Creating (-) voltage

Hi guys, i have a project to be done but i should find a way to generate negative voltage from positive voltage. The tricky part is that I'm only allowed to use resistors, capacitors, inductors, diodes and op-amps. I have an idea to use zener diodes but actually I don't know how to use. Thanks in advance.

The method is to make an oscillator with an op-amp and rectify the output to make a negative voltage.

but how do I use an op-amp if I am not allowed to use negative voltage before I generate it ?

Connect the supply pins of the op-amp, one to the positive supply voltage and one the ground voltage (usually called zero volts).

Thank you, but also there should be a way with zener diodes.

Use a rail-to-rail op amp configured as an astable multi-vibrator connected the plus supply and ground. Couple the square-wave output op amp through a capacitor into two diodes. One diode is connected anode to capacitor and cathode to ground, the other diode connected cathode to capacitor and anode to output. Add a capacitor from the output to ground to filter the resulting negative DC voltage.

The output current of this circuit is limited to about 1/2 of the maximum op amp output current.

Now that you've been handed the entire circuit, you can add zener diodes wherever you want.