Sedra/Smith Microelectronic Circuits 6/E Chapter 3: Diodes
Transcription
Sedra/Smith Microelectronic Circuits 6/E Chapter 3: Diodes
Sedra/Smith Microelectronic Circuits 6/E Chapter 3: Diodes S. C. Lin, EE National Chin-Yi University of Technology 1 【Outline】 3.1 The Ideal Diode 3.2 Terminal Characteristics of Junction Diodes 3.3 Modeling the Diode Forward Characteristic 3.4 Operation in the Reverse Breakdown Region — Zener Diodes 3.5 Rectifier Circuits 3.6 Limiting and Clamping Circuits 3.7 Special Diode Types S. C. Lin, EE National Chin-Yi University of Technology 2 3.1 The Ideal Diode i + v − (a) diode circuit symbol (b) i–v characteristic i i + v − v<0⇒i =0 (c) equivalent circuit in the reverse direction + v − i >0⇒v=0 (d) equivalent circuit in the forward direction. S. C. Lin, EE National Chin-Yi University of Technology 3 The two modes of operation of ideal diodes and the use of an external circuit. +10V 10mA 1kΩ +10V 0mA 1kΩ + + 0V − 10V − (a) the forward current (b) the reverse voltage S. C. Lin, EE National Chin-Yi University of Technology 4 Vp vi + vD − t 0 iD vi (a) Input waveform. iD + vD = vi − R + vo = vi − (c) Equivalent circuit when vi ≥ 0. 0 vi iD = 0 R + vo = 0 − vi ≤ 0 vi ≥ 0 Vp R (b) Rectifier circuit. + vD = 0 − vi + vo − (d) Equivalent circuit when vi ≤ 0 . vo t (e) Output waveform. S. C. Lin, EE National Chin-Yi University of Technology 5 Example 3.1 Fig.3.4(a), shows a circuit for charging a 12-V battery. If vs is sinusoid with 24-V peak amplitude, (a) Find the fraction of each cycle during which the diode conducts. (b) Also find the peak value of the diode current and (c) the maximum reverse-bias voltage that appears across the diode. iD 100Ω vs Method 1: 12V 24V VD 12V αθ (a ) 24sin α = 12 ⇒ α = 30o 2θ = 180o − 2α = 120o S. C. Lin, EE National Chin-Yi University of Technology 6 Method 2: 24V VD 12V θ θ (a)24 cos θ = 12 ⇒ θ = 60o Conduction angle = 2θ = 120o ▲ (b)I D (max) = 24 − 12 = 0.12A ▲ 100 (c)The maximum reverse voltage across the diode: VD (max) = 24 + 12 = 36V ▲ S. C. Lin, EE National Chin-Yi University of Technology 7 3.1.3 Another Application: Diode Logic Gates +5V vA R vB vC vY (a) OR gate vA (b) AND gate vB R vY vC vA vB vC vY vA vB vC vY S. C. Lin, EE National Chin-Yi University of Technology 8 Example 3.2 Assuming the diode to be ideal, Find the values of I and V in the circuit of Fig. (a) and (b) +10V Sol: Asssume that both diodes are conducting 10kΩ I D2 I D1 5kΩ −10V D2 VB = 0 and V = 0 + V − ID = 2 (10 − 0)V = 1mA 10kΩ Writting a node equation at B (K.C.L) 0 − (−10) I +1 = ⇒ I = 1mA 5 Results in I = 1mA, Thus D1 is conducting as originally assumed and the find resuit is I = 1mA and V = 0V ▲ S. C. Lin, EE National Chin-Yi University of Technology 9 Sol: +10V . We asssume that both diodes are conducting 5kΩ I D2 D1 I D2 then VB = 0 and V = 0 ID = + V − 2 (10 − 0)V = 2mA 5kΩ The node equation at B is 0 − (−10) ⇒ I = −1mA 10 (This is not possible) I +2= 10kΩ −10V Our assumption is not correct, we start again assuming that D1 is OFF and D2 is ON. The current I D is given by 2 10 − (−10) = 1.33mA 15 VB = −10 + 10 ⋅ 1.33 = 3.3V ID = 2 Thus D1 is reverse biased as assumed ∴ I = 0A and V = 3.3V ▲ S. C. Lin, EE National Chin-Yi University of Technology 10 3.2 Terminal Characteristics junction diode i (1) The forward-bias region determined by v > 0 1 (2) The reverse-bias region determined by − VZK < v < 0 −VZK 3 2 Expanded scale 0 0.5V 0.7V v (3) The breakdown-bias region determined by v < −VZK i + v − Figure 3.8 The diode i–v relationship with some scales expanded and others compressed in order to reveal details. S. C. Lin, EE National Chin-Yi University of Technology 11 3.2.1 The Forward-bias region ( ) i = I s ev / nV − 1 T (3.1) (a ) I s is saturation current , another name for I s is the scale current. (b) I s is a constant for a given diode at a given temperature. (c) I s is directly propotional to the across-sectional area of the diode. (d) I s For "small-signal" diode intened for low-power application, I s is on the order of 10-15A (e) I s a very strong function of temperature. As a rule of thumb, I s doubles in value for every 5oC rise in temperature. S. C. Lin, EE National Chin-Yi University of Technology 12 VT : Thermal voltage K ⋅T VT = q (3.2) where K = Boltzmann's constant = 1.38 × 10-23 Joules/Kelvin T = The absolute temperature in Kelvin = 273+temperature oC q = The magnitude of electronic charge = 1.60 × 10−19 Coulomb At room temperature(20o C), VT ≅ 25.2mV. At room temperature(27o C), VT ≅ 26mV. n n :1 ↔ 2, Depending on the material and the physical structure of the diode. S. C. Lin, EE National Chin-Yi University of Technology 13 the forward direction i >> I s i i ≅ Ise ⇒ v = nVT ln Is If the voltage is v1 (v2 ), the diode current I1 ( I 2 ) v / nVT thus i1 ≅ I s ev1 / nVT and i2 ≅ I s ev2 / nVT i2 = e( v −v ) / nV i1 2 1 T i2 i2 v2 − v1 = nVT ln = 2.3nVT log i1 i1 (3.5) This equation simply states that for a decade changes in current, the diode voltage drop changes by 2.3nVT ≈ 60mV,for n = 1 S. C. Lin, EE National Chin-Yi University of Technology 14 Example: Design the circuit in below Figure to provide an output voltage of 2.4V. Assume that diode available have 0.7V drop at 1mA and that ΔV= 0.1V/decade changes in current. Sol: Refer Fig.a to obtain vo = 2.4V +10V each diode must have a voltage drop of 0.8V thus i2 v2 − v1 = 2.3nVT log i1 R vo i2 i2 0.1 = 2.3 ⋅ 2 ⋅ 25 ⋅10 ⋅ log = 0.1log i1 i1 −3 i2 = 10i1 = 10mA The value of R is determined from (10 − 2.4)V R= = 760Ω ▲ 10mA S. C. Lin, EE National Chin-Yi University of Technology 15 T2 i > T1 −2mV/ o C I v Figure 3.9 Illustrating the temperature dependence of the diode forward characteristic. At a constant current, the voltage drop decreases by approximately 2 mV for every 1°C increase in temperature. S. C. Lin, EE National Chin-Yi University of Technology 16 