Low-power MOSFET-C 120 MHz Bessel allpass filter with extended tuning range

An integrated delay cell for a disc-drive read channel has to combine a high 120 MHz bandwidth with a low power dissipation and a large 1:6 tuning range. Because of linearity and transfer requirements, the implementation is a continuous-time third-order Bessel allpass filter. An opamp-MOSFET-C configuration is best for low power consumption but has two problems: a small tuning range and often a low bandwidth. The author gives solutions to both problems, including a patented tuning-range extension circuit. Measurements show that the design method works well. The cell dissipates 15 mW at 27°C