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richwalters 27th February 2011 09:17 AM

Low dropout 6.3v
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The circuit enclosed is one of my modified junk box designs from my prime analogue haydays of 1966......the advent of Bob Widlar and the uA709,741 etc. It caused a stir.

Notice the "jockey" soft rectifier doubler incorporated within a standard full wave rect to supply that little bit of extra voltage headroom for the op amp supply, otherwise the output cannot swing enough to supply the extra to accomodate the mosfet g/s drop voltage..only a mA or so is required.-
This little technique can greatly reduce pass transistor dissipation.
The basic current limiter softens current surges with cold tungsten resistance, so often ignored, also an ultra low Rds mosfet is an obvious candidate. As mentioned for the HV variant, the current layout is important for low noise.

As with these circuits, alot of variations, with the other half of the unused 1458 one can configure to further reduce the Vdrop of the 0.22R current limit to performance approaching the lowest dropout ic regulators.
Breadboard and simulate it.


richwalters 27th February 2011 09:50 AM

Another advantage of using an op amp is the excellent supply ripple rejection they provide (study data sheets)., so in the example shown a "rough" supply performs superbly.

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