Example 3.3 : A silicon diode said to be a 1-mA device displays a forward voltage of 0.7V at a current of 1mA. Evaluate the junction scaling constant Is in the event that n is either 1or 2, what scaling constants would apply for a 1-A diode of the same manufacture that conducts 1A at 0.7V Since i ≅ I s ev / nVT ⇒ I s = i ⋅ e − ( v / nVT ) for the 1 − mA diode. If n = 1 If n = 2 I s = 1×10−3 × e −700 / 25 = 6.9 × 10−16 A ▲ I s = 1× 10−3 × e −700 / 50 = 8.3 × 10−10 A ▲ The diode conducting 1A at o.7V corresponds to 1000 1mA in parallel with a total junction area 1000 times greater. Thus it is also 1000 times greater being 1pA and 1µA , respectively for n = 1 and n = 2 . S. C. Lin, EE National Chin-Yi University of Technology 17 3.2.2 The Reverse-Bias Region If v is negative and a few times large then VT (25mV) in magnitude the diode current becomes i ≅ − I s (Saturation current) I s : It doubles for every 10o C rise in temperature I s2 = I s1 ⋅ 2(T2 −T1 /10) 3.2.3 The Breakdown Region ⎧ Z: Zener v < −VZK ⎨ ⎩ K: Knee In the breakdown region the reverse current increase rapidly with the associated increase in voltage drop very small S. C. Lin, EE National Chin-Yi University of Technology 18 3.3 Modeling the diode forward characteristic 3.3.1 The Exponential Model. ID R + VDD VD − ⎧VDD > 0.5V Assuming that ⎨ (3.6) ⇒ I D = I s eVD / nVT ⎩ ID > Is The other equation that governs circuit operation is obtained by writing a loop equation by KVL, resulting in I D = VDD − VD R (3.7) S. C. Lin, EE National Chin-Yi University of Technology 19 3.3.2 Graphical Analysis Using the Exponential Model. i VDD R Q (Operating point) ID 0 −1 Slop = R VD VDD v Figure 3.11 Graphical analysis of the circuit in Fig. 3.10 using the exponential diode model. S. C. Lin, EE National Chin-Yi University of Technology 20 3.3.3 Iterative analysis using the exponential model Example3.4 determine the current I D and the diode voltage VD for the circuit in Fig.3.10 with VDD = 5V and R = 1kΩ, assume that the diode has a current of 1mA at voltage of 0.7V and that its voltage drop changes by 0.1V for every decade change in current. Sol : We assume that VD = 0.7V, I D = 1mA VDD − VD (5 − 0.7)V = = 4.3mA ID = R 1kΩ I V2 − V1 = 2.3 ⋅ n ⋅ VT ⋅ log 2 I1 V2 = V1 + 0.1 ⋅ log I2 ( n = 2, 2.3 ⋅ n ⋅VT ≅ 0.1V ) I1 4.3mA = 0.763V V2 = 0.7 + 0.1 ⋅ log 1mA The first iteration are I D = 4.3mA, VD = 0.763V S. C. Lin, EE National Chin-Yi University of Technology 21 We assume that VD = 0.763V, I D = 4.3mA VDD − VD (5 − 0.763)V = = 4.237mA 1kΩ R 4.237mA V2 = 0.763 + 0.1 ⋅ log = 0.762V 4.3mA ID = The second iteration are I D = 4.237mA, VD = 0.762V We assume that VD = 0.762V, I D = 4.237mA VDD − VD (5 − 0.762)V ID = = = 4.238mA R 1K Ω 4.238mA V2 = 0.762 + 0.1⋅ log = 0.762V 4.237mA Because I D did not change by much stop here and the solution is I D = 4.237mA and VD = 0.762V S. C. Lin, EE National Chin-Yi University of Technology 22 3.3.5 The