CURRENT FEEDBACK AMPLIFIERs

Transcription

1 Abstract-The need r hgh speed, wdeband amplers s the drng rce behnd the deelpment the Current Feedback Ampler (CFA). The CFA has sgncant adantages er cnentnal amplers n terms slew rate perrmance and nherently wde bandwdth. Ths paper prdes a reew the thery and desgn current eedback amplers. CUENT FEEDBACK AMPLIFIEs by Altan Haznec I. INTODUCTION Analg desgn has hstrcally been dmnated by ltage mde sgnal prcessng []. Ths s apparent n the electrncs ndustry where the Vltage Feedback Ampler (VFA) has becme ubqutus. The lesser-knwn Current Feedback Ampler (CFA) has a undamentally derent archtecture and ers sgncant perrmance adantages er the tradtnal VFA. Althugh, the underlyng crcut cncepts hae been arund r many years, mdern cmplementary bplar prcesses were requred r practcal CFAs t be realzed. The CFA was largely made ppular n the md 980 s by Cmlnear Crpratn. The early desgns were hybrd amplers lke the CLC05, whch was representate the typcal perrmance attanable. The CLC05 ered a 3dB bandwdth n excess 00MHz at lw gans (80MHz at 50 V V ) and 400V µ s slew rate wth ± 5V supply rals. The rst mnlthc dece, the EL00, was ntrduced n 987 by Elantec Inc. The EL00 acheed a 50MHZ 3dB bandwdth at a gan V V (30MHz at 0 V V ) and 500 V µ s slew rate [,, 3]. II. CUENT FEEDBACK AMPLIFIE FUNDAMENTALS The peratn the CFA s best understd by cnsderng the dealzed mdel shwn n Fgure (a). A unty gan buer s cnnected between the tw nput termnals such that n s rced t track n+. The nertng/nn-nertng nput termnal s

2 actually the utput/nput the unty gan buer, whch deally has zer utput mpedance and nnte nput mpedance. As a result, the nertng nput mpedance s zer whereas the nn-nertng nput mpedance s nnte. The utput s a lnear, current-cntrlled ltage surce wth zer utput mpedance, ( s) n z ( ) where z(s) s the transmpedance parameter (n hms) and n s the current lwng ut the nertng nput termnal. Vn+ x Vut z(s) n n Vn- (a) Vn+ Vn- x n z(s) n Vut src (b) Fgure : (a) Ideal CFA mdel; (b) Nn-nertng cnguratn

3 The basc nn-nertng cnguratn a CFA s shwn n Fgure (b). Analyss ths crcut reeals a unque relatnshp between the eedback resstr and the clsed lp bandwdth. The utput ltage s sensed and cnerted nt current, whch s ed back t the nertng nput. Feedback acts t mnmze the nertng nput current. The unty gan buer rces the ltage at the nertng termnal t n+. Cmbnng the eedback current ( ) and surce current ( src ) at the nertng nput termnal ges n src n 0 n ( ) The erall clsed lp ltage gan s then, A VCL + + z ( s) ( 3 ) As expected, the amlar clsed lp ltage gan + s btaned as z ( s) appraches nnty. Assumng dmnant ple cmpensatn, the transmpedance can be apprxmated by, where z ( s ) z + j ( 4 ) ( ω ω ) ω s the 3dB requency and z s the DC resstance. Substtutng Eq. (4) nt Eq. (3) and rearrangng ges, A VCL + z + z + ( ) ω + jω + j z + ω ωa ( 5 )

4 where the apprxmatn r A VCL s ald r z >>. The clsed lp ampler 3dB requency ( ω ) s gen by, a ω a ( z + ) ( 6 ) ω z ω Thus, r a rst rder crcut nly the eedback resstr determnes the clsed lp bandwdth. The clsed lp gan can be set usng resstr. The relatnshp between the gan and requency characterstcs s shwn n Fgure (a) [4, 5]. Gan(dB) A3 A 0dB A Wa lg(wa) (a) Gan(dB) A3 Wa3 A Wa 0dB A Wa lg(wa) (b) Fgure : (a) Frequency respnse 0 ; (b) Frequency espnse 0,, The precedng analyss suggests that the clsed lp gan des nt aect the requency respnse the crcut nce the eedback resstr has been set. Hweer, n practce, the requency respnse des ary wth the clsed lp gan due t the nte

