We can simplified greatly the fan speed regulation design using a dedicated IC such as the MAX6650 or MAX6651. The MAX6650 is designed to control a single fan. The MAX6651 has more features, it can controls a single fan and can monitor and watchdog three additional fan tachometer outputs. We can synchronize multiple MAX6651s to control multiple fans. The IC extensive programmability to accommodate a variety of fan speeds and types is included. A typical connection for the MAX6650 is depicted by figure below.

MAX6650 Fan Speed Controller circuit schematic

Take a note that from the circuit on figure below, the MAX6650/MAX6651 are designed to be used with an external pass transistor. Since the output stage of the MAX6650/MAX6651 is an analog DAC and amplifier combination, feedback is taken from the drain of the transistor to the terminal on the IC labeled “FB”. Local feedback is required by this DAC/amplifier combination. The tachometer signal of the fan is actually the feedback complete loop, and this also returned to the MAX6650/MAX6651. The fan will be driven by these device until that tacho signal matches the programmed period in the speed register in the MAX6650/MAX6651. [Source: MAXIM Application Note]