What is the role of RJ45 connectors in Ethernet?

Did not expect the RJ45 connector filter in the Ethernet has so many roles! In the equipment, when connecting RJ45 by PHY, a network transformer will be added in the middle of the RJ45 connector. Some transformer center tap to the ground. And when the power supply, the power value can be different, 3.3 V, 2.5 V, 1.8 V have. The transformer's role is analyzed as follows: 1, Middle Tap why some connected power? A little grounded? This is mainly determined by the use of PHY chip UTP driver type, this type of driver has two types, voltage-driven and current-driven. Voltage-driven will be connected to the power supply; current-driven can be directly connected to a capacitor to the ground! So for different chips, the center tap of connection, and PHY is closely related to the specific chip to see the datasheet and reference design.

2, why connect the power supply, and then different voltage? This is also determined by the UTP port level specified in the PHY chip data used. Whatever level is determined, the corresponding voltage must be connected. If it's 2.5 V, pull it up to 2.5 V, if it's 3.3 V, pull it up to 3.3 V. 3. What is the function of this transformer in the end? Can we not connect it. In theory, it can be directly connected to RJ45 without the need for a transformer, is also able to work properly. However, the transmission distance is very limited, and when different levels of access to the network, there will be an impact. And external interference on the chip is also very large. When connected to the network transformer, it is mainly used for signal level coupling. First, it can enhance the signal, make it more distant transmission; second, the chip end and external isolation, anti-interference ability greatly enhanced, and to the chip increased a great protection (such as lightning) ; third, when receiving different levels (such as some PHY chip is 2.5 V, some PHY chip is 3.3 V) of the network interface, will not affect each other's devices.