I.Cu to Electret Microphone(Microphone in dung).Electret Microphone c cu to gm mt ming cht p in trn b mt ca ming cht ny s xut hin in tch tri du khi n b nn hoc.Mt mt ca ming cht ny s c ni vi Ground(v nhm ca microphone).Mt bn kia s c ni vi cc Gate ca mt JFET,Cc Source ca JFET ni Ground(v nhm) cc D h.Khi c m thanh tc ng s xut hin in tch tri du trn b mt ming cht p in,tc l trn 2 mt ca ming cht p in s xut hin in p. ln ca n ty thuc vo nhy(Sensitivity) ca ming cht p in.Khi xut hin in p trn 2 mt ming cht,in p s c a vo chn G ca JFET khuych i v a ra ngoi ca Drain.V vy hot ng Microphone cn c cp mt in p qua tr RL vo chn Drain ca JFET.

in tr RL cp in cho JFET hot ng,t C lc b cc thnh phn mt chiu a ti b khuych i.

II,M phng Electret Microphone.

Cc thng s c bn ca mt Electret Microphone.Ta cn quan tm chnh ti nhy (Sensitivity) ca Microphone. nhy ca electret microphone l -46dB vi tn s 1kHz ta thc hin bin i n v dB v gi tr in p.-46=-20log(x/1)=>x=1*10(-46/20)=5mV.Vy khi ta tc dng mt p sut c gi tr 1 Pa s c mt in p 5mV sut hin u ra ca ming cht p in.Ta s thc hin khuych i gi tr ny qua JFET.

Chn gi tr RL=4k7 ta c tn hiu ra c khuyeesch(k vit c ch khuyeesch ) i nh sau.(Khi tnh dm t on ny qun mt ct ri)

Vi gi tr in p nh l 24mV ta c u vo cho b khuych i.

III,Khuyeesch i tn hiu Microphone. tn hiu m thanh c a vo ADC vi mc TTL(0-5V) vi gi tr tn hiu vo l 24mV ta s thc hin khuyeesch i gi tr ny t 50-100 ln v thc hin nng gi tr in p ban u khi cha c m thanh l 2.5V.(voltage Bias =2.5V).Ta s chn mch khuyeesch i m thanh dng Ic khuyeesc i thut ton(Opamp) TL072 c nhiu u im l low noise thch hp vi cc tn hiu u vo l JFET.Mch khuyeesch i tng quan nh sau.

Ta thc hin 2 tng mch khuyeesch i o pha,khi s dng 2 tng s hn ch c vic gi tr gain thp vi cc m thanh tn s nh(Bass)

H s khuyeesch i c gi tr bng.(tnh chi tit gip t)K1=-R3/R2=10.Vy tn hiu vo s c khuyeesch i ln 10 ln vi gi tr nh l 240mV.Cc gi tr in tr R4 v R5 c tc dng kh in p lch khng(gi tr in tr ti cc ca l bng nhau).

Tng 2 c cch mc ging tng 1.Nhng in tr hi tip s thay bng mt bin tr 100k chnh h s khuyeesch i nh nht l 10 v ln nht l 100.Khi h s khuyeechs i ton mch s bngK2=k1*(Rv1/R6).Vy U ra c gi tr bng:UR=k2*Uv vi gi tr in p nh l 24mV*100=2.4V gi tr output thuc mc TTL c phn dng v m ta lm mt cu chia p nng mc p nh ln.Khi dng tn hiu ra nh sau:

Vi gi tr ban u l 2.5V khi c tn hiu m thanh s dao ng quanh ngng 0-5V.Ta c c tuyn bin tn v c tuyn pha.

Vi gi tr gain xp x 40dB tng ng vi h s khuyeesch i 100 ln trong khong tn s nghe c l 16-20kHz.H s khuyeesch i khng b gim trong khong m tn.