My input is DC from a small DC motor which I use as a generator.
My input current is 300 milliamps and I need 1200 milliamps as output.
Is there some way to increase the output to 1200 milliamps ?
With out using external energy input you can transform voltage or current but you can not increase the power level. As power = voltage x current, that means, if voltage is increased then available current is increased, then available voltage but decrease. That is
Power in >= Power out so
Vin x Iin >= Vout x Iout
Usually power out will be less than power in due to inefficincies in the conversion process.
In your case, imagine that your generator makes 300 ma at 6 Volts.
Power in = 300mA x 6V = 1800 mW.
Power out must be <= 1800 mW.
For Iout = 1200 mA, Vout must be
Vout <= 1800 mW / 1200 mA = 1.5V
So at 1200 mA out the voltage out is <= 1.5V.
The conversion could be carried out by a 'buck converter' which reduces output voltage and increases output current.
Good buck converters can be in the range 90% - 95% efficient so in this example, at 90% efficient:
Input = 6V, 300 mA
Iout = 1200 ma.
VOut 6V / 4 x 90% = 1.35V
If you want more current AND more power you need to "change the laws of physics"
(Recommended :-). (Reference to 'change the laws of physics at 2m:25s but some (but not all) will find the whole 3m33s worth watching (once, anyway).)))