▁source / load ▁resist ances ). ▁For ▁a ▁parallel ▁reson ant ▁circuit ▁the ▁opposite ▁applies ; ▁small ▁induct ances ▁reduce ▁the ▁d amp ing ▁of ▁external ▁circuit ry . ▁ ▁There ▁are ▁practical ▁limits ▁to ▁the ▁increase ▁in ▁select ivity ▁with ▁changing ▁L / C ▁ratio : ▁tun ing ▁capac itors ▁of ▁large ▁values ▁can ▁be ▁difficult ▁to ▁construct ▁st ray ▁capac it ance , ▁and ▁capac it ance ▁within ▁the ▁trans ist ors ▁or ▁val ves ▁of ▁associated ▁circuit ry , ▁may ▁become ▁significant ▁( and ▁vary ▁with ▁time ) ▁the ▁series ▁resistance ▁internal ▁to ▁the ▁wire ▁in ▁the ▁co il , ▁may ▁be ▁significant ▁( for ▁parallel ▁tun ed ▁circ uits ▁especially ) ▁large ▁induct ances ▁imply ▁phys ically ▁large ▁( and ▁expensive ▁co ils ) ▁and / or ▁th inner ▁wire ▁( h ence ▁worse ▁internal ▁resistance ). ▁ ▁Therefore ▁other ▁methods ▁may ▁be ▁used ▁to ▁increase ▁select ivity , ▁such ▁as ▁Q ▁multip lier ▁circ uits ▁and ▁reg ener ative ▁rece ivers . ▁Super h eter ody ne ▁rece ivers ▁ ▁allow ▁use ▁one ▁or ▁more ▁fixed ▁intermediate ▁frequency ▁tun ed ▁circ uits ▁for ▁select ivity . ▁F ixed ▁tun ing ▁elimin ates ▁the ▁requirement ▁that ▁multiple ▁tun ing ▁stages ▁accur ately ▁match ▁while ▁being ▁adjust ed . ▁ ▁See ▁also ▁▁▁ ▁" P ract ical ▁R F ▁circuit ▁design ▁for ▁modern ▁wireless ▁systems ", ▁volume ▁I , ▁by ▁Les ▁B esser ▁and ▁Row an ▁Gil more ; ▁chapter ▁ 3 . 2 . 6 , ▁" Receiver ▁select