| 摘要 |
A current source circuit for providing a stable current into a filter element of a phase-lock-loop circuit of a clock generator. The current source circuit comprises a first resistor coupled to a voltage supply. The emitter of a first transistor is coupled to the first resistor; the base is coupled to a bias voltage, and the collector is coupled to a capacitor. The capacitor forms part of the filter of the phase-lock-loop circuit. Current flows from the voltage supply through the first resistor and first transistor into the capacitor. A second transistor has a collector coupled to the capacitor; a base; and an emitter coupled to ground via a second resistor. The second transistor and resistor causes a fixed amount of current to be sinked from the capacitor. Leakage current flowing out of the capacitor due to the inherent Rcb impedance associated with the second transistor is directed to a path provided by a third transistor. The third transistor has an emitter coupled to the base of the second transistor and a collector coupled to the emitter of the first transistor. The third transistor directs the Rcb leakage current inherent to the second transistor back into the capacitor. Thereby, the Rcb leakage current flowing out from the capacitor is canceled by the current flowing back into the capacitor via the third and first transistors. This produces a more stable current, and hence, more stable voltage being maintained by the capacitor. A more stable voltage means that the capacitor can be made smaller. In turn, this enables the phase-lock-loop to be fabricated on-chip with the rest of the clock generator, thereby minimizing its susceptibility to external noise and interferences. Furthermore, a more stable voltage across the filter element of the phase-lock-loop reduces unwanted jitter in the clock signal.
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