Simple PC fan controller PWM discrete components
The 100uF capacitor will cause the speed not to change instantly with duty-cycle. That does, however, cause the speed to change more smoothly and slowly and can contribute to a 'soft-start' effect. The time constant is about 0.1s so keep that in mind when thinking of fan speed response time. However, you can go ahead and use this circuit as a starting point and deduce if you need to change anything based on what you find.