Crossover Distortion Analysis. Crossover Distortion Analysis. Three Stage Amplifer Crossover Distortion Hole

Transcription

1 Three tae Aplifer Crossoer istortion Hole Crossoer istortion FET pec sheets Confiurations Applications Acknowledeents: Neaen, onald: Microelectronics Circuit Analysis and esin, 3 rd Edition prin 2018 Lecture 6 1 Feedback prin 2018 Lecture 5 2 Crossoer istortion Analysis Crossoer istortion Analysis 11 e in out out e 1 10 out in + 10 in 1 10 out + n 1 + n in n The distortion 0.6 in each direction or 1.2 total resultin in a hole that is: *1.2 ~ 0.02 Increasin open loop ain will reduce the crossoer distortion prin 2018 Lecture prin 2018 Lecture 5 4

2 BJT - FET BJT - JFET - MOFET Bipolar Junction Transistor Three terinal deice Collector current controlled by base current ib f(vbe) Think as current aplifier NPN and PNP Field Effect Transistor Three terinal deice Channel conduction controlled by electric field No forward biased junction i.e. no current JFETs, MOFETs epletion ode, enhanceent ode BJT JFET MOFET Circa G/I (sinal ain) Best Better Good Isolation PN Junction Metal Oxide* E Low Moderate Very sensitie Control Current Voltae Voltae Power YE No Yes *silicon dioxide prin 2018 Lecture prin 2018 Lecture 6 6 Voltae Noise * FET Faily Tree FET JFET depletion MOFET n-channel p-channel depletion enhanceent Vs(off) ate source cutoff or n-channel n-channel p-channel Vp pinch-off oltae Vs(th) ate source threshold oltae * Horowitz & Hill, Art of Electronics 3 rd Edition p prin 2018 Lecture prin 2018 Lecture 6 8

3 - Transistor Polarity Mappin* n-channel depletion n-channel JFET p-channel enhancent pnp bjt + - n-channel enhancent npn bjt p-channel JFT + MOFET MOFET & JFET Much ore finicky difficult process (to ake) than JFET s. Good news: Extreely hih input ipedance. Zero input current. Bad news: Easily blown up by E on the ate. Add protection circuit and input bias current becoes at best coparable to JFET s. Good news: Essentially infinitely fast. If you chane the ate oltae, the deice will respond instantaneously! Essentially always in static equilibriu. Bad news: It can be really hard to chane the ate oltae quickly! (especially power deices BIG BIG capacitor) Much better power deices than JFET s. (There were briefly power JFET s as output deices in audio aps. Too any blew up.) And you can t ake diital VLI out of JFET s. * Horwitz & Hill, the Art of Electronics, 3 rd Edition prin 2018 Lecture prin 2018 Lecture 6 10 MOFET s JFET MOFET ybols JFET s Very siple anufacturin process like BJT s. Much cheaper than (discrete) MOFET s. Quieter than MOFET s. Low input bias current like back biased diode. As low as 10pA. But note this doubles eery 6 de C! At hih teps a JFET op ap can hae ore input current than soe bipolar op aps! Used in icrophones, hearin aids and other hih ipedance sources (electret icrophones hae ery hih output ipedance) because of low noise and ruedness copared to MOFET s. Fast. Used on any hih speed scope probes. Was ajor adance in bias current and speed oer bipolar-input op aps. ee data sheets of (JFET input) LF356 series and copare to then bipolars. ownside is input capacitance can t be as low as soe BJT s. Wide spread in threshold oltae and zero-vs current. oeties requires sortin and selectin for a ien circuit. N-channel P-channel Bulk body terinal 2N7000 IF prin 2018 Lecture prin 2018 Lecture 6 12

