Lab 9: CMOS inverter propagation delay.

Transcription

1 Sae Universiy of New York a Sony Brook ESE 314 Elecronics Laboraory B Deparmen of Elecrical and ompuer Engineering Fall 1 Leon Sherengas Lab 9: MOS inverer propagaion delay. 1. OBJETIES Deermine inverer inpu and oupu capaciances. Deermine propagaion delay of unloaded and loaded inverer. Observe an effec of power supply volage. Observe an effec of swiching aciviy on dissipaed power.. TRODUTION The dynamic of performance of a logic-circui family is characerized by he propagaion delay of is basic inverer. The inverer propagaion delay ( P ) is defined as he average of he low-o-high ( PLH ) and he high-olow ( PHL ) propagaion delays: PLH PHL P. (1) Propagaion delays PLH and PHL are defined as he imes required for oupu volage o reach he middle beween he low and high logic levels, i.e. 5 % of DD in our case of MOS logic. Figure 1a illusraes he definiion of he propagaion delays. (a) (b) Figure 1. (a) Definiion of propagaion delays of logic inverer; (b) ircui illusraing capaciances deermining he inverer propagaion delay. The propagaion delay of he MOS inverer is deermined by he ime i akes o charge and discharge he capaciances presen in he logic circui. Figure 1b shows he circui for analysis of he propagaion delay of he inverer under condiion ha i is driving an idenical inverer. To make he analysis racable i is convenien o replace capaciances aached o he oupu node of primary inverer (Q1-Q in Figure 1b) wih equivalen capaciances beween oupu node and ground. This is considerable simplificaion bu individual consideraion of every capacior including nonlinear capaciances in he MOS ransisor model makes a manual analysis virually impossible. Hence, for our purposes we adop his simplified model ha is adequae for qualiaive 1

2 Sae Universiy of New York a Sony Brook ESE 314 Elecronics Laboraory B Deparmen of Elecrical and ompuer Engineering Fall 1 Leon Sherengas analysis and allows making esimaion of MOS inverer propagaion delay. Figure shows he inverer driving inverer circui where all capaciors are lumped ogeher o form hree equivalen capaciors conneced beween oupu and ground of he primary inverer. in ou ou w in The oal load capaciance of he primary inverer is: Figure. L, () Where: gd1 gd db1 db W, (3), (4) g3 g4 and W wiring or any oher exra load capaciance. The physical meaning of each of he ransisor capaciances was discussed during lecure ime. The propagaion delay of he MOS inverer ha is loaded only wih is own oupu capaciance is called PLH PHL inrinsic delay: P. (5) In he simplified analysis he NMOS and PMOS ransisors can be replaced by equivalen resisances R EQN and R EQP when charging and discharging he resuling ino (anoher significan simplificaion): PLH.69 R, (6) EQP PHL.69 R. (7) EQN Thus i is assumed ha PMOS acs as a R EQP when is charged o DD when oupu is swiched from lowo-high and NMOS acs as a R EQN when is discharged when oupu is swiched from high-o-low. Propagaion delay of he loaded inverer can be expressed hrough inrinsic delay: W P P 1. (8) The values of he equivalen resisances and even L can depend on DD hus he propagaion delay is affeced by power supply volage (being smaller for higher DD ). I should be noed, however, ha his dependence of he propagaion delay on power supply volage is less pronounced in MOSFETs wih shor channels where drain curren sauraion is achieved due o velociy sauraion raher han due o channel pinch-off a drain.

3 Sae Universiy of New York a Sony Brook ESE 314 Elecronics Laboraory B Deparmen of Elecrical and ompuer Engineering Fall 1 Leon Sherengas Measuremen of he and can be performed using he following approaches: 1. Measuremen of. Once he propagaion delay of an unloaded inverer is known (inrinsic propagaion delay) one can deermine he from equaion (8) by measuring propagaion delay of he loaded inverer (Figure 3). The load capaciance should be dominan, i.e. W >>, and mus be known wih high precision. in 5 6 ou ou w Figure 3.. Measuremen of. Inpu capaciance of he inverer can be deermined by driving i wih square wave inpu from low oupu impedance funcion generaor conneced in series wih known resisance (Figure 4). learly, he Rs is supposed o be measured precisely. Inpu capaciance of an inverer creaes a series R circui wih he source resisance. When s swiches from low-o-high, in will exponenially rise from zero o DD : s in DD 1 exp (9) Rs Rs in 3 4 in Power dissipaion Figure 4. Dynamic power dissipaion of he digial logic circui depends on power supply volage DD, on circui capaciances (for MOS inverer sudied - L ) and on swiching aciviy f. Saic and direc pah power dissipaions are ofen smaller han Pdyn and can be negleced in he basic analysis. Pdyn is given by: DD Pdyn f (1) L In his lab we will observe he dependence of he average dissipaed power in he unloaded and loaded inverer as a funcion of he swiching frequency. 3

