| 摘要 |
Matched semiconductor junctions on a single integrated circuit chip are connected in a current mirror arrangement to provide a low power AC level detector arrangement using only one resistor. Two transistors are identical except for junction area and each is in series with a constant current source. Since all other physical characteristics are identical, when identical voltages are applied, the junction currents are related by the ratio of their areas. By fabricating the area of the second emitter-base junction to be less than the area of the first, a portion of the current from the second current source is diverted therefrom to the base of a third transistor, forcing the latter into saturation. Accordingly, when an AC input signal of at least threshold level is coupled to the base of the second transistor, the current in the second transistor is driven to the level of its source. The current formerly diverted to the third transistor now drops to zero and the voltage across the third transistor rises immediately to its supply voltage. The input threshold level required to shut off the third transistor can be varied by different chip design parameters or by modifying a current source with an external digital control.
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