Forum: Analog Circuits Darlington NPN emitter voltage breaks down with minimal load

von Patrick G. (sblive)

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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...


von Patrick G. (sblive)

Attached files:

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Also see attached "schematic" (sort of).

von Harry U. (harryup)

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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

von Forgetti (Guest)

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>What am I missing? I'm sure it's simple...

Forgot the internal biasing resistors of Darlington??

von William Kuipers (Guest)

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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.


von Wooschder (Guest)

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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.


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