Constant-Voltage-Drop Model. (Rapid analysis) iD (mA) VD = 0.7V ⎧ iD = 0, ⎨ ⎩iD = (vD − 0.7V) / R VD vD ≤ 0.7V vD ≥ 0.7V vD (V) S. C. Lin, EE National Chin-Yi University of Technology 23 The constant-voltage-drop model of the diode forward characteristics and its equivalent-circuit representation. iD (mA) 12 ID 10 + 8 vD 6 − VD = 0.7V VD = 0.7V 4 2 0 0.2 0.4 0.6 0.8 S. C. Lin, EE National Chin-Yi University of Technology 1.0 v (V) D 24 Example: Repeat the problem in example 3.4, utilizing the constant-voltage-drop model whose parameters are given in Fig 3.12 . Find I D and VD . ID ID R + VDD VD − + vD − VD = 0.7V Sol: VDD − VD (5 − 0.7)V ID = = = 4.3mA R 1kΩ VD = VD = 0.7V S. C. Lin, EE National Chin-Yi University of Technology 25 3.3.7 The Diode Small-Signal Model. iD = I S evD / nVT (3.8) vD (t ) = VD + vd (t ) (3.9) S. C. Lin, EE National Chin-Yi University of Technology 26 iD (t ) = I S evD (t ) / nVT = I S e[VD + vd (t )] / nVT = I S eVD / nVT ⋅ evd (t ) / nVT ID = I D ⋅ evd (t ) / nVT (3.12) If the signal vd (t ) is kept sufficiently small vd (t ) such that << 1 nVT Thus, the approximate expression ⎛ vd ⎞ iD (t ) = I D ⋅ ⎜1 + ⎟ ⎝ nVT ⎠ (3.14) Small-signal approximation S. C. Lin, EE National Chin-Yi University of Technology 27 We have iD (t ) = I D + ID vd nVT (3.15) id ID vd nVT vd nVT rd ≡ = id ID where id = (3.17) (3.18) The rd is diode smal-signal resistance, and called incremental resistance) In Fig.3-13(b), which is equal to the slope of the tangent to the i - v curve at the operating point Q is equal to the small-signal conductance. that is, rd = 1 ⎡ ∂id ⎤ ⎢ ∂v ⎥ ⎣ d ⎦i d (3.19) =ID S. C. Lin, EE National Chin-Yi University of Technology 28 Example 3.6: Consider the circuit shown in Fig 3.15 a string of three diode is used to provide a constant voltage of about 2.1V, we want to calculate the percentage change in this regulated voltage cause by : (a) ±10% change in the power-supply voltage (b) Connection of a 1kΩ load resistance. Assume n=1 Sol: with no load (10 − 2.1)V = 7.9mA I= 1kΩ nVT 25mV = = 3.2Ω rd = I 7.9mA The total incremental resistance r = 3rd = 9.6Ω S. C. Lin, EE National Chin-Yi University of Technology 29 r 0.0096 = ±1 = ±9.5mV ▲ (a) ∆vo = ±1 r+R 0.0096 + 1 This implies a change of about ± 3.2mV per diode. (b)When a load resistance of 1kΩ is connected across the diode string. it draws a current of 2.1mA ∴∆vo = −2.1× r = −2.1× 9.6 = −20mV. ▲ this implies that the voltage across each diode decreses by about 6.7mV. S. C. Lin, EE National Chin-Yi University of Technology 30 3.4 Operation in the reverse breakdown region -Zener diode (1) rZ is the inverse of the slop of the almost-linear i - v curve at point Q (2) rZ : increase resistance (3) Typically, rZ i s in the range of a few ohms to a few ten ohms S. C. Lin, EE National Chin-Yi University of Technology 31 Vz = Vzo + I z rz S. C. Lin, EE National Chin-Yi University of Technology (3.20) 32 3.4.2 Use of the Zener