This paper presents a broadband transformer design based on three coils with mutual magnetic coupling for cascode amplifiers. To demonstrate benefits of the proposed tri-coil transformer, a single-stage cascode low noise amplifier is designed in 40nm CMOS process. The LNA with proposed tri-coil transformer exhibits significant stability and bandwidth enhancement. The simulated results show that the LNA achieves the 3-dB bandwidth of 27GHz, and factional bandwidth of 52.4%. At 54GHz, the simulated noise figure and gain of single-stage LNA are 3.6dB and 11.07dB, respectively.