the electrodes 12 and 13 to the input control terminal 70 as well as by making equal the variable impedances of the electrodes 12 and 13 to ground there is achieved a better circuit balancing which in turn entails less required semiconductor area on which to build the circuit less power loss and greater circuit stability on the other hand moderate difference between the varactor parameter say one varactor being twice as large as the other can be useful in some instances as for modifying the relationship between signal amplitude at terminal 26 relative to terminal 27 capacitors 28 and 29 optionally can be connected to the output terminal 26 adn 27 respectively in order to modify permanently the resonant frequency f r of the circuit 100 as well as to increase the linearity of u03b4f r vs v c at some expense of pull range however advantageously the capacitances of both capacitors 28 and 29 though not necessarily equal are both less than those of varactors 30 and 40 by at least a factor of about five fig2 shows another embodiment of the invention elements shown therein which are similar to or the same as those shown in fig1 are denoted by the same reference numerals as shown in.