To manage the temperature, we usually use a fan. However, the fan usually produces audible noise. The following circuit is designed to overcome this kind of problem. The first circuit reduces the noise by controlling the cooling fan speed as a function of temperature. An IC is used to combine temperature sensing with a PWM output. The mechanism is like this : if the temperature of the processor increases, the fan will then speed up, and vice versa.
The first circuit is improved by the second one. The second circuit is designated to eliminate noise caused by the removal of PWM pulses. Basically, this second circuit is an inverting amplifier with high output-current capability. It converts the PWM output of IC1 to a DC power supply for the cooling fan. This circuit operates from 12V, IC1 operates at 3.3V and the power supply voltage for full-speed operation of the fan is 12V. R1, R2, and R3 provides feedback around the amplifier and voltage gain between the PWM output and fan. Q2′s drain is driven to assume the corresponding voltage to accommodate the gain. Capacitor C3 and C1 produce an effective DC output by filtering the PWM waveform. Eventhough it produces a small amount of ripple, it is acceptable and inaudible. [source : maxim-ic.com]