P. Bruschi: Project guidelines PSM Project guidelines.

Transcription

1 Projec guidelines. 1. Rules for he execuion of he projecs Projecs are opional. Their aim is o improve he sudens knowledge of he basic full-cusom design flow. The final score of he exam is no affeced by he choice of carrying ou a projec. Projecs can be carried ou by groups of up o 5 sudens. Projec can be assigned o single suden. The complexiy of he projec increases wih he number of sudens of he group. Projecs can be assigned in any period of he year. Verificaion of he projecs consiss of wo phases: (1) he suden are received by he eacher once hey have finished he schemaic design flow and performed he required simulaion. If he resul is saisfacory, he eacher auhorizes hem o complee he design wih he layou drawing. () Once he layou is compleed and he verificaion seps (DRC, LVS) are passed, he sudens send a shor repor o he eacher by (pdf forma). The repor should include: -) brief descripion of he ask and of he crieria used for he design (wo pages max). -) snapsho of he schemaic view, aken from LTSpice schemaic edior; -) ll he required simulaion resuls (plos) in graphical forma; -) snapsho of he layou -) copy of he lvs repor; ll figures in he repor should be provided of a small capion, indicaing he conen of he figure.. Sudens design ask..1 Inroducion Projecs generally have he following characerisics ha grealy simplify he sudens design work: 1) The opology is assigned ) The bias curren is assigned. 3) The lenghs of all mosfes are fixed o he same value (ypically 1 m). 4) few design choices (e.g. mirror raios) are given. Once he given design choices have been implemened, he circui opology is such ha he drain currens of all mosfes can be derived from he bias curren by very simple calculaions ha do no involve process parameers. The sudens have o deermine he aspec raios (/L raios) of all mosfes (i.e. he, since L is given), in order o guaranee accepable dc operaion (operaing poin and inpu/oupu ranges). In mos cases his reduces o seing he overdrive volage (V-V) o a value, which is: 1) Large enough o bias he mosfe in srong inversion region. 1

2 ) Small enough o have a negligible impac on he inpu and oupu ranges For hese reasons he sudens are asked o se he V V of all mosfes o a value close o 00 mv. Deviaions from his rule are accepable, provided ha he overdrive volage says wihin he [100 mv- 300 mv] inerval. This applies o all mosfes whose V-V is a free parameer ha can be se as small as possible, o increase he inpu and/or oupu ranges and allow operaion a small power supply volages. This rule does no apply o paricular devices whose V-V should be made larger han he oher ones, in order o obain a correc operaing poin. These paricular mosfees, if presen, are indicaed o he sudens when he projec is assigned. Jus for an example, consider he wide swing cascode curren mirror where he V-V of a mosfe in he inpu branch is made larger (ypically doubled) han he oher overdrive volages.. mehod o se all V-V o he arge (00 mv) value in a simple way. Firs of all, le us consider ha he V-V of a mosfe in srong inversion (and sauraion) region can be approximaed by: V V I D (1) where C OX () L Using hese expressions, we can deermine he /L of all mosfes in he circui, since, as saed in he inroducion: (i) he curren in all mosfe is known if he bias curren is given; (ii) he bias curren is assigned. In pracice, i is more convenien o find he /L of a chosen mosfe and hen proceed o all oher mosfes by simple proporionaliy rules. Le us suppose ha he chosen mosfe is n-ype and call i M. Then consider anoher mosfe M and call K he raio: ID/ID. Then: L L L K K μ μ if n p M if M is n - ype is p - ype Generally, if possible, i is convenien o sar from an n-ype mosfe and close i in diode configuraion, biasing i wih he quiescen curren ha i is expeced o receive in he projec. To do ha, simply use he es circui shown in Fig. 1.

