Designing and testing various power supplies circuit can be helpful in more complicated circuit design or lower the cost of the power supply. For example we showed how Arduino or any small microcontrollor board or circuit can be powered with a low cost few readily available components transistor and a zener diode in simple Arduino voltage regulator circuit blog post. Here is another simple voltage regulator design that uses op-amp and shunt transistor with zener diode to output fixed +5V DC from +12V or above voltage source which can be battery DC or unregulated voltage source.
Here the zener diode of 2.5V acts as voltage reference to the LM358N op-amp. The op-amp is itself powered by the battery source and not from the output of the voltage regulator. Here 15V power supply, zener diode with current at break down voltage of 50mA and a load resistor of 270Ohm were necessary in order that the output is +5V. Changes in these changed the output voltage. Also the same circuit with TL072 op-amp was not working, even if the power supply voltage was varied. The operational amplifier in the voltage supply circuit is used as error amplifier which is a differential amplifier that amplifies the difference between the signals at its two terminals. The reference voltage Vz set by the zener diode is connected to the +ve terminal and the feedback signal from the output via the voltage divider network set by equal resistors R1 and R2 is connected to -ve terminal of the op-amp. When the load somehow increases, then that is feedback to the op-amp which amplifies the difference between the signal and since one of the input is at constant voltage at 2.5V set by zener, the output voltage at the op-amp output is also reduced which then causes the shunt switch transistor 2N2222A to conduct more and hence increase the output voltage. So decrease in output voltage causes increase in the output voltage due the feedback mechanism, amplification and switching action of the transistor. Thus the output voltage remains at 5V regardless of the output load.
The op-amp power supply has be considered while making this circuit or any circuit that uses op-amp. You can to consider the op-amp voltage swing rating,the minimum voltage input etc from the op-amp datasheet especially with the shunt type regulator. Series voltage regulator design with op-amp seems to work with lower power supply voltage than their shunt voltage regulator counterpart.
