Hi! I am trying to build a simple low noise PSU (Vref/DAC-OpAmp-NPN) and everything works nicely, the emitter voltage follows the Vref/DAC voltage, until I load the emitter (only 1k), which is when the emitter voltage breaks down massively. It is (or rather was? ;)) my understanding that the OpAmp will output whatever voltage is necessary (up to ~3,5V in this case as I am using a 5V single supply "Rail-to-Rail" (yeah right as if -1,5V were "rail"...) OpAmp) to see that same voltage appear on it's inverting input. I am basically adhering to the LT1028 DS - http://cds.linear.com/docs/Datasheet/1028fa.pdf and using the listed 2N6387. The NPN DS has no pinout (just numbers), but I assumed BCE. Difference is only that input (C) is from a 12V lab PSU and I am using unity gain with a different, unity-gain stable OpAmp (OPA140). I tried varying the B resistor (I assume it needs ~3mA from DS and because the DS uses 10V+330Ohm) from 100-500 Ohm, but to no avail... What am I missing? I'm sure it's simple... Thanks!
Also see attached "schematic" (sort of).
Attached files:
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JustForYouMyFriend.png
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hi Patrick, you're so right, it's really simply. Sure that the pins of the darlington are connected well? Seems not so, the voltage at the emitter is as stable as the voltage at the collector. Take a look to the voltage of the base, it should be 0,8 to 1,2 V higher than the needed output. What current is required? Perhaps there's no need for a darlington type, i think, the OPA140 can drive 20mA or more, so you can get an output current of min. 2 Amps. Kindly regards, harry
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#2422668
>What am I missing? I'm sure it's simple...
Forgot the internal biasing resistors of Darlington??
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#2719380
Suggestion: Perhaps a high current flows trough the input protection diodes of the opamp. Between the + and - input of the opamp are two antiparallel 'virtual' diode that start to conduct at difference voltages > 1.4 Volt. Use a series resistor e.g. 2k2 at each of the + and - inputs to limit the current. Perhaps the circuit might be instable (oscillating) in some conditins, but that should not be destructive for the darlington. regards, William
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#2719993
Hi! If you are not sure about connecting the darlington transistor (B, C and E): Most Multimeters have a transistor tester. Maybe u can use it to determine correct pinning. Yours, Stefan
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