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Old 31st Jul 2020, 5:23 pm   #16
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,244
Default Re: Device to simulate a dual rail power supply from a single DC source

Just be aware that that is designed for a particular application, and probably won't be suitable for general purpose use.

The problem with it is simple: it can only sink current.

So if the positive half of the circuit being supplied takes more current than the negative half, no problem. That will be the case in the product that it lives in. TR5 conducts and sinks current to the -ve rail as necessary to bring the "0V" back to the centre.

But if the negative half of the circuit takes more, there is nothing to source current to that half. So the mid-point "0V" will move down towards the negative rail, and there's nothing the poor op-amp and PNP transistor can do to change it...

The fix is simple - add an NPN transistor. Join the bases and emitters together, and tie the collector to the positive rail. In other works, follow the op-amp with an unbiased output stage. The unbiased arrangement might cause a "dead spot" that needs attention, but that's easy enough to fix if necessary.

Hope that makes sense - not trying to be negative (no pun intended! )

Mark
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