3 Fig. 1. Tes circui used o design he chosen reference mosfe M. The curren source value is se o he drain curren ha mus flow hrough M in he quiescen (res) condiion, s a saring poin, calculae he (/L) raio using equaions (1) and (): I D 1 L V V COX (3) where V-V is he arge overdrive volage (00 mv for he projec purpose) and Cox is given in he process manual (DRM of PSM05 for he projecs). The /L esimae founds in his way has been marked by an aserisk, since i has generally o be adjused. Indeed, he aspec raio calculaed in his way does no produce he desired V-V, due o he poor accuracy of he mosfe square laws. To check his, se M in he circui of Fig.1 wih (/L) and run an operaing poin simulaion (.op). Call (V-V) he value found in his way. This will be generally slighly differen from our arge value (00 mv). Refine he /L raio by using equaions (1) and (): L L V V V V (4) fer his refinemen, he obained /L generally produces a V-V which is sufficienly close o he arge value. For he aim of he projec, we will consider ha he arge is obained if he acual V-V obained by simulaing he circui of Fig.1 differs from he arge by less han ± 0 mv..3 Effecs of changing he main circui parameers Mos of he described circui design choices are indicaed o he sudens wih no explanaion. mong hese choices here are: (i) he bias curren; (ii) he mosfe lengh; (iii) equal overdrive volage for all devices. In he nex paragraphs, hese choices are briefly reviewed in order o le he sudens know which circui performances and rade-offs are relaed o he parameers involved. ias curren. Increasing he bias curren produces he following advanages: (a) The circui ges faser. This is due o he fac ha mos circui singulariies (poles and zeroes) are proporional o gmi/cpi, where gmi is he ransconducance of i-h ransisor, while Cpi is he 3

4 parasiic capaciance conneced o is drain. If V-V is fixed by design, he gm can be convenienly expressed as: gmi=idi/vtei where IDi and VTEi are he drain curren and equivalen hermal volage of ransisor Mi. Increasing he bias curren, increases he quiescen curren of all mosfe, shifing heir singulariies o higher frequencies. This is an approximae and raher qualiaive approach, bu is useful o undersand he relaionship beween quiescen curren and circui speed. (b) If we are dealing wih an amplifier, he inpu referred hermal noise densiy gess smaller. The drawback of increasing he curren is mainly increased power consumpion. Since his is an imporan parameer, a rade-off has o be made beween he speed and inpu noise performance and power consumpion. Channel lengh (L) Increasing he channel lengh produces he following advanages: (a) Parameer decreases, improving he oupu resisances and hen volage gain of amplifiers. Curren mirrors behave more ideally, since he oupu curren depends less on he oupu volage due o he increased oupu resisance. (b) The gae area increases (if L is increased, also should be increased of he same facor, o mainain he /L raio consan). This improves device maching (reducing amplifier inpu offses) and reduces he flicker noise componens. The drawbacks of increasing he device lengh are clearly: (a) Increased area occupaion (circui layous ges bigger). (b) Reduced speed: increased gae areas resul in larger parasiic capaciances. Overdrive volages e have chosen o se all overdrive volages o he same value (00 mv in our case), excep for hose ha are consrained o assume a paricular value or mee a paricular relaionship. This choice is no opimal, since he overdrive volages of differen mosfes play differen roles in deermining he circui behavior in erms of noise, offse and frequency response. For example, in mos differenial amplifier opologies, an opimal noise reducion is obained when he overdrive volage of he inpu differenial pair(s) is minimized, while he overdrive volage of mosfe in curren mirrors is maximized. Such an analysis is beyond he aim of he PSM course, hen we have chosen o equalize he V-V in order o simplify device sizing. Furhermore, he choice of adoping a small overdrive volage increases he inpu and oupu ranges and faciliaes operaion wih small power supply volages..4 mehod for providing independen common mode and differenial mode inpu o a differenial amplifier. The following circui can be used o explicily se he common mode and differenial mode volages a he inpu of an amplifier: 4

5 Fig.. mehod o se he differenial mode and common mode volage by means of independen and dependen (conrolled) generaors. On he righ, he LTSpice implemenaion is shown, where V 1 and V are he non-invering and invering volages, respecively. The independen volage sources indicaed wih Vd and Vc ses he differenial mode and common mode volage, respecively. The differenial volage produced by Vd, presen a node Vd, is added o Vc by he dependen sources E1 and E. The gain of he laer is se o 0.5 and -0.5, respecively, so ha: Vd Vd Vin Vc ; Vin Vc (5) as required. 5