▁capac itor ▁is ▁charged ▁until ▁its ▁voltage ▁equals ▁the ▁sample d ▁voltage ▁of ▁an ▁incoming ▁weak ▁signal , ▁and ▁if ▁the ▁capac itor ' s ▁capac it ance ▁is ▁then ▁reduced ▁( say , ▁by ▁manually ▁moving ▁the ▁pl ates ▁further ▁apart ), ▁then ▁the ▁voltage ▁across ▁the ▁capac itor ▁will ▁increase . ▁In ▁this ▁way , ▁the ▁voltage ▁of ▁the ▁weak ▁signal ▁is ▁ampl ified . ▁ ▁If ▁the ▁capac itor ▁is ▁a ▁var ic ap ▁di ode , ▁then ▁" mov ing ▁the ▁pl ates " ▁can ▁be ▁done ▁simply ▁by ▁applying ▁time - v ary ing ▁DC ▁voltage ▁to ▁the ▁var ic ap ▁di ode . ▁This ▁driving ▁voltage ▁usually ▁comes ▁from ▁another ▁oscill ator ▁— ▁sometimes ▁called ▁a ▁" p ump ". ▁ ▁The ▁resulting ▁output ▁signal ▁contains ▁frequencies ▁that ▁are ▁the ▁sum ▁and ▁difference ▁of ▁the ▁input ▁signal ▁( f 1 ) ▁and ▁the ▁p ump ▁signal ▁( f 2 ): ▁( f 1 ▁+ ▁f 2 ) ▁and ▁( f 1 ▁- ▁f 2 ). ▁ ▁A ▁practical ▁paramet ric ▁oscill ator ▁needs ▁the ▁following ▁connections : ▁one ▁for ▁the ▁" common " ▁or ▁" ground ", ▁one ▁to ▁feed ▁the ▁p ump , ▁one ▁to ▁retrieve ▁the ▁output , ▁and ▁maybe ▁a ▁fourth ▁one ▁for ▁bias ing . ▁A ▁paramet ric ▁ampl ifier ▁needs ▁a ▁fifth ▁port ▁to ▁input ▁the ▁signal ▁being ▁ampl ified . ▁Since ▁a ▁var actor ▁di ode ▁has ▁only ▁two ▁connections , ▁it ▁can ▁only ▁be ▁a ▁part ▁of ▁an ▁L C