a circuit that includes a series arrangement of a capacitive element and an active component forming an equivalent resistor . a dc current source is provided for altering the conductance of the active component and fixing the mean level of a ac voltage . such a voltage is applied to the terminals of the series arrangement , and the biased voltage having the desired mean level is tapped off the node between the capacitive element and the active component . a capacitive voltage divider is also provided for modulating the dc current passing through the active component , with a fraction of the ac voltage to be biased . the circuit is particularly applicable to the determination of the mean level of the voltage produced by a quartz oscillator .