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I want to make a voltage regulator from one or two BJTs and a zener diode. However, I would like to be able to make full use of supply voltages when they are lower than the voltage that the regulator would otherwise supply. Perhaps a circuit with a transistor that would saturate. The problem with the circuit below is the 0.7v drop of the transistor, plus the additional drop required across R1. Please could someone recommend a circuit?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I think if TI, ADI, LT etc could make a low drop-out regulator from one or two BJTs and a zener diode they would be happy folk. \$\endgroup\$ – Andy aka Aug 1 '13 at 17:31
  • \$\begingroup\$ Ah thank you, I didn't realize it was a difficult problem. \$\endgroup\$ – CL22 Aug 1 '13 at 17:43
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Your circuit is a half decent regulator and would be good for powering most things that need a small current. The problem, as you quite rightly point out, is the 0.7V lost between input and output.

Low drop-out regulators are really quite common-place and some work really well down to a few milli volts between input and output BUT, they are far more complex than what you think and cannot be made from a couple of transistors, a handful of resistors and a zener diode.

However, in pursuant with your wishes, the best idea I can come up with is to construct a small single transistor colpitts oscillator that runs from the incoming power supply and, via two capacitors and two diodes can create a voltage that is bigger than the incoming voltage - this can then feed the zener diode via a resistor ensuring that the BJT is always biased on at the zener voltage even when the supply is quite close to the zener voltage.

Probably the best you could hope for is a volt-drop across the transistor of 0.1 to 0.2V - but that is better than 0.7V but not as good as some commercially available devices from the usual vendors.

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Check the datasheet of an LDO regulator. Things won't get as simple as one or two transistors though. The stadard trick is to do the comparing at a lower voltage (using a long tailed pair), and to drive a PNP series transistor.

This design can be found at http://www.discovercircuits.com/Andy/Discrete_LDO_Voltage_Regulator.htm :

enter image description here

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