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# Simple voltage regulator

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It is versatile.

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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.

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We had this discussion 7 months ago.

Have you forgotten?

http://www.electronics-lab.com/forum/index.php?topic=38954.0

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The schematic shown has a gain of 10. I changed the gain to 10,000 in this circuit. I don't know how unstable that would make it. I guess I'll have to match the opamp and output transistors.

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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.

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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I went ahead and built this circuit today. The input resistor voltage changed very little. The output transistor voltages changed quite a bit.

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I went ahead and built this circuit today. The input resistor voltage changed very little. The output transistor voltages changed quite a bit.

You make no sense because your circuit has no input resistor. Its input is the (+) input of the opamp.

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