| 摘要 |
The laser driver circuit includes an array of differential pairs 50 and 52. Each one of the differential pairs is coupled to a corresponding tail current source 67 and 69. The total modulation current for the laser driver circuit is developed as the sum of the output current from the array of differential pairs 50 and 52. Each of the tail current sources 67 and 69 generates a subrange of the total range of modulation current. The tail current in a given differential pair will then only vary over a small subrange of the total modulation current range, and the device size in each pair may be optimized to keep the emitter current density near the level that gives optimum bandwidth. This is equivalent to electrically increasing the emitter area of the composite differential pair as the total modulation current is increased, keeping current density approximately constant at its optimal level.
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