| 摘要 |
A VCO has n inverters connected to each other in a ring, wherein the inverters have a depletion-mode FET. A selector control circuit produces a select signal based upon a threshold voltage of a selector depletion-mode FET to select one of an output derived from a (n-i)-th inverter, or another output signal derived from the n-th inverter. Because the selector FET and the inverter FETs are made using the same process steps and thus have the same device parameters, the selector FET has the same threshold voltage variation as the inverter FETs. Based upon the variation in the FET threshold voltage, the selected output is provided to an input of a first inverter. The VCO operates at a constant frequency by changing the total number of inverters in the ring to compensate for the variation in the threshold voltage of the depletion-mode FETs in the inverters, as determined by the depletion-mode FET in the selector circuit.
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