I try to make controller for 12V fan, where I can vary voltage from 3V to 12V on this fan. It works OK, but it makes electrical noise on 12V power line. How could I get rid of it? How to filter it, or how to modify my circuit? this is the noise:
UPDATE: Fan draws 220mA @ 12V, 60mA @ 3V. This is the modified circuit, according to answers. Noise is lowered down to ~20mV peak-peak: Added 100nF across pins 1 and 2 of opamp, 100nF cap across power pins of opamp, 2200uF cap from 12V to ground.
UPDATE2: I put 3k resistor from Q1 emitter to ground and removed capacitor that was directly on fan connector (there is just 2200uF on 12V line). This lowered noise once again.