4.1 The Ideal Diode. Reading Assignment: pp Before we get started with ideal diodes, let s first recall linear device behavior!

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1 1/25/2012 secton3_1the_ideal_ode 1/2 4.1 The Ideal ode Readng Assgnment: pp Before we get started wth deal dodes, let s frst recall lnear dece behaor! HO: LINEAR EVICE BEHAVIOR Now, the deal dode s our frst electronc crcut dece. It s asymmetrc, and non-lnear. Q: Is t also made from sem-conductors? A: Actually, deal dodes are entrely made from our magnatons! HO: THE IEAL IOE Q: I see the math, and I see the plots, but I m stll not sure what an deal dode actually does. A: Perhaps an analogy would help! HO: IOE MECHANICAL ANALOGY Jm Stles The Unersty of Kansas ept. of EECS

2 1/25/2012 secton3_1the_ideal_ode 2/2 Q: So we can use these thngs n crcuts? A: Sure! But we wll fnd analyzng the crcut s problematc. HO: IEAL IOE CIRCUITS Q: So just how do we analyze deal dode crcuts? A: Just carefully and precsely and patently follow the steps delneated n Ideal ode Crcut Analyss Gude HO: THE IEAL IOE CIRCUIT ANALYSIS GUIE Q: Can you ge us a few examples to help llustrate ths procedure? A: EXAMPLE: A SIMPLE IEAL IOE CIRCUIT EXAMPLE: ANALYSIS OF A COMPLEX IEAL IOE CIRCUIT Jm Stles The Unersty of Kansas ept. of EECS

3 1/25/2012 Lnear ece Behaor 1/2 Lnear ece Behaor 1) Recall the crcut behaor of a resstor: R R 2) If R = 0, then we hae a short crcut: Jm Stles The Un. of Kansas ept. of EECS

4 1/25/2012 Lnear ece Behaor 2/2 3) If R=, then we hae a open crcut: = 0 4) We can also plot the behaor of both current and oltage sources: (ma) (ma) 2V + - 1mA Jm Stles The Un. of Kansas ept. of EECS

5 1/25/2012 The Ideal ode present 1/15 The Ideal ode odes are perhaps the most fundamental of all non-lnear crcut elements. * Lke a resstor, nductor, or capactor, a dode s a two-termnal dece. * Thus, ts behaor s characterzed by the relatonshp between the current through the dece, and the oltage across t. * Unlke lnear two-termnal deces, the relatonshp between dode oltage and dode current s a lttle complcated! To begn, we consder the deal dode. As the name mples, ths dece s deally how a dode would behae f we knew how to actually buld an deal dode. Jm Stles The Un. of Kansas ept. of EECS

6 1/25/2012 The Ideal ode present 2/15 Uncorns and leprechauns are more lkely Q: You mean we re gong to study a dece that doesn t really exst? What s the pont n that? A: Lke uncorns or leprechauns or Kansas Cty Chefs playoff ctores, deal dodes are mythcal they only exst n our mnd. Howeer, studyng these magnary deces s mportant and useful, as we wll acqure sklls necessary to understand and analyze juncton dodes. Juncton dodes we can buld they do exst and n many ways they behae smlarly to deal dodes. Jm Stles The Un. of Kansas ept. of EECS

7 1/25/2012 The Ideal ode present 3/15 It s green lke a leprechaun So frst, the deal dode crcut symbol: Note that t s asymmetrc t has an A sde and a B sde! One termnal s called the anode, and the other s called the cathode. anode cathode Resole to commt ths to memory! Jm Stles The Un. of Kansas ept. of EECS

8 1/25/2012 The Ideal ode present 4/15 The standard deal dode notaton Wth respect to the current through and oltage across ths dece, the standard notaton s: + - Thus, we can conclude: 1. Ideal dode current s poste, f flowng from the anode, to the cathode. 2. Ideal dode oltage s poste, f the anode potental s greater than the cathode potental. Resole to commt ths to memory as well! Jm Stles The Un. of Kansas ept. of EECS

9 1/25/2012 The Ideal ode present 5/15 It s complcated n a complcated way Q: So, what s the relatonshp between deal dode oltage and deal dode current? A: It s a lttle complcated. Q: I see, snce an deal dode s a non-lnear dece, the deal dode oltage and deal dode current s probably related by some crazy non-lnear functon, lke: Rght? ( ) 62.3( ) 2 3 = π - + e A: It s not complcated lke that; t s complcated n a dfferent sort of way. Q:???? A: For an deal dode, we fnd that f current s flowng from the anode to the cathode (.e., ³ 0 ), then the oltage across the deal dode s zero(.e., = 0) regardless of how much poste current s flowng: ³ Jm Stles The Un. of Kansas ept. of EECS

