http://imgur.com/0OkAQ

I built the above circuit. The 5v square wave is a 555 oscillator circuit. The sine wave is a function generator. Tl072CN is an non saturated op amp.

The timer has a 2.5v DC offset and I need to modify the circuit so that it uses -15v dc from the power supply to remove the offset here.

Any help here? I'm not really sure how to start. All I find is about using bypass filters or capacitors to remove DC voltage. How would a negative voltage remove dc offset? Thanks.

 

The timer chip gives its output from lowest voltage to highest voltage supplied to the 555 chip. To "center" this, give the timer chip equal power supply voltages above and below "ground".