. ▁This ▁can ▁be ▁accomplished ▁by ▁placing ▁a ▁DC ▁blocking ▁capac itor ▁with ▁a ▁capac it ance ▁about ▁ 1 0 0   times ▁greater ▁than ▁the ▁maximum ▁capac it ance ▁of ▁the ▁var ic ap ▁di ode ▁in ▁series ▁with ▁it ▁and ▁by ▁applying ▁DC ▁from ▁a ▁high ▁imped ance ▁source ▁to ▁the ▁node ▁between ▁the ▁var ic ap ▁c ath ode ▁and ▁the ▁blocking ▁capac itor ▁as ▁shown ▁in ▁the ▁upper ▁left ▁circuit ▁in ▁the ▁accomp any ing ▁diagram . ▁ ▁Since ▁no ▁significant ▁DC ▁current ▁flows ▁in ▁the ▁var ic ap , ▁the ▁value ▁of ▁the ▁res istor ▁connecting ▁its ▁c ath ode ▁back ▁to ▁the ▁DC ▁control ▁voltage ▁res istor ▁can ▁be ▁somewhere ▁in ▁the ▁range ▁of ▁ 2 2   k Ω ▁to ▁ 1 5 0   k Ω ▁and ▁the ▁blocking ▁capac itor ▁somewhere ▁in ▁the ▁range ▁of ▁ 5 – 1 0 0   n F . ▁Sometimes , ▁with ▁very ▁high - Q ▁tun ed ▁circ uits , ▁an ▁induct or ▁is ▁placed ▁in ▁series ▁with ▁the ▁res istor ▁to ▁increase ▁the ▁source ▁imped ance ▁of ▁the ▁control ▁voltage ▁so ▁as ▁not ▁to ▁load ▁the ▁tun ed ▁circuit ▁and ▁decrease ▁its ▁Q . ▁ ▁Another ▁common ▁configuration ▁uses ▁two ▁back - to - back ▁( an ode ▁to ▁an ode ) ▁var ic ap ▁di odes . ▁( See ▁lower ▁left ▁circuit ▁in ▁diagram .) ▁The ▁second ▁var ic ap ▁effectively ▁rep laces ▁the ▁blocking ▁capac itor ▁in ▁the ▁first ▁circuit . ▁This ▁reduces