4 Sae Universiy of New York a Sony Brook ESE 314 Elecronics Laboraory B Deparmen of Elecrical and ompuer Engineering Fall 1 Leon Sherengas 3. PRELIMARY LAB 3.1. Wha frequency of he inpu square wave signal (5 % duy cycle) would you selec o measure (by an oscilloscope) 1 ns low-o-high and high-o-low propagaion delays of he inverer. Seleced frequency should allow for simulaneous observaion of low-o-high and high-o-low ransiion on oscilloscope screen. 3.. For circui in Figure 4. Assume ha in = 1 pf and Rs = 1 kω. Wha frequency of he square wave inpu would be adequae o measure he R ime consan of he circui based on exponenial charging or discharging of in? 3.3. Assume inverer wih inrinsic propagaion delay of 1 ns. Assume in = 1 pf, ou = 1 pf (Figure ). For W =, 1 and 1 pf find a propagaion delay of he inverer driving inverer. Deermine he maximum swiching frequency of he circui for each L Find he dynamic power dissipaion for he inverers from 3.3 operaed a heir maximum swiching frequencies. Assume DD = 5. Wha is average value of he D curren flowing from power supply o ground in corresponding MOS inverer circuis? 3.5. ompare he dynamic power dissipaion for he inverers from 3.3 operaed a he same frequency - maximum swiching frequency of he slowes inverer. Assume DD = 5. Wha is average value of he D curren flowing from power supply o ground in corresponding MOS inverer circuis? 4

5 Sae Universiy of New York a Sony Brook ESE 314 Elecronics Laboraory B Deparmen of Elecrical and ompuer Engineering Fall 1 Leon Sherengas 4. EXPERIMENT. We will use D47 MOS array o consruc inverers for his lab. MOSFETs in D47 are no mached; hence, expec asymmeric inverer volage ransfer characerisics and differen equivalen resisances for NMOS and PMOS (see daashee) onsruc wo separae MOS inverers using single D47 chip. Use DD = 5. Deermine an inrinsic propagaion delay for each inverer. Namely: (1) apply -o-5 square wave o inpu; () visualize boh inpu and oupu waveforms on oscilloscope screen; (3) selec frequency of inpu signal o see clearly low-o-high and high-o-low ransiion a he oupu (1 o 5 MHz); (4) record inpu and oupu waveforms; (5) measure PLH and PHL and calculae P according o (5) for each inverer. Esimae he maximum swiching frequency for he unloaded inverers. ommen on maching beween NMOS and PMOS. 4.. onsruc circui from Figure 3 using one of he inverers. Measure propagaion delay of he loaded inverer for L = 1 pf and L = 1 nf. Be sure o measure acual values of capaciances. Beware ha inpu frequency will need o be reduced subsanially for his measuremens. alculae assuming ha L dominaes he propagaion delay. Esimae he maximum swiching frequency of he loaded inverer onsruc circui from Figure 4 using anoher inverer. Use Rs 1 kω. Be sure o measure acual values of resisance. Selec inpu frequency so he ransiion ime of in for charging and discharging can be observed on oscilloscope screen. Deermine using equaion (9) onsruc circui from Figure using inverer from 4. as a primary and he one from 4.3 as a secondary. Do no connec W. Measure propagaion delay. ommen on relaive roles of and in deermining a propagaion delay of he inverer driving inverer circui Repea 4.1 for one of he inverers using DD =. Be sure o change inpu square wave o -o-. Explain he change in inrinsic propagaion delay hange DD back o 5 and inpu square wave o -o-5. Measure average (D) curren flowing from power supply ( DD ) o ground when varying he inpu frequency from 1 khz o 1 MHz. Deermine he power dissipaed by inverer a each frequency. Repea for inverer loaded wih L = 1 pf. ompare he measured power dissipaion wih he predicion of equaion (1). 4. REPORT The repor should include he lab goals, shor descripion of he work, he experimenal and simulaed daa presened in plos, he daa analysis and comparison followed by conclusions. Please follow he seps in he experimenal par and clearly presen all he resuls of measuremens. Be creaive; ry o find somehing ineresing o commen on. 5