Section J8b: FET Low Frequency Response

Transcription

1 ection J8b: FET ow Frequency epone In thi ection of our tudie, we re o to reiit the baic FET aplifier confiuration but with an additional twit The baic confiuration are the ae a we etiated ection J6 of the WebT note, with the iilaritie and difference a noted the table below: Then External capacitor bypa & coupl Ideal hort for ac Tranitor output Very lare reitance, r o ay uually be ored Input and output eitance characteritic, out FET pproxiation i, o i i Now Non-ideal keep all al circuit OK, we ll keep thi one Ipedance Z, Z out OK, unle otherwie noted Jut like we did for the BJT confiuration, we re o to tart by look at each of the baic aplifier tae ter of analyi and fih with trateie for de for a pecific low frequency characteritic ll aplifier are preented a capacitie-coupled to tae that ay occur before and after ecall that thi i the eaiet way to enure dc iolation, but ay not be feaible certa circutance or under certa condition Note: the circuit that follow, the actual al ource and it aociated ource reitance ource hae been cluded reiouly, we knew that thi ource and reitance wa there but we jut tarted our etiation with the put to the tranitor Thi hould not caue too uch heartburn the analyi proce i the ae and the relationhip between and i a oltae diider ce we e already ot an FET circuit the ource le, don t et confued between the ource of the tranitor itelf and the reitance aociated with the al ource hown a orry, I know it confu! ow Frequency epone of the oon-ource plifier To facilitate the analyi of the FET aplifier confiuration, the ot coplicated confiuration i addreed firt Thi will allow u to detere all tie contant and the analyi of the ipler confiuration will ole the eliation of appropriate ter lo recall that JFET and MOFET circuit are analyzed the ae way thi alo hold here that the tie contant do not depend on the type of FET

2 The JFET ipleentation of the coon-ource aplifier i ien to the left below, and the odified all al odel i to the riht below baed on Fiure 9a and 9b of your text ett the put ource,, equal to zero reult the circuit ien to the riht that we will ue for analyi purpoe U thi circuit, and the oberation that the dependent current ource i opened ce, we can fd the equialent reitance een by,, and our old friend, the Method of hort ircuit Tie ontant : ett and equal to fity hort circuit, the equialent reitance een by i, where for the confiuration : ett the ipedance of and be equal to zero, the equialent reitance een by i

3 : ett the ipedance of and be equal to zero, the equialent reitance een by i Note that, jut a for the cae of the coon-eitter aplifier, the bypa capacitor troduce a zero at Z Z The coupl acitor each troduce a z cap ero at zero frequency The tie contant for the aplifier are therefore defed by ; ;, and the lower corner frequency i crudely approxiated by, ith a better approxiation ien by -ource aplifier, where only part of the ource reitance i bypaed, i ien by w Z Z The idband oltae a for the coon idband, and the noralized frequency dependent a expreion i ien by Z idband

4 ow Frequency epone of the ource-follower plifier The eneric circuit for the FET F aplifier u an n-channel JFET i illutrated the fiure to the left below The odified all al equialent circuit i hown below and to the riht baed on Fiure a and b of your text The ource-follower, or coondra, FET aplifier i iilar to the EF aplifier, it BJT counterpart It alo ha two tie contant, one of which i uch larer than the other The circuit ued to derie the tie contant of the circuit capacitor i ien to the riht : The equialent reitance een by the coupl capacitor i found by ett equal to fity, or, where the put reitance for an F aplifier wa preiouly defed a : The equialent reitance een by the coupl capacitor i equal to

5 Note that thi equialent reitance i not directly oberable fro the aboe fiure Intead, the Theen oltae and current are defed, with the equialent reitance be equal to the Theen reitance e for a ien Frequency haracteritic e an FET aplifier for a pecified low frequency repone i a outled ection H3 for BJT aplifier and i reproduced follow: pproach : If the pole can be eparated by at leat a decade, we let one doant pole produce the entire 3dB drop Thi i iilar to the cae of a le pole, ce there i irtually no teraction between the two the frequency oe to zero, the doant pole at the hiher frequency will defe the corner frequency before the econd pole be to take effect pproach : If the put and output reitance are approxiately equal, we et the two pole frequencie to be equal; ie, we hae a double pole Thi ean that each pole contribute eenly at the break pot or, equialently, that each pole contribute a 5dB drop o that the total decreae will be 3dB at the deired corner frequency For exaple, a noralized oltae a expreion with each pole at frequency, i ien by idband If we further pecify that we want thi noralized a to hae a anitude of 77-3dB at a corner frequency, j idband j j ol for the frequency of the double pole ter of the pecified corner frequency, we et Equation 4 55 Note that the actual frequency of the double pole i below the pecified de corner frequency Thi i to be expected, ce the pole are

6 teract if each had been located at, there would hae been a 6dB drop tead of the 3dB deired pproach 3: The firt two approache achieed the deired corner frequency by controll pole placeent In contrat, thi ethod chooe equal capacitor alue, a technique that will allow for terchan of coponent a retrict ourele to a two-pole yte with pole frequencie and, we et the expreion for the noralized a anitude equal to 77 at the pecified corner frequency, : idband j j j ol the aboe expreion for, we et 4 If and are both le than, the lat ter the aboe expreion ay be nelected and we can ole for a Equation 7, Modified Once we et to thi pot, it depend on the actual copoition of the tie contant aociated with and The tratey i to et the capacitor of each pole equal Know the reitie coponent the equialent reitance and the deired corner frequency, the capacitor alue i the only unknown Equation 7 the odified erion aboe and ay be calculated