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Interview Question :: How do you choose an op-amp for this circuit


Need a little help picking an op-amp that will work in my circuit. Here is a link to an image of the circuit I have created http://i162.photobucket.com/albums/t259/...

As you can see I am trying to create a variable voltage power supply. The idea is to control the voltage with the potentiometer. I have ~5.471V coming into the input of my op amp. I want to get roughly 15V at the output (which is how I chose the 5.6K and 10K resistor values). Everything works fine in my circuit simulation program if I use an "ideal" op amp (I get a max voltage of ~15V at the 1.8K load). However if I switch the op-amp to say an LF411C the max voltage at the load drops to ~12.18V. I am afraid I do not know enough to properly choose the correct op-amp. I have incorporated some multi-meters into the circuit so you can see the various currents and voltages. If more information is required please let me know. Otherwise which op-amp should I choose?

Thanks
circuit simulation
Answers to "How do you choose an op-amp for this circuit"
RE: How do you choose an op-amp for this circuit?

Part of the problem is that you're using an NPN emitter-follower which will have more dropout than a PNP pass transistor style. The other part is counting on rail-to-rail swing of the op-amp.



If you use PNP then the dropout is less. Then, use current to control it. I've linked a page below that uses this topology with ordinary LM324 op-amps. It's a bit more complex but it will go all the way to zero and near the Vin rail. There's a lot of extra stuff in this design but the accompanying explanation covers the basic feedback loop.



Note also that this design would work equally well with a P-channel FET which would reduce your dropout even further.
 
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RE: How do you choose an op-amp for this circuit?

Opamps do not work at the rails. That is, if you have 15 volts for the +Vcc, the opamp will only work up to about 13 volts. Same for the negative supply. You will also lose about 0.7 volts in the 3n2222, which will limit you to about 12 volts.



Crank up the supply to 18 volts on the plus side.



If you want the output to go down to zero, you will have to generate a -3 volts supply for the -Vcc for the op amp.





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