▁to ▁Vienna ▁and ▁Moscow , ▁and ▁were ▁predicted ▁to ▁have ▁a ▁useful ▁future ▁( Er n st ▁Alexand erson , ▁ 1 9 1 6 ). ▁These ▁early ▁paramet ric ▁ampl ifiers ▁used ▁the ▁non linear ity ▁of ▁an ▁iron - core ▁induct or , ▁so ▁they ▁could ▁only ▁function ▁at ▁low ▁frequencies . ▁ ▁In ▁ 1 9 4 8 ▁Ald ert ▁van ▁der ▁Ziel ▁pointed ▁out ▁a ▁major ▁advantage ▁of ▁the ▁paramet ric ▁ampl ifier : ▁because ▁it ▁used ▁a ▁variable ▁react ance ▁instead ▁of ▁a ▁resistance ▁for ▁ampl ification ▁it ▁had ▁inher ently ▁low ▁noise . ▁A ▁paramet ric ▁ampl ifier ▁used ▁as ▁the ▁front ▁end ▁of ▁a ▁radio ▁receiver ▁could ▁ampl ify ▁a ▁weak ▁signal ▁while ▁introdu cing ▁very ▁little ▁noise . ▁In ▁ 1 9 5 2 ▁Harrison ▁R owe ▁at ▁Bell ▁L abs ▁extended ▁some ▁ 1 9 3 4 ▁mathematical ▁work ▁on ▁p ump ed ▁oscill ations ▁by ▁Jack ▁Man ley ▁and ▁published ▁the ▁modern ▁mathematical ▁theory ▁of ▁paramet ric ▁oscill ations , ▁the ▁Man ley - Row e ▁relations . ▁ ▁The ▁var actor ▁di ode ▁invent ed ▁in ▁ 1 9 5 6 ▁had ▁a ▁non linear ▁capac it ance ▁that ▁was ▁us able ▁into ▁mic row ave ▁frequencies . ▁The ▁var actor ▁paramet ric ▁ampl ifier ▁was ▁developed ▁by ▁Mar ion ▁H ines ▁in ▁ 1 9 5 6 ▁at ▁Western ▁Electric . ▁At ▁the ▁time ▁it ▁was ▁invent ed ▁mic row aves ▁were ▁just ▁being ▁explo ited , ▁and ▁the ▁var actor