10 1/25/2012 The Ideal ode present 6/15 Q: Hey! That s not complcated at all! See what I mean? The oltage across t s zero, regardless of the current flowng through t that descrbes a short crcut: Rght? A: Not exactly. For the oltage across an deal dode to be zero, the current through the deal dode must be poste. Current must be flowng from the anode to the cathode. Q: What happens f the deal dode current s negate? What happens f t flows from cathode to anode? A: Ideal dode current can t be negate t cannot flow from cathode to anode! Jm Stles The Un. of Kansas ept. of EECS

11 1/25/2012 The Ideal ode present 7/15 Q: Sure t can! The adjecte s engmatc Put a negate oltage across the deal dode make the cathode potental hgher than the anode potental and I ll bet you get a bunch of current flowng from cathode to anode. Rght? A: Wrong! =? It turns out, f we place a negate oltage any negate oltage across an deal dode (.e., 0), then no current wll flow through the dode (.e., = 0): = Q: What the heck? Ths now sounds lke an open crcut the current through t s zero, regardless of the oltage across t. A: Yes, but an deal dode behaes as an open crcut only when the oltage across t s negate. Jm Stles The Un. of Kansas ept. of EECS

12 1/25/2012 The Ideal ode present 8/15 It s a wonderful dece So, an deal dode sort of behaes lke a short-crcut, and t also sort of behaes lke an open-crcut. Q: Look, I don't know how you know these thngs, but f you know the dece equaton for ths goofy component, you'll tell me. Tell me Clarence; please, tell me! A: You're not gong to lke t. There are two equatons and they re both condtonal: = 0 f 0 = 0 f ³ 0 Both equatons are necessary to fully descrbe an deal dode the two equatons are not nconsstent, nor are they n conflct. Jm Stles The Un. of Kansas ept. of EECS

13 1/25/2012 The Ideal ode present 9/15 Plots are always helpful Perhaps the best way to see ths s to plot the expressons. The frst condtonal equaton looks lke one-half of the open-crcut cure = 0 = 0 f Jm Stles The Un. of Kansas ept. of EECS

14 1/25/2012 The Ideal ode present 10/15 Half-a-short The second condtonal equaton looks lke one-half of the short-crcut cure : = 0 f ³ 0 ³ Jm Stles The Un. of Kansas ept. of EECS

15 1/25/2012 The Ideal ode present 11/15 At least t s contnuous Together, we see the plot for the deal dode to be a contnuous one: The alues of deal dode current and deal dode oltage must be represented by a pont on ths green cure. Thus, the deal dode oltage can neer be poste, and the deal dode current can neer be negate! Jm Stles The Un. of Kansas ept. of EECS

16 1/25/2012 The Ideal ode present 12/15 Because I sad so Q: Why can t they be? Why can t I place a poste oltage across an deal dode? Why can t I put a negate current through an deal dode? A: For the same reason you can t place a poste oltage across a short crcut, or a negate current through an open crcut! Q: So how do we deal wth these two condtonal dece equatons? A: As we sad, the alues of deal dode current and deal dode oltage must be represented by a pont on the green cure. Thus, that pont wll ether be on: 1. the ertcal lne, where the deal dode oltage s zero ( = 0), and the deal dode current s poste ( ³ 0 ), or 2. the horzontal lne, where the deal dode current s zero ( = 0), and the deal dode oltage s negate ( 0). Jm Stles The Un. of Kansas ept. of EECS

17 1/25/2012 The Ideal ode present 13/15 They frst tred to ge us forwardbas43 We ge each of these two possbltes a name: Forward Bas Mode - when the deal dode oltage s zero ( = 0), and the deal dode current s poste ( ³ 0 ), or Reerse Bas Mode - when the deal dode current s zero ( = 0), and the deal dode oltage s negate ( 0). Forward Bas Mode Reerse Bas Mode Jm Stles The Un. of Kansas ept. of EECS

18 1/25/2012 The Ideal ode present 14/15 Commt ths to memory: Trust me: memorze ths stuff * for the forward bas mode, the deal dode acts lke a condtonal short crcut the oltage across t s zero, proded that the current through t s poste. * for the reerse bas mode, the deal dode acts lke a condtonal open crcut the current through t s zero, proded that the oltage across t s negate. Forward Bas Mode Reerse Bas Mode Jm Stles The Un. of Kansas ept. of EECS

19 1/25/2012 The Ideal ode present 15/15 Why can t you all be more lke ths student? Behold the deal dode the crcut dece wth a splt personalty! Is t a short? Or s t an open? nald 0 reerse based forward based no bas nald nald 0 nald nald nald Jm Stles The Un. of Kansas ept. of EECS