懂得并运用电压基准解析

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20、T F bsg(MW H twmacsugmtad tte gtt , GcMg5WANJ2-1OX7LJWCApplicati on Note 82 The last steps in elude a pbase diffusi on and an emitter diffusion, Iocated at the center of the Zener (see Figure A2. The emitter becomes the cathode, whereas the comb ined isolati on and base diffusi on serve as the ano d

21、e. Breakdown occurs at the bottom of the of the cathode, where the emitter and isolation + base dopa nt concen trati ons are richest. Lighter dop ing concen trati ons result in a ACTIVE ZENER AREA ISOLATION ANODE higher breakdow n voltage at the iso-buried layer, base-epi and iso-epi junctions, and

22、at the outer fringes of the emitter diffusion, ensuring that these areas are not active whe n the buried jun cti on is biased into breakdow n. The result is an extremely stable subsurface breakdow n mecha nism that has n ear-theoretical no ise and is un affected by surface con tam in ati on or oxide

23、 effects CATHODE p + ISO EPI p-BASE n + EMITTER p + ISO n + BURIED LAYER p -SUBSTRATE AN82 Fa2 Figure A-2. An Emitter Diffusi on Forms the Cathode. Breakdow n Occurs Un der the Cen ter of the Emitter, Where Both Emitter and Iso + Base Dopa nt Concen trati ons are Highest APPENDIX B ?VBE: INTEGRATED

24、CIRCUIT WORKHORSE It is, perhaps, a cruel fate for IC desig ners that no si ngle IC device or structure is inv aria nt with cha nges in temperature. Various comb in ati ons of devices have bee n devised to stabilize circuits aga inst cha nges in temperature. As expla ined in the text, Zen er-based r

25、efere nces use a Zener and a forward-biased diode conn ected in series to achieve n ear-zero temperature coefficient and a bandgap relies on a ?VBE in series with a forwardbiased diode. An in dispe nsable tech nique in in tegrated circuit desig n, the ?VBE is not widely known in other fields. Before

26、 explaining the theory of ?VBE, let s skip ahead to the two mostimporta nt results: two ide ntical diode (or base emitter junctions running differe nt curre nts produce differe nt voltage drops. The ratio of the curre nts con trols the absolute value of the offset voltage. Further, this offset has a

27、 predictable, positive temperature coefficie nt of approximately 3.4 V/ Qifor each room-temperature millivolt of offset. By combining the positive TC of a ?VBE with the n egative TC of a diode drop, a zero TC ban dgap refere nee is formed. As we shall soon see, it takes a ?VBE offset of 650mV to car

28、ei the-2.18mV/ C TC of a hypothetical diode.* In formatio n fur ni shed by Lin ear Tech no logyCorporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Lin ear Tech no logy Corporati on makes no represe ntati on that the in terc onn ecti on of its circu

29、its as described here in will not infringe on exist ing pate nt rights. AN82-11Applicati on Note 82 Two tran sistors (or diodes produce an offset give n by the following equation: ?VBE = VBE1 VBE2 = (kT/q ln(JE1/JE2 (1 where ?VB E = offset voltage, k = Boltzmann s constant (1.3833?Jo0les/K, T-= abso

30、lute temperature (298K at room, q = charge of an electro n (1.6 ? 10 19 Coulombs, and JE = emitter curre nt den sity. The actual un its of area used to calculate JE1 and JE2 can cel each other, so that only the area ratio is important. Similarly, only the current ratio is important. If we restrict o

31、urselves to using two identical transistors, Equation (1 reduces to ?VBE = VBE1 -VBE2 = (kT/q ln(IC1/IC2 (2 where IC = collector current (see Figure B . The temperature coefficient is given by TC = d?VBE/dT = (k/q ln(IC1/IC2 where k/q = 86.3 V/ C. (3 Calculating the current ratio required to produce

32、 +2.18mV/C (corresp onding to 650mV offset we find that it is unman ageably large, about 94 ? 1010:1. In practice, a much smaller offset is gen erated by a ?VBE cell and the n amplified to 650mV. As an example, see Figure B2. Usi ng a 10:1 curre nt ratio,* we find a room temperature offset from Equati on (2 of 59.2mV, and a temperature coefficie nt of 199 M C. Applying a gain of slightly less than eleven brings us to 650mV and +2.18mV/ C. Adding a PNP emitter follower