A continuous-time 4th-order equiripple linear phase G
m-C filter with an automatic tuning circuit is presented. A high speed OTA based on the inverter structure is realized. The combined CMFF and CMFB circuit ensures the input and output common-mode stability. The gain performance could be maintained by combining a negative resistor at the output nodes. Transconductance tuning can be achieved by adjusting the bulk voltage by using the Deep-NWELL technology. Through the use of the OTA as a building block with a modified automatic tuning scheme, the filter -3dB cutoff frequency is 1GHz with the group delay less than 4% variation up to 1.5fc frequency. The -43dB of IM3 at filter cutoff frequency is obtained with -4dbm two tone signals. Implemented in 0.18-μm CMOS process, the chip occupies 1mm2 and consumes 175mW at a 1.5-V supply voltage.