4 JFET: Junction FET ybol iple Model of MOFET G G N channel JFET G G P channel JFET G + V s - MOFET ade VLI (icroprocessors and eories) possible. Very hih input resistance Voltae controlled deice ~25 V ax operatin ~0 ate current G off state V s < V t G on state V s V t 2N7000 on 7.5 IF9110 on prin prin MOFET: Gain & non-linearity source Heaily doped (n-type or p-type) diffusions Very thin (<20Å) hih-quality io 2 insulatin layer isolates ate fro channel reion. Channel reion: electric field fro chares on ate locally inerts type of substrate to create a conductin channel between source and drain. L bulk W ate Polysilicon wire Inter-layer io 2 insulation drain I W/L oped (p-type or n-type) silicon substrate MOFETs (etal-oxide-seiconductor field-effect transistors) are four-terinal oltae-controlled switches. Current flows between the diffusion terinals if the oltae on the ate terinal is lare enouh to create a conductin channel, otherwise the osfet is off and the diffusion terinals are not connected Chris Teran FETs as switches The four terinals of a Field Effect Transistor (ate, source, drain and bulk) connect to conductors that enerate a coplicated set of electric fields in the channel reion which depend on the relatie oltaes of each terinal. ate source drain INVEION: A sufficiently stron ertical field will attract enouh electrons to the surface to create a conductin n-type channel between the source and drain. The ate oltae when the channel first fors is called the threshold oltae -- the osfet switch oes fro off to on. E h N+ N+ p bulk E inersion happens here CONUCTION: If a channel exists, a horizontal field will cause a drift current fro the drain to the source Chris Teran

5 Four states of MOFET for different Vs and Vds What s the difference between the drain and the source? MOFET s can be syetrical and drain and source interchaneable. Especially inside IC s. But discrete deices (with few exceptions) hae input protection networks on the ate to protect aainst E. Also, the substrate ust connect soewhere. Once the input protection clapin and the substrate are connected to a terinal, that ust be the source. Oliier eleae and Peter cott (CC BY-A 3.0) prin 2018 Lecture prin 2018 Lecture 6 18 Classic ideal MOFET characteristics Flat cures in saturation reion assue lon channel ideal MOFET cures continued Triode ode, or linear ode, or ohic reion. aturation or actie ode. K n transconductance paraeter As the channel lenth becoes short, these equations becoe inaccurate. At the channel ends, source and drain reions causin frinin effects and istort the electric fields fro the ideal case used to derie aboe eq s. MOFET BJT For analo desin, lon-channel MOFET s can offer extreely hih output Ipedance, akin excellent stiff current sources. Miniu eoetry transistors used in diital VLI do not hae such flat cures prin 2018 Lecture prin 2018 Lecture 6 20

6 Channel Lenth Modulation: Early Voltae JFET p-channel prin 2018 Lecture prin 2018 Lecture N7000 n-channel Need ate-source cutoff oltae prin 2018 Lecture prin 2018 Lecture 6 24

7 2N7000 2N7000 C tie constant for V s? Wide process spread Vs(th) : C iss C G C oss C C irs C G Estiatin MOFET Paraeters fro the ata heet prin 2018 Lecture prin 2018 Lecture 6 26 MOFET Confiurations Basic FET Circuits Coon source Coon drain Analo switch - oltae controlled iital loic icroprocessor, VLI, AIC Power switchin preferred oer BJT Variable resistors use linear reion of drain cure Current sources General replaceent for bjt (in soe cases) Coon ate prin 2018 Lecture prin 2018 Lecture 6 28

8 iple NMO all-inal Equialent Circuit Coon-ource Confiuration C analysis: Couplin capacitor is assued to be open. o o r ( i G 2K ( V i i ) n n 1 r [ λk ( V d s GQ GQ V V TN ) 2 ) ] 2 1 TN K I n [ λi Q ] 1 Q AC analysis: Couplin capacitor is assued to be a short. C oltae supply is set to zero olts prin 2018 Lecture 6 29 all-inal Equialent Circuit Coon ource Neaen Ch 4.3 More eneralized coon source with source deeneration and equations: A V o V i ( r o )( i i + i ) Usae: oltae aplifier, transconductance aplifier prin 2018 Lecture 6 32

9 Coon rain ource Follower Neaen Ch 4.4 NMO ource-follower or Coon rain Aplifier Usae: oltae buffer prin 2018 Lecture 6 33 all-inal Equialent Circuit for ource Follower Coon Gate Neaen Ch 4.5 A ( o i 1 + ro i + i r ) Usae: Hih frequency aplifier prin 2018 Lecture 6 36

