| 摘要 |
To provide a stabilised current to a current modulator or the like in a limited voltage headroom environment, the present invention supports a current mirror having first and second mirror transistors (34, 36). Bases of the transistors are coupled together through a resistive element (R1, 102), with base leakage current (Ibase) to the first transistor generating a voltage drop (IbaseR1) across the resistive element (R1, 102). The voltage drop acts to boost a voltage through an emitter resistor (R3) of an output stage of the current mirror, the output stage realised by at least the second mirror transistor (36). With boosting of the voltage through the emitter resistor (R3) there is a corresponding boost in output current (Iout, 52) defined by the voltage across emitter resistor (R3). As such, the circuit of the preferred embodiment effectively increases the operating voltage and hence the voltage headroom derived from a supply voltage Vcc provided to the bases of the first and second mirror transistors (34, 36). The circuit, as exemplified in <cross-reference target="DRAWINGS">FIG. 4</cross-reference>, is particularly useful in the context of providing current to laser diodes for accurate extinction ratio control, especially in the context of silicon-germanium (SiGe) bipolar technology.
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