What's the voltage used for and how much current do you need?
For a "signal" (very low current) you can double a voltage with a simple op-amp circuit, but you need to power the op-amp with slightly more than you are trying to get out of it.
There are fairly simple capacitor-oscillator based voltage doublers if you need a small amount of power and you don't need efficiency (they do require a chip or other active circuitry).
If you need to "power" a circuit, you need an inductor-based voltage booster (again they require active circuitry). These are very efficient, but of course you can't get more power out than you put in... So if you double the voltage, you cut the current (approximately) in half. But normally if you are gong to power something, you start-out with enough voltage... Boosting (or inverting) the power supply voltage is usually a "last resort", like if you need to run something off a car battery and it needs more than 12V, etc.
Actually its a pretty small circuit so I'm using 2AA battery. I need about 6v in my application
4 AA batteries. ;)