I am trying to tune this circuit, dimensioning the values of resistors Ra,Rb and the capacitor C... The circuit work in this way: a voltage Vin regulates the current flowing in the inductor, via the VCO block. The operational give a Vctrl to NE555 configured as astable oscillator that (adjusting the duty cycle) will impose the right current. This current could be red from the OP Amp through the feedback 2 ohm resistor. I ask you what is the quickest and fastest method to get this circuit work. I followed this procedure: First i calculated the time constant of the inductor on the drain, with L/R formula. Then i chose a frequency far away the frequency of the pole, to get a "low pass effect" of the current in the inductor, about 100kHz. So, I supposed that the op amp is not rail to rail, and then I did a calculation to impose that with Vin = 1V the charge of the capacitor (from Vdd/3 to about 11V, the rail of the OP Amp) would last about 90% of the discharge time, then getting a current of 90% of the maximum current that can flow in the drain (a kind of 12V/22Ω). Then I tried with a different value of Vin (0.1V), and verified if the duty cycle would be reduced.
I know that it looks so confusing, so please i ask you how do you dimension this circuit.