5 utput mpedance ( ) the nput unty gan buer. The mdel the CFA ampler ncrpratng s shwn n Fgure 3. The expressn r the nertng nput current can be dered rm the llwng equatns, n ( 7 ) src n n+ n ( 8 ) n n + n ( 9 ) n n+ n ( 0 ) n n+ n+ ( ) ( + + ( )) ( ) + ( + )( + + ( )) ( 3 ) + The 3dB requency s gen by, ω a zω z ω + z ω ( + ) + AVCL ( 4 ) Cnsequently, the eect s t decrease the 3dB requency r ncreasng clsed lp gan. Ths decrease n bandwdth s nt as drastc as that bsered n the VFA.

6 Vn+ x n z(s) x Vut n src Fgure 3: CFA small sgnal mdel A mdern sngle stage CFA s shwn n Fgure 4. The majrty cmmercal CFAs are a aratn ths crcut. The nput unty gan buer cnssts transstrs Q-Q4. Q3 and Q4 are cngured as a cmplementary par, push pull stage wth lw utput mpedance. The bas r ths stage s generated by passng a cnstant current thrugh Q, such that Q3,4 are just n the edge the acte peratng regn. Q5 and Q6 sense the cllectr currents C3 and C 4, whch are mrrred t the hgh mpedance nde (Z) by Q7 and Q8 respectely. Thus, the eecte current lwng nt nde Z ( C 7 C8 ) s a replca n. The utput stage cnsstng Q9-Q buers the ltage at nde Z t the utput, thereby prdng a lw utput mpedance. Dmnant ple cmpensatn s realzed by placng a capactr between the hgh mpedance nde and grund. The mpedance at nde Z s determned by usng the equalent small sgnal mdel and applyng a test ltage at that nde. The mpedance s then dened as the rat X / X and s gen by, z x ( s) r 7 r 8 rb c x ( sc ) z ( sc ) ( 5 ) where r 7,8 represents the nte utput mpedance seen lkng nt the cllectr Q7,8 and r b s the utput stage nput mpedance. Under wrst-case cndtns (.e. wth the c

7 utput cnnected t grund) the mpedance seen lkng nt the bases Q9 and Q0 can be determned by usng the resstance relectn rule, [ r + ( β + ) r ] [ r + ( β ) r ] r b + ( 6 ) π 9 9 π π0 0 π III. STABILITY ANALYSIS As the eedback resstance (r mpedance) s reduced, the ampler bandwdth ncreases. Hweer, there s a crrespndng lss phase margn due t the hgher rder ples z(s), whch were gnred n Eq. (5). I the negate the lp gan equals, r alternately the phase margn s zer and the ampler gan s greater than, the CFA wll becme unstable. Een the phase respnse als t reach the crsser pnt, the CFA may stll exhbt unacceptable peakng and rngng. Therere, n rder t guarantee stablty a mnmum eedback resstance ( ) s requred [6]. where A mre accurate representatn z(s) was presented n [7], z ( s ) z ( 7 ) ( + sτ )( + sτ ) τ s the dmnant ple requency, ( ) mn z C, and c τ mdels the hgher requency ples due t the current mrrrs. Fr a maxmally lat requency respnse the eedback resstr shuld be chsen such that τ C. The 3dB requency s then c gen by, ω τ a ( 8 ) Fr an arbtrary phase margn φ M, the alue the eedback resstr can be determned rm,

8 C c τ tan φ M + ( 9) and Eq. (3). A cmmn mscnceptn s that purely capacte eedback wll cause the CFA t scllate. The reasnng s that the equalent mpedance the eedback capactr at hgher requences s less than mn. Neertheless, a relately stable crcut wth capacte eedback can be realzed the llwng cndtn s satsed [7], C C c tanφ >> M + tan φm Ib Q5 Q7 Ib Vn+ Q Q3 Vn- Z Q9 Q Vut Ib Q Q3 Q6 Q4 n Q8 Cc Ib Q0 Q src Fgure 4: A typcal mdern sngle stage CFA

