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Simple voltage regulator


Kevin Weddle
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Kevin, you need to learn about negative feedback.

Your simple voltage regulator has no voltage gain. Its voltage gain is a little less than 1.
Its 200k resistor does almost nothing (it reduces the voltage gain from exactly 1 to a little less than 1).
The opamp is pretty fast but the darlington connected transistors are probably pretty slow causing phase shift and causing the circuit to oscillate.

post-1706-14279144901004_thumb.png

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It appears as though you've confused the non-inverting and inverting amplifier topologies.

As we said many times in the thread linked to above, you need to build a non-inverting amplifier with the negative feedback taken from the output side of the buffer.

Read the documents linked below. If you don't understand them, read them again. Then if it's still not clear ask here.
https://www.onsemi.com/pub/Collateral/SR004AN-D.PDF
http://www.ti.com/lit/an/snva558/snva558.pdf

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Kevin, if you are referring to your first circuit in this thread, it cannot work.

The Vbe of the Darlington will always be around 1.5V so the voltage across the 20K resistor will be about 135mV.
Ignoring the input offset, the two input voltages of the op-amp must be the same, so the output voltage will therefore settle at Vset minus 135mV, and there is no way you can get it any higher.

If you change the values of the two feedback resistors to alter its gain, the output voltage will lie somewhere between Vset and Vset minus Vbe, and as Vbe varies with load, then so will the output voltage.

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