Supplement Reading on Diode Circuits. edu/~ee40/fa09/handouts/ee40_mos_circuit.pdf

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1 EE40 Lec 18 Diode Circuits Reading: Chap. 10 of Hambley Supplement Reading on Diode Circuits edu/~ee40/fa09/handouts/ee40_mos_circuit.pdf Slide 1

2 Diodes Circuits Load Line Analysis Analysis of Diode Circuits by assumed states Diode Logic Circuits Wave Shaping Circuits Rectifying Circuits Slide 2

3 SOLVING CIRCUITS WITH NONLINEAR ELEMENTS Look at circuits with a nonlinear element like this: Linear circuit I L V L - - I NL V NL Nonlinear element A nonlinear element with its own I-V relationship, attached to a linear circuit with its own I-V relationship. 1. I L = f L (V L ) (linear circuit I-V relationship) 2. I NL = g NL (V NL ) (nonlinear element I-V relationship) 3. I NL = -I L 4. V NL = V L Slide 3

4 SOLVING CIRCUITS WITH NONLINEAR ELEMENTS The 4 equations can be reduced to 2 equations in I NL and V NL I NL = -f L (V NL ) - the linear loadline I NL = g NL (V NL ) which we can equate and solve for V NL, or graph the two equations and solve for the intersection. Slide 4

5 1 kω EXAMPLE I NL = -15 I Given : I 0 10 A. 2 V L - V L V_ NL Find V NL _ 1. I L = (V L - 2) / I NL = ( ) V / e NL 1 3. I NL = -I Substitute 1 and 2 in 3 L ( ) 4. V NL = V V e NL / = [ (VNL 2)/1000] L Solve by iteration, V NL ~ 0.725V Slide 5

6 linear nonlinear Graphical Solution Loadline: I= - (V-2)/ I_NL Diode I-V 0.725V V_NL Slide 6

7 Piecewise linear Model of Nonlinear Devices -5.5V intercept 1.5V intercept Segment A : i = v / 400 Segment B : i = (v 1.5) / 100 Segment C : i = (v 5.5) / 800 Slide 7

8 I D Ideal Diode Model of PN Diode Circuit symbol I-V characteristic Switch model I D (A) V D forward bias reverse bias V D (V) I D V D Diode behaves like a switch: closed in forward bias mode open in reverse bias mode used when voltage of interest >> 0.6V Slide 8

9 Piecewise Linear Model Circuit symbol I-V characteristic Switch model I D I D (A) I D V D forward bias V Don reverse bias V D (V) V Don V D For a Si pn diode, V Don 0.6 V Diode behaves like a voltage source in series with a switch: closed in forward bias mode open in reverse bias mode Slide 9

10 Zener Diode A Zener diode is designed to operate in the breakdown mode. (l k ) t I D (A) reverse (leakage) current forward current breakdown voltage V BD V D (V) v s (t) >15V for all t R t v s (t) V BD = 15V v o (t) integrated circuit it Slide 10

11 Piecewise-linear Model of a Zener Diode Slide 11

12 Diode Circuit Analysis by Assumed Diode States 1) Specify Ideal Diode Model or Piecewise-Linear Diode Model I D (A) I D (A) reverse bias forward bias V D (V) reverse bias forward bias V Don 2) Each diode can be ON or OFF 3) Circuit containing n diodes will have 2 n states 4) The combination of states that works for ALL diodes d (consistent t with KVL and KCL) will be the solution Slide 12

13 Example Analysis by assumed Diode States 1.75mA D1=on D2=on 0.5mA D1=off D2=off D1=off D2=on 10 3 D1=on D2=off 6 3 Slide 13

14 Transfer Function of Diode Circuits Piecewise-Linear Model with 0.6V voltage drop Slide 14

15 Diode Logic: AND Gate AND gate Piecewise-Linear Model with 0.6V voltage drop V cc Inputs A and B vary between 0 Volts ( low ) and V cc ( high ) R AND V OUT Between what voltage levels does C vary with V CC =5V A C 5 B EOC Slope =1 Shift 0.7V Up Output t voltage C is high h only if both A and B are high V IN Slide 15

16 Diode Logic: OR Gate OR gate A B Piecewise-Linear Model with 0.6V voltage drop C C VOUT R OR 5 Inputs A and B vary between 0 Volts ( low ) and V cc ( high ) h ) Between what voltage levels does C vary with V CC =5V? EOC Output voltage C is high if either (or both) A and B are high Slope =1 Shift 0.7V Down V 5 V IN Slide 16

17 Diode Logic: Incompatibility and Decay Signal Decays with each stage (Not regenerative) AND gate output voltage is high only if both A and B are high OR gate output voltage is high if either (or both) A and B are high V cc A R AND B A C AND R OR C OR B 0.6V drop Slide 17

18 Clipper Circuits Assume forward diode has 0 voltage drop Slide 18

19 Peak Detector Circuit Assume the ideal (perfect rectifier) model. V i (t) V i i( (t) C V C (t) V i t Idea: The capacitor charges due to one way current behavior of the diode. V C (t) V C Slide 19

20 Peak Detector with Load Resister Slide 20

21 Level Shift Circuit -V C V IN V C V IN C V OUT t - - V OUT 1 3 V OUT =V C V IN 2 t 1) Diode =open, V C =0, V OUT = V IN 2) Diode =short, V C = -V IN, V OUT =0, 3) Diode =open, V C = -V IN (min), V OUT =V IN V C Slide 21

22 Clamp Circuit (level shifter) Max of v in (t)=5 sin(ωt) is shifted by -5V by the diode-voltage d source combination Slide 22

23 Voltage Doubler Circuit -V C1 C 1 V IN R 1 V OUT V IN C 2 V C21 - R 2 V OUT Level Shift Peak Detect See Homework problem Output is the peak to peak voltage of the input. Slide 23

24 Half Wave Rectifier Equivalent circuit V >0.6V, diode = short circuit V o = V I V < 0.6V, diode = open circuit Vo =0 Slide 24

25 Adding a capacitor: what does it do? V m sin (ωt) C R V 0 - Slide 25

26 Half-Wave Rectifier Current charging up capacitor Slide 26

27 Full Wave Rectifier Slide 27

28 Small Signal Linear Equivalent Circuit Suppose the nonlinear device has the functional dependence I = i(v) is biased with a DC voltage v G at the Q- point (quiescent point). A small differential voltage v is added on top of v G. Using Taylor series expansion di i(v Q v) = i(v Q ) v... dv v Q We can define a dynamic resistance r at the Q point r ii 1 di dv v Q v r Slide 28 i v G v i Tangent line v

29 Small Signal Model of Diode i v r Q1 Q2 i i v v Slide 29

30 Small Signal Model Example V C and R C Determines r d at Q point of diode Slide 30

31 Small Signal Model Example The large capacitors and DC bias source are effective shorts for the ac signal in small-signal signal circuits * See Hambley for an application of voltage controlled Attenuator Slide 31