9 IV. SLEW ATE Cnsder a pste gng ltage appled t the nn-nertng nput termnal r the crcut n Fgure 4. The base ltage Q, ncreases by the same amunt whereas the nertng nput ltage ncreases by an amunt almst equal n magntude. Snce the nertng nput ltage s >0V, Q3 wll surce an errr current nt the external eedback resstr netwrk, whch s mrrred t the hgh mpedance nde. Thus, the ltage at the utput s equal t the errr current multpled by the equalent nde mpedance. The slew rate depends n the ablty Q7,8 t charge r dscharge the cmpensatn capactr. Ntce, that the nput stage bas des nt lmt the aalable chargng current. The errr current that Q7/Q8 surces/snks s dynamc. Therere, t the rst rder, the slew rate s nnte. Ultmately, hweer, the slew rate s lmted by the parastc transstr capactances and the nte base currents n the nput and utput buer stages. Current bst crcutry can be used t reduce the eect the parastc capactances and prde an rder magntude ncrease n the slew rate. Fgure 5(b) shws an mpred slew rate buer stage. Fr a pste nput ltage step, as Q begns t slew due t Cjsp and Cjcn, Q surces mre current t charge Cjsn and Cjsp. Ths extra current s recrculated thrugh Q7, whch ncrease the bas current r Q. Smlarly, r a negate nput ltage step, Q snks mre current t charge Cjsp and Cjsn, whch s re-crculated thrugh Q8 and ncreases the bas current r Q [8].

10 T Current Mrrr T Current Mrrr Q7 Q5 Q Cjsp Cjcn Q3 Q7 Q5 Q Q9 Cjsp Cjcn Q3 Vn Q Vut lad Vn Q Q0 Vut lad Q4 Q4 Q8 Q6 Cjsn Cjcp Q8 Q6 Cjsn Cjcp T Current Mrrr T Current Mrrr (a) (b) Fgure 5: (a) Smple CFA nput stage; (b) CFA nput stage wth current bst crcutry V. DC OFFSET EO The asymmetrcal crcut archtecture the CFA makes t ery dcult at best t mnmze the nput set ltage. The DC set errr rm the nput t the utput can be expressed as, V ut Vn 0V Vs AVCL + I ( 0 ) B where V s s the nput unty gan buer set ltage and I B s the nertng nput bas current. The errr due t gan ndependent errr. Generally, the V s scales wth the clsed lp DC gan ( A VCL ) but I B adds a I B errr dmnates r lw A VCL whereas the V s errr dmnates r hgh A VCL. A typcal ur-transstr CFA nput stage buer crcut s shwn n Fgure 5(a). Summng the ltage drps rm the base Q t the emtter Q3 ges, V s n+ n VBE3 VBE ( ) Neglectng the nte base currents Eq. () can be re-wrtten as,

11 V s V T I ln I 3 I I Sn Sp V T I ln I Sp Sn ( ) where I I 3 I b under quescent cndtns. As a result, the nput set ltage depends n hw clsely the base emtter ltages the pnp transstrs match thse the npn transstrs. An alternate ur-transstr unty gan buer crcut wth mpred set ltage s shwn n Fgure 6. Ths crcut uses dde-cnnected transstrs the crrect type t bas the cmplementary par utput stage such that lke transstrs are matched wth lke transstrs. One majr drawback ths crcut hweer, s the relately lw nput mpedance [9]. Q7 T Current Mrrr Q5 Q Q3 Vn Q Q4 Vut Q8 Q6 T Current Mrrr Fgure 6: Lw set ltage nput buer A number actrs are respnsble r the nertng nput bas current (reer t Fgure 4); () msmatch between I B and I B, () current mrrr errrs, (3) derences between α3 and α 4. Hgher gan current mrrrs such as the Wlsn mrrr can be used t sgncantly reduce I B. Hweer, these current mrrrs exhbt lcal resnance, whch decreases the erall ampler bandwdth. VI. CMOS CFAs The majrty mdern cmmercal CFAs are based n bplar transstr crcuts. Hweer, a nel CMOS realzatn a CFA was recently reprted n [0]. One the

