The DC voltage produce by this circuit is depend on the load; the heavier the load (more current to the load) means the capacitor discharge time will be more faster and as a result the DC voltage output level will be drop. The circuit above is called unregulated AC to DC converter; because it could not maintain its voltage output level, this kind of power source could not be use in electronic circuit that required constant voltage level in order to operate properly such as in digital and microcontroller circuit. Therefore we need what is called Voltage Regulator Circuit and at the same time works as the DC to DC voltage step down.
The first one is the analog voltage regulator, this type of regulator operate the transistor in its linear region (current gainer). The current supplied to the transistor base lead is depend on the voltage different between the reference voltage and output voltage apply to the error amplifier input; for example when the output voltage is greater than the reference voltage, than the error amplifier will make the transistor to conduct less, this mean the voltage drop across the collector and emitter (Vce) will be increase this will make the output voltage to decrease and vice verse. With output voltage being continuously compared to the reference voltage (close loop feedback) by the error amplifier, this kind of circuit could maintain its voltage output level constantly. The disadvantage of using this type of voltage regulator is the power dissipation (power lost as a heat) on the transistor is high especially when we want to use 5 Volt output from 11.33 volt from the unregulated DC source or drain lot of current from it.
The second one is the most efficient voltage regulator as this type of voltage regulator operates the transistor in its saturate region or known as a switching voltage regulator. The working principal is the same as the analog one, but instead of using the constant current to the transistor base lead; this voltage regulator type use pulse current or known as PWM (Pulse Width Modulation) to the transistor base lead and this make the transistor to turn on and off according to the PWM duty cycle supplied by the error amplifier. Therefore by changing the PWM duty cycle we could change the average voltage drop across the collector and emitter (Vce). The inductor (L) is used to release its energy to the load through the diode (D) when the transistor is turn off; and when the transistor is turn on then the transistor will supply the current to the load and the inductor will store the energy in the electromagnetic form; in other word the inductor and diode will ensure that the load will always get a constant current when the transistor is turn off.
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