Generally most joule thief circuits are simply a bad designed boost convertor (Thereby giving the illusion of powering devices that require much higher than the voltage of the battery), if you choose the right components this can operate to very low source voltages in the order of hundreds if mV (As long as the transistors are able to turn on and off still)
The frequency is harder to control as it is based on vary basic components, variences in the inductance, capacitors, temperature etc will all have an impact in a simple circuit like this and if your driving a few LEDs or something then who cares.
If you need Ghz frequency then you should stop looking at joule thief circuits and focus your search more on RF LC tank oscillators and a RF transistor to go with it, your inductors will be very different from the joule thief
Generally most joule thief circuits are simply a bad designed boost convertor (Thereby giving the illusion of powering devices that require much higher than the voltage of the battery), if you choose the right components this can operate to very low source voltages in the order of hundreds if mV (As long as the transistors are able to turn on and off still)
The frequency is harder to control as it is based on vary basic components, variences in the inductance, capacitors, temperature etc will all have an impact in a simple circuit like this and if your driving a few LEDs or something then who cares.
If you need Ghz frequency then you should stop looking at joule thief circuits and focus your search more on RF LC tank oscillators and a RF transistor to go with it, your inductors will be very different from the joule thief
