| 摘要 |
A line driver circuit capable of operating in 100Base-T mode and 10Base-T mode includes a 1:1 transformer having a primary winding, the primary winding having a center tap. A current source couples to the center tap. A current steering circuit steers drive current from the current source in first and second directions to provide a differential output signal at the output nodes. The current steering circuit includes two switches. Input signals control the opening and closing of the switches such that the switches are never both open at the same time. Thus, during a time period, one switch closes to pull one output node up toward the supply voltage, while the other switch opens to pull the other output node down toward circuit ground. Since the current source couples to the center tap of the transformer, the current source is only pulled down to a voltage half that of the supply voltage rather than to circuit ground. Therefore, the current source has sufficient headroom to drive the output current.
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