Circuit Converts PWM Fan Drive to Linear and Reduces Acoustic Noise

We usually use a fan to manage the temperature, but it produces audible noise at full speed. To reduce this noise we can use this circuit. The first circuit reduces the noise by controlling the cooling-fan speed as a function of temperature. It uses an IC that combines temperature sensing with a PWM output. When the temperature of the processor increases, the fan speed up  and the fan spins slowly when the temperature decreases, it produces little acoustic noise. Here is the circuit :

The second circuit is a improvement of the first circuit designed to eliminate noise caused by the removal of PWM pulses. This circuit is an essentially an inverting amplifier with high output-current capability. This circuit 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 12 V. R3, R2, and R1 provide feedback around the amplifier and voltage gain between the PWM output and fan. Q2’s drain is driven to to assume the corresponding voltage, to accommodate the gain. An effective DC output voltage is produced by capacitors C3 and  C1 by  filtering the PWM waveform. It produces a small amount of ripple, but it is inaudible and acceptable. [Source: maxim-ic.com]