I'm searching for a ramp generator circuit which is using only op amps (I guess I need an integrator & differentiator for that). The output should be like in the image below.
The standard method for doing what you want is to configure an op-amp as an integrator with a constant input current that sets your ramp rate, then discharge the integrator capacitor when it's time for the ramp to reset back to the start level.
How quickly must the ramp reset back to the start level? The device that you choose to discharge the integrator capacitor determines the reset time - older designs used a J-FET configured as a switch but the ON-resistance slows down the reset.
There are now modern switches available that will discharge a relatively-large capacitor in very little time.
You can use a 555 like this: -
It relies on a transistor (Q2) acting as a constant current generator to charge the capacitor linearly. Normal discharge method via internal transistor. Q2 constant current charge replaces the resistor normally fitted.
Or you can use the classic voltage to frequency convertor: -
The saw output is at "out1" (the output of the resetting integrator). You can even tweak operating frequency via a voltage on "in"