Whits, EE 3 Lctur 0 Pag of 8 Lctur 0: Emittr Followr and Diffrntial Amplifirs Th nxt two amplifir circuits w will discuss ar ry important to lctrical nginring in gnral, and to th NorCal 40A spcifically. Howr, nithr of ths amplifirs appars in discrt form in th NorCal 40A. Instad, you will find ths amplifirs prforming thir important functions insid ICs. Emittr Followr (aka Common Collctor) Amplifir A typical mittr followr amplifir is shown in Fig. 9.: V CC - V o + R s Q Output + V bb V V i R - Thr ar two big diffrncs btwn this amplifir and th common mittr amplifir:. thr is no collctor rsistor,. th output oltag is takn at th mittr. 006 Kith W. Whits
Whits, EE 3 Lctur 0 Pag of 8 Thr ar four important charactristics of th mittr followr amplifir (prsntd hr without driation):. oltag gain,. currnt gain >, 3. high input impdanc, 4. low output impdanc ( Ω). Consquntly, th mittr followr is usful as. a buffr amplifir,. an almost idal oltag sourc. In th NorCal 40A, mittr followrs can b found intrnally in th:. Audio Amplifir U3 (LM 386). S th quialnt schmatic on p. 399.. Oscillator circuits of th Product Dtctor U and th Transmit Mixr U4. Both ar SA60 ICs. S th quialnt circuit shown in Fig. 4 on p. 49 of th txt. Diffrntial Amplifir This is probably a nw circuit for you. Th diffrntial amplifir is an intrsting circuit in that it amplifis only a diffrnc in th two input oltags.
Whits, EE 3 Lctur 0 Pag 3 of 8 Actually, you usd diffrntial amplifirs for yars now, though you probably didn t know it. A diffrntial amplifir appars as th input circuit for an oprational amplifir. It is this circuit that gis ris to th familiar o = A( + ) rlationship for th op amp (whr A is th opn-loop gain). Th diffrntial amplifir also appars in th Audio Amplifir and th SA60 mixr ICs in th NorCal 40A. In th lattr cas, th diff amps appar in th form of Gilbrt Clls (s p. 7). W will spnd som tim hr on th opration of th diffrntial amplifir, considring its importanc to th mixing procss. A typical diffrntial amplifir is shown in Fig. 9.3: V cc R c i c + o - i c R c i Q Q i R R i i i t "tail" R t
Whits, EE 3 Lctur 0 Pag 4 of 8 It s important that th circuit ha matchd transistors and rsistors for satisfactory prformanc (mor spcifically, to nsur symmtry in th circuit). This diff amp is a modratly complicatd circuit to analyz. A rlatily simpl mthod of analysis, howr, is to considr two spcial cass of input signals:. i = i, calld th diffrntial (or odd ) input, =, calld th common-mod (or n ) input.. i i Aftr dtrmining th rspons of th diff amp to ach of ths two xcitations, arbitrary combinations of inputs can b analyzd as wightd combinations of ths two. I. Diffrntial Input, i = i: For ths input oltags, i = i it = i + i = 0 (9.53),(9.54) With ach amplifir ffctily groundd at R t, thn w can us th common-mittr amplifir gain G = R (9.3) to gi = i and = i R R (9.55),(9.56) Th output oltag for this spcific input combination is dfind as th diffrntial output oltag d as d o i i R R ()
Whits, EE 3 Lctur 0 Pag 5 of 8 which is writtn d R c = id (9.57) R whr id i i is th diffrntial input oltag. Thrfor, th diffrntial gain G d is d G = d = R (9.59) id Not that this is th sam gain for just on half of th diffrntial amplifir. II. Common-Mod Input, i = i: For ths input oltags, i = i it = i + i (9.6),(9.63) Applying KVL through th transistor bass to R t and thn to ground, th input oltags can b xprssd as i = Ri + Ri t t = ( R + Rt) i (9.64) i = Ri + Ri t t = ( R + Rt) i (9.65) Th last qualitis us th rlationships it = i and it = i, rspctily. Nxt, using KVL from V cc to (ac signals only) gis = Ri cc Ri c = i R + R Q (9.64) acti Similarly, it can b shown that = R c i R + Rt t (9.66) (9.67) Notic that with this common-mod input, both and ar qual. Consquntly, th output oltag is
Whits, EE 3 Lctur 0 Pag 6 of 8 o = = 0 This last rsult clarly shows that th diffrntial amplifir dos not amplify signals that ar common to both inputs. Cool! Sinc ths oltags and ar th sam, w dfin ithr of thm as th common-mod oltag c c = = so that + = c. () Using (9.66) or (9.67), c = ic R + R (9.68) whr ic = i = i. Hnc, th common-mod gain G c is G c c = (9.69) R + R ic t t Diffrntial Amplifirs in th SA60 Mixrs As mntiond priously, th diffrntial amplifir plays a critical rol in th SA60 mixr. Spcifically, th diff amp appars as th two input trminals and (s p. 49). Howr, in th NorCal 40A, only on diff amp input is connctd to th signal (SA60 pin ). Th othr input (pin ) is
Whits, EE 3 Lctur 0 Pag 7 of 8 connctd to ground (through a dc block capacitor). This input configuration is not on of th two considrd arlir. W can account for this typ of input, howr, simply as a wightd sum of diffrntial and common-mod inputs. That is, in ordr to account for th inputs i = i and i = 0, us () and () to yild:. id = i i = i 0 = i (9.70) i+ i i + 0 i. ic = = = (9.7) Lt s chck that wightd sums of ths two inputs (9.70) and (9.7) ar indd quialnt to th dsird inputs i = i and =. i 0 First, calculat (9.70)+ (9.7) (i.., th sum id + ic ) giing i+ i i i i + = i + or, i = i (input to Q is indd i ). Nxt, calculat (9.7)-(9.70) (i.., th sum ic id ) giing i+ i i ( i i) = i or, i = 0 (input to Q is indd 0).
Whits, EE 3 Lctur 0 Pag 8 of 8 Summary of Common and Diffrntial Inputs Th chck w just prformd illustrats th usfulnss of th common and diffrntial input analysis. W bgan with Thn w askd: What id and ic (diffrntial and commonmod inputs) yild th sam and as for th non-symmtric inputs shown abo? Th answrs, as w just saw, ar i id = i and ic =. Expanding ths two rsults, w find from (9.59) that d = (9.7) = G d id = G d i and c = = (9.73) i = G c ic= Gc