Recently the author had to adapt a standard circuit configuration (which often uses an npn bipolar) so as to operate as a low-dropout (LDO) regulator. The circuit shown here uses that rarity, a depletion-mode MOSFET to implement the LDO function. What to do when you have to derive an analogue supply voltage (close to +5 V) from an existing ‘digital’ 5-volt rail, ensuring sufficient decoupling between the two? One answer is to step up and then use a linear regulator to step back down. However, if around 4.5 volts will suffice then an alternative is a home-made LDO regulator. The circuit is usually a fairly standard shape typically a npn transistor (with base-current limiting resistor) is used. Circuit diagram : LDO Regulator Circuit Diagram Initially, it would appear that this design suffices after all, the text books say the saturation voltage is around 0.2 V. Unfortunately, this is no longer true when the collector is tied directl...