This paper presents a 4^th-order active-RC low-pass filter (LPF) with tunable cut-off frequency (20-50 MHz). To achieve high in-band linearity performance, a three-stage opamp with an operational transconductance amplifier (OTA) output stage is adopted. The first two stages of the opamp only introduce about 45° phase shift using Miller-feedforward hybrid compensation scheme, which guarantees the stability of the filter. The designed opamp can achieve 47 dB gain and 1.4 GHz gain-bandwidth products (GBW) for the filter with 50 MHz cut-off frequency. The post-layout simulation results show that the filter consumes 5.7 mW from 1.1 V supply voltage. The in-band integrated input-referred noise (IRN) is 473 μVrms. The filter achieves 29.8 dBm IIP3 at 30 MHz. Due to the high in-band linearity performance, 153.4 dB · J^-1 of figure-of-merit is achieved in this design.