as a Shunt Regulator Example 3.7: The 6.8V Zener diode in the circuit of Fig.3.19(a) is specified to have VZ = 6.8V at I Z = 5mA, rz = 20Ω and I ZK = 0.2mA, the V + is 10V ± 1V. S. C. Lin, EE National Chin-Yi University of Technology 33 (a) Find Vo with no load and with V + at its nominal value. Sol: first we must determine the value of V zo Vzo = Vz − I z rz = 6.8 − 5 ⋅10−3 × 20 = 6.7 V 10 − 6.7 Iz = I = = 6.35mA 0.5 + 0.02 Thus Vo =Vz = Vzo + Iz rz = 6.83V ▲ (b) Find change in Vo resulting from the ± 1V change in V+ . Sol: ∆ Vo = ∆V + rz 20 = ±1 × = ±38.5mV R + rz 500 + 20 line regulation ≡ ∆Vo / ∆Vi = ±38.5mV/1V = ±38.5mV/V ▲ S. C. Lin, EE National Chin-Yi University of Technology 34 (c) Find change in Vo resulting from connecting a load resistance RL . that draws a current I L = 1mA, and hence find the load regulation (∆Vo / ∆I L ) in mV/mA. Sol: ∆ Vo = ∆I z ⋅ rz = 20Ω × −1mA = −20mV load regulation ≡ ∆Vo / ∆I L = −20mV/mA ▲ (d)Find change in Vo when RL = 2kΩ. Sol: RL = 2kΩ, I RL ≈ ∆I z ≈ −6.8V / 2kΩ = −3.4mA ∆ Vo = ∆I z ⋅ rz = −20 × 3.4 = −68mV ▲ S. C. Lin, EE National Chin-Yi University of Technology 35 (e)Find the value of Vo when RL = 0.5kΩ. Sol: RL = 0.5kΩ, would draw a I L = 6.8 / 0.5 = 13.6mA.This is impassible. because I supplied through R is only 6.4mA, therefore, RL the zener must be cut-off. Hence Vo = V = 5V ▲ R + RL + (f) What is the minmum value of RL for which the diode still operates region in the breakdown . Sol: I z = I zK = 0.2mA ⇒ Vz = VzK ≈ Vzo = 6.7V 9 − 6.7 I (min) = = 4.6mA 0.5 I (max) = (4.6 − 0.2)mA = 4.4mA, ∴ RL = 6.7 = 1.5kΩ ▲ 4.4 S. C. Lin, EE National Chin-Yi University of Technology 36 3.4.3 Temperature effects, T.C or Temco (1) VZ < 5V, dVz / dT < 0. Negative T.C (2) VZ > 5V, dVz / dT > 0. Positive T.C (3) VZ = 5V, dVz / dT = 0. Zero T.C S. C. Lin, EE National Chin-Yi University of Technology 37 3.5 Rectifier Circuits Figure 3.20 Block diagram of a dc power supply. (1) The Half-wave rectifier. (2) The full-wave rectifier (a) The center tap rectifier (b) The bridge rectifier S. C. Lin, EE National Chin-Yi University of Technology 38 3.5.1 Half-Wave Rectifier. (a) Half-wave rectifier. (b) Equivalent circuit of the half-wave rectifier with the diode replaced with its battery-plus-resistance model. S. C. Lin, EE National Chin-Yi University of Technology 39 vS < VDO ⎧vO = 0, ⎪ R ⎨ v = ⎪ O R + r (vS − VDO ), vS ≥ VDO D ⎩ (c) Transfer characteristic of the rectifier circuit. VD φi = sin Vm −1 φi (d) Input and output waveforms, assuming that rD << R. S. C. Lin, EE National Chin-Yi University of Technology 40 We assume vS = Vm sin α thus 1 π VDC = vS dα ∫ 0 2π 1 π Vm sin αdα = ∫ 0 2π Vm π = (− cos α 0 ) 2π Vm = 0.318Vm = π 1/ 2 ⎡1 ⎤ Vrms = ⎢ ∫ vS 2 dα ⎥ ⎣ 2π 0 ⎦ 1 π 2 2 2 Vrms = Vm sin αdα ∫ 0 2π Vm 2 π 1 1 = ( − cos 2α )dα ∫ 2π 0 2 2 π π Vm 2 1 1 = ( α − sin 2α ) 2π 2 4 0 Vm 2 = ⇒ Vrms = 0.5Vm 4 PIV = Vm (PIV : Peak inverse voltage) VDO φi = sin Vm −1 