A bidirectional transceiver (Bi-TRx) integrated with a novel transmission envelope detector, which can detect the presence of received signals to generate a mode control signal automatically, is proposed in this letter. Implemented in a 0.18-mu m complementary metal-oxide-semiconductor technology, the Bi-TRx occupies an area of 1.08 mm(2) and dissipates 42 and 49 mW in transmitter (Tx) and receiver (Rx) modes, respectively. It achieves a 3-dB bandwidth of 4.4 and 4 GHz in Tx and Rx modes, respectively, while the disabled outputs for the Tx and Rx modes are isolated with 41 and 68 dB, respectively, from the enabled outputs.