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I see that this works in theory, but I my gut feeling says there is a drawback or caveat of which I'm not aware. Basically what I want to achieve is described best by the circuit diagram below.

schematic

simulate this circuit – Schematic created using CircuitLab

I have a power supply (2A @ 12VDC) that I want to use to make a variable voltage power supply. In the voltage follower configuration, whatever is on the non-inverting terminal on the op-amp is present on the output terminal. The extra current at the output terminal needed to keep the voltage up is provided by the op-amp's supply. Since I'm using a linear pot as a voltage divider to set the input voltage to the op-amp, will this work as an effective variable power supply from 0V to almost 12V? If not, why can this configuration not be used this way?

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  • \$\begingroup\$ I should have noted that the op-amp I want to use is a high current op-amp. Specifically the OPA548 from TI \$\endgroup\$
    – Jacob Calvert
    Commented Apr 24, 2015 at 17:50
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    \$\begingroup\$ Did you notice figure 16 in the datasheet? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Apr 24, 2015 at 17:57
  • \$\begingroup\$ One drawback is the cost of OPA548. Look into something like LM317, it is specific to your type of application and is a lot cheaper. Another thing to look into is that both devices mentioned would stop short of going near the upper and lower power rails. \$\endgroup\$
    – rioraxe
    Commented Apr 24, 2015 at 18:18
  • \$\begingroup\$ Also, figure 2 in the datasheet is probably important for you. \$\endgroup\$
    – The Photon
    Commented Apr 24, 2015 at 18:27
  • \$\begingroup\$ I ultimately decided to use an LM317 for the design. The LM317 does what I need it to, can handle the current, and is much cheaper than the OPA548. \$\endgroup\$
    – Jacob Calvert
    Commented Apr 25, 2015 at 19:20

2 Answers 2

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It can be used but for most op-amps the maximum current you would be able to drive from the output will be limited to less than 50mA. Op-amps are not miracle workers! On the other hand you can attach an NPN and PNP transistor in a push-pull configuration and get more current: -

enter image description here
(source: northwestern.edu)

Or, you could use an OPA541 - it is an op-amp and can supply up to 10 amps: -

enter image description here

enter image description here

I guess this one is a bit of a miracle worker!

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The problem is that the output current from most op amps is weak at best. If you use the op amp to control a P-type high-side transistor instead then you'll have yourself a LDO regulator.

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