S. C. Lin, EE National Chin-Yi University of Technology 41 3.5.2 The full-wave rectifier (a) circuit (b) transfer characteristic assuming a constant-voltage-drop model for the diodes S. C. Lin, EE National Chin-Yi University of Technology 42 (c) input and output waveforms. S. C. Lin, EE National Chin-Yi University of Technology 43 We assume vS = Vm sin α thus VDC ⎧1 π ⎫ = 2 × ⎨ ∫ vS dα ⎬ ⎭ ⎩ 2π 0 2 ×1 π = Vm sin αdα ∫ 0 2π Vm π = (− cos α 0 ) π 2 × Vm = 0.636Vm = π 1/ 2 ⎡ 2 ×1 π 2 ⎤ Vrms = ⎢ vS dα ⎥ ∫ 0 ⎣ 2π ⎦ 2 ×1 π 2 2 2 Vrms = Vm sin αdα ∫ 0 2π 2 × Vm 2 π 1 1 = ( − cos 2α )dα ∫ 2π 0 2 2 π Vm 2 1 1 = ( α − sin 2α ) 4 π 2 0 Vm 2 = ⇒ Vrms = 0.707Vm 2 PIV = 2Vm (PIV : Peak inverse voltage) VD φi = sin Vm −1 S. C. Lin, EE National Chin-Yi University of Technology 44 3.5.3 The Bridge Rectifier Figure 3.27 The bridge rectifier: (a) circuit; (b) input and output waveforms. S. C. Lin, EE National Chin-Yi University of Technology 45 3.5.4 The Rectifier with a Filter Capacitor-The Peak Detector A simple circuit used to illustrate the effect of a filter capacitor S. C. Lin, EE National Chin-Yi University of Technology 46 S. C. Lin, EE National Chin-Yi University of Technology 47 S. C. Lin, EE National Chin-Yi University of Technology 48 We assumption that RC >> T in Fig3.29(b) The load current iL = vO / R dv The diode current iD = iC + iL = C I + iL dt (1) Derivative the Vr ( rms ) During the diode cutoff interval vo = VP − Vr VP − Vr = VP e −T / RC Vr = VP V VP T = P ⇒ Vr ( rms ) = RC fRC 2 3 fRC (2) Derivative the conduction interval ∆t VP cos(ω∆t ) = VP − Vr 1 The angle ω∆t is very small , ∴ cos(ω∆t ) ≈ 1 − (ω∆t ) 2 2 ⎡ 1 ⎤ VP ⎢1 − (ω∆t ) 2 ⎥ = VP − Vr ⇒ ω∆t = 2Vr / VP (3.30) ⎣ 2 ⎦ We note that when Vr << VP the conduction angle ω∆t will be small S. C. Lin, EE National Chin-Yi University of Technology 49 (3) Dertermine the average diode current during conduction, iDav the charge that the diode supplies to the capacitor QSupplied = iC ( av ) ⋅ ∆t (a) the charge that the capacitor loses Qlost = C ⋅Vr (b) C ⋅ Vr The(a) equal to (b) ⇒ iC ( av ) = ∆t VP VP Using(3.29a ) Vr = , ⇒C = fRC Vr fR 2Vr and(3.30) ω∆t = 2Vr / VP ⇒ ∆t = ω VP VP ⋅V r f V rR 2π VP VP C ⋅Vr we obtain: iC ( av ) = = ⋅ = I L ⋅ π ⋅ 2VP / Vr = ∆t 2Vr 2Vr R 2π f iD ( av ) = iC ( av ) + I L = I L ⎡⎣1 + π 2VP / Vr ⎤⎦ VP (3.31) S. C. Lin, EE National Chin-Yi University of Technology 50 (4) Dertermine the peak value of the diode current , iD (max) iD (max) dvI =C + iL dt dvI d (t = t1 = −∆t ) ⇒ C = C (VP cos(−ω∆t ) ) = ωCVP sin(ω∆t ) dt dt (ω∆t )3 + = ω ⋅ C ⋅ VP ⋅ (ω∆t ) [sin(ω∆t )] = ω∆t + 3! 