10 Coparison of 3 Basic Aplifiers Confiuration Voltae Gain Current Gain Input esistance Output esistance Coon ource A > 1 ource Follower A 1 Moderate to * TH hih * TH Low Cascode Confiurations All hae the sae purpose to decouple the input terinal (of the botto deice) fro capacitie feedback fro the output by takin the output fro a second deice. Botto deice: Current ain (no appreciable oltae ain) Top deice: Voltae ain (no current ain) Cobines coon-eitter/source/cathode with coon-base/ate/rid. esult is like a sinle coon-eitter/source/cathode deice with drastically reduced Miller capacitance fro the output to the input Coon Gate A > 1 A i 1 Low Moderate to hih * eterined by biasin resistors prin 2018 Lecture 6 37 BJT JFET MOFET Vacuu tube triode prin 2018 Lecture 6 38 inle deices with cascode like construction JFET Aplifier Confiurations Tetrode (tet for 4 terinal) acuu tube adds a fourth rid called a screen to shield the rid and cathode fro the anode iilar MOFET deice incorporates a second ate. Useful for F circuits. Coon ource Aplifier Coon rain Aplifier [ource Follower] Coon Gate Aplifier * For polarized [electrolytic] input couplin capacitor, the "+" should be oriented towards the ost positie C oltae. For exaple, if there is -2V on the ate, and -8V associated with Vin, then the capacitor orientation should be reersed as shown. The input couplin cap for the coon ate confiuration will ost often be a polarized electrolytic, since the ipedance at the ource of the JFET is only 1/ in parallel with prin 2018 Lecture prin 2018 Lecture 6 40

11 Coon ource JFET (bypassed source resistor) Coon rain Aplifier (ource Follower) A out in s L A 1+ s L + s or s s [ 1+ ] L A L A out in s + s s s s ; [ 1+ ] 1+ A prin 2018 Lecture prin 2018 Lecture 6 42 Coon Gate Aplifier Output esistance ource Follower i + V s _ I test s d V s + V test eoe and replace it with a test AC oltae enerator hort the input sinal V i and replace it with its source resistance i. ole for I test, which is a consequence of applyin the test enerator V test, and for V test in ters of the hybrid-π paraeters. _ To correctly calculate the alue of a bypass capacitor for s, use the parallel cobination of r o and. A out in s s i + L i L i + ; i if i 0, V V test s r o I V test s 1 then A L prin 2018 Lecture prin 2018 Lecture 6 44

12 Low Frequency Hybrid π Model OK, now what can we do with these thins? sinal in: -10 to +10 MOFET analo switch prin 2018 Lecture prin 2018 Lecture 6 46 OK, now what can we do with these thins? JFET follower This scheatic fro the now obsolete Intersil 7662 datasheet shows how a flyin capacitor enerates a neatie oltae fro a positie oltae. lihtly different connections can double a oltae instead of inertin it. A JFET follower usin atched (dual) JFET s. The botto JFET autoatically enerates just the riht aount of current to bias the top one so Vin is approxiately equal to Vout. Horowitz Hill, 3 rd Edition p prin 2018 Lecture prin 2018 Lecture 6 48

13 JFET ariable attenuator n-channel JFET Current ource 2N5459 The olby B noise reduction circuit used this circuit as a Variable attenuator. By addin ½ the drain oltae back to the ate oltae linearizes the JFET resistance. V ) 2 1 2κ ( V G V th r 6k Control oltae (neatie) P4392 Fro An introduction to electronics, Cabride Uni Press prin 2018 Lecture prin 2018 Lecture 6 50 P-Channel JFET Current ource Neat Circuit Ideas Fro Make a classic phase shift oscillator (3 staes of 60 de phase shift each any three diital loic inerters will usually do) so you can WATCH the oscillation run around the loop! Works with any odd nuber of staes. Question : Is this uaranteed to start up? Why? And what if you had a lare (odd) nuber of staes can you start a skinny pulse oin around the loop? Will it stay skinny or widen and turn into 50-50% duty cycle? 2N prin 2018 Lecture prin 2018 Lecture 6 52

