How to provide feedback for inverter?

Hi, you have to increase in your power source. As P=VI, in your case P is constant (or may be some limitation),you are increasing load, means increase in current naturaly voltage will drop. you can take an out put current signal and compare it with your set value and give that eror signal to the firing pulse generator circuitory. It will limit you output current and hence maintain output voltage. Hope it will solve your problem.

thanks for the professional idea you gave to me. i will try it exactly the way you suggested. please, i will really appreciate any further assistance you can still provide.

the tri-inverter oscillator is known as the poor man's oscillator for a reason, it doesn't give you much to work with. That would be a good modulation source, but for the pulse/driver circuitry, I have had more success with the TL494, a pulse width modulation chip. It has two diode steered operational amplifiers to help with your load sensative feedback issues.