voltage regulator

[Cali-Sam]

my understanding is that you can basically make a voltage regulator out of a pot, an op-amp and a transistor, just like Dave's dummy load but without the load resistor..... the problem is that everywhere i look, i see a voltage reference being used and connected to one of the op-amp's inputs.....
why do i need a voltage reference? can i get away with a  pot instead?
without a reference, you can drive the voltage down to 0 volts, why doesn't everybody use this? why do people use a regulators IC's that have 1.2v minimum voltage?.......
 

[c4757p]

The voltage reference is what the output is regulated with respect to. If you use a pot, an op amp and a transistor (assuming I'm picturing what you're picturing), the output will always be some fixed ratio to the input. If you use a reference, the output can be fixed no matter what the input does.

Now, typically, a voltage regulator "contains" an op amp (in its block diagram, at least), and the voltage reference is connected to the (+) input, and the output is divided by a voltage divider and that goes to the (-) input. This gives you voltages above the reference voltage. If you move the voltage divider to the reference side, you get voltages below the reference. This topology can be used for the very few times when you need a voltage under 1.2V.

And I suppose the best answer to the "why" would be "almost nothing needs voltages under 1.2V".

[Galaxyrise]

Getting 1V via linear regulation is pretty inefficient since your regulator (and rectifier, if you're starting from AC) will likely be dissipating at least as much power as the load.   It's much more efficient to get low voltages using a DC-DC converter instead, and if you go to the PMIC - Voltage Regulators - DC DC Switching Regulators area of digikey, you'll find a lot of options that support <1V output.

Variable linear bench power supplies do use something like an op-amp and transistor as the regulator element so they can generate low voltages with low noise.  But they are generally quite bulky, partly to deal with the heat generated in that scenario.