

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 (v_CEi_C) 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%.

Advantages:

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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