2Vr VP = 2πf ⋅ ⋅ VP ⋅ = I L ⋅ 2π ⋅ 2VP / Vr fVr R VP iD (max) = iC + I L = I L ⎣⎡1 + 2π 2VP / Vr ⎦⎤ S. C. Lin, EE National Chin-Yi University of Technology (3.32) 51 the full-wave rectifier circuit. ∵ f r = 2 fi VP − Vr = VP e −T / 2 RC VP T /2 = Vr = VP RC 2 fRC VP Vr ( rms ) = 4 3 fRC (3.33) S. C. Lin, EE National Chin-Yi University of Technology 52 2.4VP Vr ( rms ) = ( f = 60Hz, RL = kΩ, C = µF ) RC when the ripple small then 6.5% ⇒ VP = VDC 2.4VDC 2.4 I DC Vr ( rms ) = = RC C Vr ( rms ) 2.4 r% = % = VDC RC If r % > 6.5% V 1 ⎡ 4.17 ⎤ ⋅ VP VDC = VP − Vr = VP − P = ⎢1 − ⎥ 2 4 fRC ⎣ RC ⎦ iD ( av ) = iC ( av ) + I L = I L ⎡⎣1 + π VP / 2Vr ⎤⎦ (3.34) iD (max) = iC + I L = I L ⎡⎣1 + 2π VP / 2Vr ⎤⎦ (3.35) S. C. Lin, EE National Chin-Yi University of Technology 53 Figure 3.27 The “superdiode” precision half-wave rectifier and its almost-ideal transfer characteristic. Note that when vI > 0 and the diode conducts, the op amp supplies the load current, and the source is conveniently buffered, an added advantage. Not shown are the op-amp power supplies. S. C. Lin, EE National Chin-Yi University of Technology 54 3.6 limiting and clamping circuits 3.6.1 limiter circuit. L− L+ ≤ vI ≤ K K vO = KvI Ingeneral K >1 In this section have K ≤1 L+ (a ) If vI > (the upper threshold), K the vO is limited or clamping to the upper limiting level L+ L− (b) If vI < (the lower limited threshold), K the vO is limited to the lower limiting level L− . S. C. Lin, EE National Chin-Yi University of Technology 55 Hard limiting. Soft limiting. S. C. Lin, EE National Chin-Yi University of Technology 56 S. C. Lin, EE National Chin-Yi University of Technology 57 Example.3.26 Assuming the diode to be ideal, describe the transfer characteristic of the circuit shown in Figure E3.26. (2) − 5V ≤ vI ≤ 5V, both diode are cut-off and vO = vI Figure E3.26 Sol: (1) vI ≤ −5V, D1 : conducts, D2 cut-off 10 10 vO = vI −5 10 + 10 10 + 10 = 0.5vI − 2.5 (3) vI ≥ 5V, D1 : cut-off, D2 conducts 10 10 vO = vI +5 10 + 10 10 + 10 = 0.5vI + 2.5 S. C. Lin, EE National Chin-Yi University of Technology 58 . vo vO = 0.5vI + 2.5 vI vO = 0.5vI − 2.5 S. C. Lin, EE National Chin-Yi University of Technology 59 Example : Assuming the diode to be ideal, describe the transfer characteristic of the circuit shown in Fig.a. 0 < vi ≤ 150V A D2 100kΩ vi (1)For 0 <Vi ≤ 25V, 200kΩ D1 + + − − vo both diodes are cut-off vO = 25V (2)For 25V<Vi < 137.5V, D1 conducts and D2 cut-off Sol: For VA ≥ 100V, both diodes are conducts 2 25 vi ⋅ + = 100 ⇒ vi = 137.5V 3 3 vO = VA = 2 25 vI + (V) 3 3 (3)For 137.5V< Vi < 150V, both diodes are conducts vO = 100V S. C. Lin, EE National Chin-Yi University of Technology 60 vo vO = 2 25 vi + 3 3 2 3 vi S. C. Lin, EE National Chin-Yi University of Technology 61 3.6.2 The Clamped Capacitor or DC Restorer Figure 3.32 The clamped capacitor or dc restorer with a square-wave input and no load. S. C. Lin, EE National Chin-Yi University of Technology 62 Figure 3.33 The clamped capacitor with a load resistance R. S. C. Lin, EE National Chin-Yi University of Technology 63 3.6.3 Voltage Doubler Figure 3.34 Voltage doubler: (a) circuit; (b) waveform of the voltage across D1. S. C. Lin, EE National Chin-Yi University of Technology 64 S. C. Lin, EE National Chin-Yi University of Technology 65