vacuum tubes amplifier

7 AMPLIFIERS VOLTAGE A M PL IF IE R S 7~1 Cascading The transist or and va cuum-t ube circuits discus se d in previous cha pte rs are ide ally sui ted to amplify voltage sig nal s wit h ~inim~m waveform di st or ti on. Ga in factors gr eate r than those poss ibleWI th a si ngle-s tage ampli fi er ar e obtained by cascading sever al ampli fier st ag es . Th e output of one amplifier stage isamplif ie d byano ther st age or stagesuntil the desi red si gnal voltage level is achi eved. Consider , for example, the two-stage cascaded tr iode amplif ier, Fi g. 7 - 1. Two i nd iv id ual c ir cu it s s imi la r t o t ho se d isc us se d i n r-- --- --- --, --- -< ). V CC3 - : 12AX7 r O-__ -+ _L ~_fO_k_~f_L_f_~ _L _L_f_~_~_f__<) FIGURE 7-1 Two- st age ampl if ier using triodes con nec ted in cas cad e. The pr in ci pa l a pp li ca ti on s o f t ra ns is to rs and vacuum tubes ar e b as ed l1li thei r ab il ity to amplify el ec tric si gn als. So me circuits amplify minute vo lt ag e si gn als byfactors of ma ny mi Ui on while others i nc re as e t he e l ec tr ic powe r of a signal in order to operate a me chanical devi ce such as an electr ic mo tor. StiU ot her circuits ampl ify curren ts In ea ch of these applic at ions the fr eq uency range of th e input signal is imp or ta nt . D if fe re nt ci r cu it s ha ve been de vew ped for de amplific at io n an d for us e at high radi o frequencies. Chap. 5 ar e connected wit h the coupling capacitor Ce2• This capa ci tor pass es the ampl ifi ed ac signal fr om VI to the grid of V2• At the sa me time itblocks the positiveplate voltage of VI from the gr id of the second triode. Similarly,capac it ors G el and G e3 isolate the input and output circ ui ts insofar asdc potentials ar e concerned. The en tire ac equi va lent circuit of this amplif ie r may be dr awn us ing the principles discussed inCha p. 5. The performance of the syste mis det ermined from a comple te ac- cir cui t ana lys is. Actually, th is p ro ce du r e isunwieldy be ca us e of the number of loops in the n et wo rk a nd i s r ar el y a tt emp te d. Rat he r, t h e c ir c ui t is analyzed in se ve ral sepa ra te steps, each of which has a mi ni mum of ma the- ma ti cal co mp lex it y. This has the further ad va nt ag e t ha t t he i m- port ant ef fect s ca n be isolated and more cl earl y examined. For example, the re actances of both cathode bypass capaci tors a re a ss ume d sma ll e no ugh to be negligible . Acco r di ng ly, th ese components ar e absent in the ac equivalent ci rcui t of the ampl ifi er, Fig. 7- 2. Th e r ea ct an ce s o f th e coupling capacitors are al so i g- Most often, the sig nal1evel is increased in several suc ces siv e amplifi er sta ge s to at ta in th e d es ir ed o ut pu t si gn a l m ag ni tu de . In this case , t he inte raction of amplifier st ageJ must be considered an d fa ir ly complicated networks ar e involved. Fortun t el y, th e te ch niq~f of ci rc uit an al ys is develo e d in pr ev ious chapte rs, in pa rt ic ul ar the ac equivalent circuits for tu be s an d transistors, ar e sujficient for a sati sf actory understa nd ing of comple te ampl if ie r ci rc ui u.

vacuum tubes amplifier

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