Hello, thanks for your replies.
DVDdoug:
I don't know what a CA buzzer is....
Sorry, by AC I mean that the buzzer doesn't have an internal oscilator so it doesn't work with DC (Direct Current). Fortunately the Arduino "tone" sentence creates a squared signal.
[u]Ohm's Law[/u].
You don't "push current". You put-out a known voltage, and the amount of current depends on the voltage and the resistance/impedance. (i.e. resistance is "resistance to current flow".)
By Ohn's law I had expected a current of 16mA, as the voltage is 5V and the buzzer internal impedance is 300 ohms.
The transistor needs to be connected to a higher voltage. The 5V from the Arduino (through a current limiting resistor) into the base of the transistor turns-on current flow through the collector & emitter of the transistor, and through you buzzer.
I cannot connect the buzzer to a higher voltage, it is a constraint of the design. That's why I wanted to increase current and so power, I thought it would be possible to increase volume (power) by using the same voltage and increasing current with a transistor. I can change the buzzer, but all the design must be as lighter as possible: it will fly in a structure of 150gr, and currently the electronics are adding 50 extra gr, including battery.