Down-converting voltage

Question

I'm trying to reduce the voltage of a power supply for a certain load without affecting the current the load draws. Since the op-amps I could find were not able to provide sufficient current, I added a NPN transistor to the output of the op amp as can be seen in the figure below. I simulated the circuit and it worked perfectly. Would such a circuit function as desired in practice?

Answer 1

It should work within the limitations of the opamp and the transistor.

The opamp limitations include its power supply range, input and output voltage range, and output current capability.

The transistor limitations include its current handling capability, and power dissipation at the worst case operating point.

You also have to make sure that the output current divided by the gain of the transistor at that operating point is something the opamp can source.

Added

Note that your circuit will make the output voltage some fraction of the input voltage. That doesn't matter much if the input voltage is well regulated. If it varies, however, then the output voltage will vary proportionally with it. It's not clear if that's what you want.

If what you really want is a lower and fixed output voltage from a higher but possibly varying input voltage, then a off the shelf linear regulator would be simpler than doing it yourself.

If you want a standard output voltage, like 5 V, 9 V, 12 V, for example, then you can even get linear regulators pre-set to that fixed voltage. The old standard 7805 is one 5 V example of this.

If you want some unusual output voltage, then use a adjustable regulator, like the LM317. It does all the regulating, and you set the output voltage with a resistor divider.