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As I rolled my simpleified (removed the overload protection circuit) home-built PMOS LDO circuit on a breadboard, I found out something strange is going on.

schematic

simulate this circuit – Schematic created using CircuitLab

Now what is going on? Why am I seeing 0.01 V across my 1 kΩ load? Also, why is the sampling voltage divider emitting 0.04 V? Or is it because of the OPA2134 amplifier? Did I get a fake one from the Shenzhen supplier?

Am I treading the waters of leakage currents of semiconductors?

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Both the IRF4905 and the 2N7002 have a non-zero drain to source current when the gate to source potential is zero. For the IRF4095 the drain to source leakage current is 25 uA with a VDS=-55 V. 25 uA across the output load resistance of 903 Ohms (1 k || 9.4 k) gives 23 mV. This is reasonably similar to the 10 mV you measure, especially since the VDS is different than the datasheet and the VGS is not precisely 0. I can't explain why the node of the sampling divider is reading 0.04 V. I would expect that node to be 1/2 Vout (the op-amp has an input bias current of 5 pA).

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Get rid of that 1uF capacitor on the op-amp output. It's probably oscillating internally at a few tens of kHz.

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    \$\begingroup\$ Without a scope, how to tell if it is actually oscillating? \$\endgroup\$ – Maxthon Chan Dec 26 '14 at 21:33

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