This paper presents a CMOS bidirectional amplifier for time division duplexing systems. By switching supply and ground voltages of a common gate amplifier, the bias current and amplification direction can be switched between forward and backward modes. Depending on the amplification direction, CMOS switches parallel with matching inductors perform switchable matching circuits. The source and drain voltages of CMOS switches are switched by the bias direction, and the CMOS switches can be automatically turned on and off with a fixed gate voltage. The measured peak gain is 9.8 dB at 7.4 GHz with input and output return losses lower than -6.4 dB. The measured output 1-dB power compression point and average noise figure are 4.2 dBm and 7.7 dB including probe loss, respectively. The designed circuit and layout are directional input-output symmetric, and therefore the amplification characteristics of the forward and backward modes are identical. The measured insertion phase difference between forward and backward modes is <6°. The chip measures 470×544 μm2 without pads.