28 GHz RF Front-End Structure Using CG LNA as a Switch

This letter proposes a compact 28-GHz front-end (FE) structure that uses a common-gate (CG) low-noise amplifier (LNA) as a transmit/receive switch. The power amplifier (PA) output shares a dc bias with the CG LNA input. When the shared dc bias is high, the state of the LNA becomes passive, and the LNA operates as a turned-off shunt switch in the transmitter (TX) mode. The proposed CG LNA switch allows high isolation and linearity with a TX path insertion loss of 0.07 dB at 28 GHz. On the contrary, when the shared dc bias is low, the state of the PA becomes passive and it results in a receiver (RX) path insertion loss of 0.51 dB at 28 GHz. The path losses of the CG LNA switch are minimized by the RF floating bodies of the PA and the CG LNA. This is implemented in a 65-nm CMOS process occupying a 0.215-mm² core area. The measured saturation power is 15.3 dBm with 25.1% peak power-added efficiency in the TX mode, and the measured noise figure is 4.9 dB in the RX mode.