I = α I I. Bipolar Junction Transistors (BJTs) 2.15 The Emitter-Coupled Pair. By using KVL: V

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1 Bpolar Juncton ransstors (BJs).5 he Emtter-oupled Par By usng KL: Wth the transstors based n the forward-acte mode, the reerse saturaton current of the collector-base juncton s neglgble. / α F ES e / α F ES e then where ( ) / / e d e d / s called dfferental sgnal. By usng KL: whch leads to ( E + E ) F + α F Solng for yelds Smlarly, we hae + α + e α F + d + e F d / / /α / α F Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3

2 We obsere that ncreasng poste alues of d of magntude greater than 4 cause and to approach α F and zero, respectely. he cures for 0 o - o, and o o are depcted here Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3

3 Bpolar Juncton ransstors (BJs).6 Emtter-oupled Logc (EL) he buldng block of emtter-coupled logc (EL) s the Emtter-oupled Par ( or current swtch) crcut whch conssts of matched components Dependng on how much hgher or lower the nput oltage s compared to EF, the reference current wll swtch to one of the legs creatng a oltage or Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3

4 Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3 Bpolar Juncton ransstors (BJs) he preous fgure showed the deal case for swtchng the currents between the two legs, but n real BJs current wll be present n both legs dependng upon of each BJ n the par Snce and he collector current dfference s gen by: tanh α S S exp, exp tanh exp exp exp exp + + E E α + + or.6 Emtter-oupled Logc (EL)

5 Bpolar Juncton ransstors (BJs).6 Emtter-oupled Logc (EL) urrent Swtch Analyss for > EF Gen the crcut shown under the gen bas condtons ( s 300m larger than EF ), the majorty of current wll flow n the left leg E E 0 α α E E α 0 urrent Swtch Analyss for < EF Gen the crcut shown under the gen bas condtons ( s 300m less than EF ), the majorty of current wll flow n the rght leg E E 0 α α E E 0 α O O EF EF Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3

6 Bpolar Juncton ransstors (BJs).6 Emtter-oupled Logc (EL) he Emtter-oupled Logc (EL) Gate he outputs of the preous current swtch hae the alue of ether 0 or 0.6 he dfference of the nput and output of the current swtch s exactly one base-emtter oltage drop For a complete EL gate, the oltages are shfted by a base-emtter drop as shown n the fgure N N EF B 0 β H F + + L Leel shfters For -0.7 For -.3 O O EF EF Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3

7 Bpolar Juncton ransstors (BJs).6 Emtter-oupled Logc (EL) EL Gate Benefts EL gates produce both true and complemented outputs EL gates are fast snce the BJs are always n forward acte mode, and t only takes a few tenths of a olt to get the output to change states, hence reducng the dynamc power EL gates prode near constant power supply current for all states thereby generatng less nose from the other crcuts connected to the supply Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3

8 Bpolar Juncton ransstors (BJs).6 Emtter-oupled Logc (EL) he EL O-NO Gate A 3-nput EL O-NO Gate A 3-nput EL NO Gate A -nput EL O Gate Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3

9 Bpolar Juncton ransstors (BJs) emperature ompensaton.6 Emtter-oupled Logc (EL) Dode emperature oeffcent Dode oltage under forward bas: D k D S{exp -}, q akng the derate wth respect to temperature yelds d D d k q ln D k q S S d S d D D ln D + k S q ln D + k S q ln D d S S d D GO 3 Assumng D >> S, S n, and GO s the slcon bandgap energy at 0K. For a typcal slcon dode wth D 0.65, E G.e, and 0.05 yelds d D d ( ) 300K Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3 /K -.8m/K -.8m/ K n E k S 3 G 6 B exp cm GO E q G

10 Bpolar Juncton ransstors (BJs).6 Emtter-oupled Logc (EL) emperature ompensaton Snce the of the BJ changes by approxmately.8m/k, t s obous that when s used to replace the current swtch current source, that E wll ary wth temperature he technque shown below can temperature compensate (track) the araton E mA. 7kΩ + D Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3

11 ommon-base haracterstcs.7 Statc, Parastc and Second-Order Effects Slop / r µ he alue of the order of β r µ r o s n Aalanche Breakdown c decrease as BJ becomes forward-based. Slop / r o Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3

12 ommon-emtter haracterstcs Slop > / r o Slop / r o B EO > 50 B EBO 6 ~ 8 Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3

13 he ransstor ß Slop > / r o h FE Q β dc he D ß BQ h fe β ac B E constant he A ß he dfference n alue between dc ß and ac ß s usually small, and we wll not normally dstngush between the two. Howeer, the alue of ß depends on the current leel n the dece. Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3

14 he Hgh-Frequency Hybrd-π Model ypcally, p s n the range of a few pf to a few tens of pf m s n the range of a fracton of a pf to a few of pf r x s a few tens of ohms, and ts alue depends on the current leel n a rather complcated manner. Snce r x << r π, ts effect s neglgble at low frequences, but not n hgh frequences. Electronc rcuts, Dept. of Elec. Eng., he hnese Unersty of Hong Kong, Prof. K.-L. Wu Lesson &3