14 More fro sae web site note sinle-ended drie iplies this otor has coutator brushes. I had wronly assued these drills used brushless otors. Iportant basic power confiuration The H-bride Note how the hih-side MOFET s are drien by leel shift. Four drie sinals required. Note the trade-off in switchin speed ersus static power dissipation in leel shifter. The 10k resistor will not turn off the IF9Z30 ery fast. But otor dries don t operate at ery hih frequencies prin 2018 Lecture prin 2018 Lecture 6 54 Continuin fro this web site This is a reat suary of MOFET failure odes AKA (Also Known As) What NOT to do with a MOFET. WHY MOFETs FAIL There are quite a few possible causes for deice failures, here are a few of the ost iportant reasons: Oer-oltae: MOFETs hae ery little tolerance to oer-oltae. aae to deices ay result een if the oltae ratin is exceeded for as little as a few nanoseconds. MOFET deices should be rated conseratiely for the anticipated oltae leels and careful attention should be paid to suppressin any oltae spikes or rinin. Proloned current oerload: Hih aerae current causes considerable theral dissipation in MOFET deices een thouh the on-resistance is relatiely low. If the current is ery hih and heatsinkin is poor, the deice can be destroyed by excessie teperature rise. MOFET deices can be paralleled directly to share hih load currents. Transient current oerload:massie current oerload, een for short duration, can cause proressie daae to the deice with little noticeable teperature rise prior to failure prin 2018 Lecture 6 55 MOFET failure odes continued hoot-throuh - cross conduction: If the control sinals to two opposin MOFETs oerlap, a situation can occur where both MOFETs are switched on toether. This effectiely short-circuits the supply and is known as a shootthrouh condition. If this occurs, the supply decouplin capacitor is dischared rapidly throuh both deices eery tie a switchin transition occurs. This results in ery short but incredibly intense current pulses throuh both switchin deices. Allow a dead tie between switchin transitions, durin which neither MOFET is turned on. This allows tie for one deice to turn off before the opposite deice is turned on. No free-wheel current path: When switchin current throuh any inductie load (such as a Tesla Coil) a back EMF is produced when the current is turned off. It is essential to proide a path for this current to free-wheel in the tie when the switchin deice is not conductin the load current. This current is usually directed throuh a free-wheel diode connected antiparallel with the switchin deice. When a MOFET is eployed as the switchin deice, the desiner ets the free-wheel diode "for free" in the for of the MOFETs intrinsic body diode. This soles one proble, but creates a whole new one prin 2018 Lecture 6 56

15 MOFET failure odes continued MOFET failure odes continued Excessie ate drie: If the MOFET ate is drien with too hih a oltae, then the ate oxide insulation can be punctured renderin the deice useless. Gate-source oltaes in excess of +/- 15 olts are likely to cause daae to the ate insulation and lead to failure. Care should be taken to ensure that the ate drie sinal is free fro any narrow oltae spikes that could exceed the axiu allowable ate oltae. *** WAIT A MINUTE! This author fails to point out that practically all discrete MOFET s hae a oltae clap on the input. The actual failure echanis is usually you elt the clapin zener, and the puddle of olten silicon fors a short. The MOFET ay be fine, but the ate is now shorted to the source, which akes it kind of hard to use. Insufficient ate drie - incoplete turn on: MOFET deices are only capable of switchin lare aounts of power because they are desined to dissipate inial power when they are turned on. It is the responsibility of the desiner to ensure that the MOFET deice is turned hard on to iniise dissipation durin conduction. If the deice is not fully turned on then the deice will hae a hih resistance durin conduction and will dissipate considerable power as heat. A ate oltae of between 10 and 15 olts ensures full turn-on with ost MOFET deices. ***NOTE: The reference to ate oltaes of between 10 and 15 olts applies to older or hiher oltae power deices (like 20 to 200V). The newer power parts hae lon been based on the latest diital process: i.e., they re desined for 5V. Newer power MOFET s hae uaranteed on resistance at lower Vs oltaes consistent with use in 3.3V loic inputs, and hae Vds absolute axiu ratins of 6V or 7V, and siilar abs ax Vs ratins. Modern loic requires lots of power conersion deices operatin at these low oltaes prin 2018 Lecture prin 2018 Lecture 6 58 For further readin and possible inspiration for your projects, read Ji Willias app notes! You otta loe a uy who titles an app note (#25) : The aboe title is not happenstance and was arried at after considerable deliberation Mysterious odes, sudden, seeinly inexplicable failures, peculiar reulation characteristics and just plain explosions are coon occurrences. iodes conduct the wron way. Thins et hot that shouldn t. Capacitors act like resistors, fuses don t blow and transistors do. The output is at round, and the round terinal shows olts of noise. Added to this poisonous brew is the reulator s feedback loop, sapled in nature and replete with uncertain phase shifts. Eerythin, of course, aries with line and load conditions and the tie of day, or so it sees. In the face of such enace, what are Eeryan and the poets to do? prin 2018 Lecture prin 2018 Lecture 6 60