A varactor circuit with a linearised capacitance vs. control voltage characteristic

摘要

A circuit 100 (fig.2) includes varactor diodes 110, 112, 114 selected such that the sum of their maximum capacitances is equal to the required total capacitance of the circuit 100. One electrode of each of the varactors 110, 112, 114 is respectively connected to a dc offset voltage source V1, V2, V3. The other electrodes of the varactors are connected via resistor 118 to a common control voltage input 116. The offset voltages are arranged so that V3 V2 V1, and are chosen so that there is an overlap between the high capacitance range of one varactor and the low capacitance range of another of the varactors. As the control voltage at 116 is increased, each of the varactors is sequentially operated. The resulting C/V curve is more linear than that of a single varactor (fig.4), i.e. the dC/dV curve is flatter (fig.5). Applications are described in voltage controlled oscillators, filters, tuned amplifiers and modulators. The capacitance may also be made to follow a square law, e.g. to linearise tuning of an LC circuit in an RF VCO.