The Junction Diode Forward Bias Equation

Transcription

1 8//005 he Juncton ode Forward Bas Equaton.doc /6 he Juncton ode Forward Bas Equaton In forward bas, we hae learned that the dode current can be related to the dode oltage usng the followng approxmaton: I e I e, proded that 5 mv. We can nert ths approxmaton to alternately express n terms of dode current : Ie e I I ln I Jm tles he Un. of Kansas ept. of EEC

2 8//005 he Juncton ode Forward Bas Equaton.doc /6 Now, say a oltage across some juncton dode results n a current. Lkewse, dfferent oltage across ths same dode a dode of course results n a dfferent current. We can defne the dfference between these two oltages as, and then usng the aboe equaton can express ths oltage dfference as: I I I I Ykes! Look at what ths equaton says: * he dfference n the two oltages s dependent on the rato of the two currents. * hs oltage dfference s ndependent of scale current I s. We can lkewse nert the aboe equaton and express the rato of the two currents n terms of the dfference of the two oltages: Jm tles he Un. of Kansas ept. of EEC

3 8//005 he Juncton ode Forward Bas Equaton.doc 3/6 ln ( ) V ln n ( ) exp Agan, we fnd that ths expresson s ndependent of scale current I s. Q: top wastng my tme wth these pontless deratons! Are these expressons een remotely useful!?! A: hese expressons are often ery useful! Frequently, nstead of explctly prodng dece parameters n and I s, a juncton dode s specfed by statng n, and then a statement of the specfc dode current resultng from a specfc dode oltage. For example, a juncton dode mght be specfed as: A juncton dode wth n pulls ma of current at a oltage 0.6 V. Jm tles he Un. of Kansas ept. of EEC

4 8//005 he Juncton ode Forward Bas Equaton.doc 4/6 he aboe statement completely specfes the performance of ths partcular juncton dode we can now determne the current flowng through ths dode for any other alue of dode oltage. Lkewse, we can fnd the oltage across the dode for any other dode current alue. For example, say we wsh to fnd the current through the juncton dode specfed aboe when a potental dfference of 0.7 V s placed across t. We hae two optons for fndng ths current: Opton : We know that n and that ma when 0.6 V. hus, we can use ths nformaton to sole for scale current I s : I e I e I e I 755x0. ma Now, we use the forward-based juncton dode equaton to determne the current through ths dece at the new oltage of 0.7 V: I e ( 755x0 ) 09. ma e Jm tles he Un. of Kansas ept. of EEC

5 8//005 he Juncton ode Forward Bas Equaton.doc 5/6 Opton Here, we drectly determne the current at 0.7 usng one of the expressons dered earler n ths handout! Usng ma, 0.6, and 0.7 V we can fnd current as: ( ) exp ( ) exp ( ). exp ma Opton (usng the equatons we dered n ths handout) s obously qucker and easer (note n opton we dd not hae to deal wth annoyng numbers lke 7.55 x 0 -!). Fnally, we should also note that juncton dodes are often specfed smply as a ma dode or a 0 ma dode or a 00 ma dode. hese statement mplctly prode the dode current at the standard dode test oltage of 0.7 V. Q: But what about the alue of juncton dode dealty factor n? Jm tles he Un. of Kansas ept. of EEC

6 8//005 he Juncton ode Forward Bas Equaton.doc 6/6 A: If no alue of n s proded (and there s not suffcent nformaton gen to determne t), we typcally just assume that n. For example, consder the followng problem: etermne the oltage across a 00 ma juncton dode when there s ma of current flowng through t. A 00 ma juncton dode smply means a juncton dode that wll hae a current of 00 ma flowng through t ( 00 ma) f the oltage across t s 0.7 V. We wll assume that n, snce no other nformaton about that parameter was gen. hus, usng 0.7, 00mA, and ma, we can determne the alue of : 07. ( 005. ) V EXCELEN! Jm tles he Un. of Kansas ept. of EEC