12 majr dsadantages MOS transstrs cmpared t BJTs, s the reduced transcnductance. The crcut n Fgure 7(a) can be ewed as a cmpund MOS transstr wth hgh transcnductance. G M s the cmpund gate and S M s the cmpund surce. The rst rder transcnductance rm G M t S M gnrng the utput cnductance M, s gen by, g m gmg g αg m m m gm where α s the current transer rat the current mrrr. g m M8 M7 M6 M M5 M Vn Vut a: M M M3 M4 (a) (b) Fgure 7: (a) Cmpund transstr; (b) CMOS ltage llwer Fgure 7(b) shws the CMOS mplementatn a ltage llwer crcut presented n [0]. M-M4/M5-M8 rms a p-type/n-type cmpund transstr. Gen that the gate M3/M4 s ted t the surce M4/M3 the gate-surce ltage M3 and M4 are rced t be equal. Assumng all lke transstrs are the same sze, the gate

13 surce ltage M and M are almst equal snce I d4 equals I d and I d3 equals I d. Therere, the utput ltage s rced t be equal t the nput ltage. Ths ltage llwer was subsequently used n the desgn the CMOS CFA shwn n Fgure 8 [0, ]. Ths crcut s ery smlar n prncple t the ampler n Fgure 4. The nput and utput stage buers are based n the archtecture Fgure 7(b). M9-M and M3-M6 are regulated cascde current mrrrs. A cmpensatn capactr s placed between the hgh mpedance nde (Z) and grund r stablty. The crcut was abrcated usng 0.6um dgtal CMOS technlgy. M8 M4 M7 M5 M6 M3 M6 M3 M M5 Vb M4 M Vn+ M Vn- Cc Vb M0 M7 Vut M M9 M8 M3 M M M9 M4 M0 Fgure 8: CMOS current eedback ampler VII. CONCLUSION The CFA s nt a replacement r the VFA nr s t an engneerng cursty. Untl a specc applcatn s cnsdered, the CFA s nt nherently better r wrse than the VFA. The mst sgncant adantage the CFA er cnentnal amplers s the

14 hgh slew rate perrmance and wde bandwdth. It s nterestng t nte that sme VFAs tuted as hgh speed amplers are essentally a CFA wth a ltage gan blck cnnected between the nertng nput termnal and the utput the nput stage buer []. EFEENCES [] C. Tumazu, F.J. Ldgey, & D.G. Hagh, Analgue IC desgn: the current-mde apprach [] Elantec Inc, EL00C 50MHz Current Feedback Ampler, Datasheet, 996 Tarb Curt, Mlptas, CA [3] Farchld Semcnductr Crpratn, KH05 Oerdre-Prtected Wdeband Op Amp, Datasheet [4] Arne Buck, Current-Feedback Myths Debunked, Applcatn Nte OA-0, July 99, Natnal Semcnductr Crpratn [5] Edwn W. Greenech, Analg Integrated Crcuts, Chapman & Hall, 997 [6] Serg Franc, Analytcal Fundatns Current-Feedback Amplers [7] Jrayuth Mahattanakul & Chrs Tumazu, A Theretcal Study the Stablty Hgh Frequency Current Feedback Op-Amp Integratrs, Trans. Crcut Syst. I, l. 43, pp-, 996 [8] Duglas Lee Smth, Hgh Speed Operatnal Amler Archtectures, IEEE 993 Bplar Crcuts and Technlgy Meetng 9. [9] Steen O. Smth, The Gd, the Bad and the Ugly: Current Feedback-Techncal Cntrbutns and Lmtatns, 993 IEEE [0] K. Manetaks & C. Tumazu, Current-eedback pamp sutable r CMOS VLSI technlgy,, Electrncs Letters, l. 3, n., 6 th June 996

15 [] K. Manetaks, C. Tumazu & C. Papaasslu, A Hgh Frequency CMOS Current Feedback Opamp, 997 [] Natnal Semcnductr Crpratn, LM77 Very Hgh Speed, Hgh Output Current, Vltage Feedback Ampler, Datasheet, 999