Tài liệu Malvino-EP-03- Lý thuyết Diode pptx

Chương 3 Chương 3 Lý thuyết diode Lý thuyết diode Từ Vựng (1) Từ Vựng (1) • anode anode • bulk resistance = điện trở khối th d •ca th o d e • diode • ideal diode = diode lý tưởng • knee voltage = điệnápgối knee voltage điện áp gối • linear device = dụng cụ tuyến tính ldli đờ tải • l oa d li ne = đ ư ờ ng tải Từ Vựng (2) Từ Vựng (2) • maximum forward current = dòng thuận maximum forward current = dòng thuận cực đại • nonlinear device = dụng cụ phi tuyến nonlinear device dụng cụ phi tuyến • Ohmic resistance = điện trở Ohm ti đị h ứ ô ất • power ra ti ng = đị n h m ứ c c ô ng su ất • up-down analysis = phân tích tăng-giảm Nội dung chương 3 Nội dung chương 3 3-1 Các ý tưởng cơ bản 3-2 Diode lý tưởng 3-3 Xấp xỉ bậc 2 3-4 Xấp xỉ bậc 3 3-5 Trounleshooting 3-6 Phân tích mạch tăng-giảm 3-7 Đọc bản g dữ liệu g 3-8 Cách tính điện trở khối 3-9 Điện trở DC của diode 3-10 Đườn g tải g 3-11 Diode dán bề mặt Properties of DiodesProperties of Diodes Figure 1.10 Figure 1.10 –– The Diode Transconductance CurveThe Diode Transconductance Curve 22 •• VV DD = Bias Voltage= Bias Voltage •• II DD = Current through = Current through II DD (mA)(mA) Diode. IDiode. I DD is Negative is Negative for Reverse Bias and for Reverse Bias and Positive for Forward Positive for Forward BiBi II Bi as Bi as •• II SS = Saturation = Saturation CurrentCurrent VV BRBR II SS •• VV BRBR = Breakdown = Breakdown VoltageVoltage VV DD ~V~V φφ •• VV φφ = Barrier Potential = Barrier Potential VoltageVoltage Kristin Ackerson, Virginia Tech EEKristin Ackerson, Virginia Tech EE Spring 2002Spring 2002 (nA)(nA) Properties of DiodesProperties of Diodes The Shockley EquationThe Shockley Equation •• The transconductance curve on the previous slide is characterized by The transconductance curve on the previous slide is characterized by the following equation:the following equation: II DD = I= I SS (e(e VV DD // ηη VV TT –– 1)1) •• As described in the last slide, IAs described in the last slide, I DD is the current through the diode, Iis the current through the diode, I SS is is th t ti t d Vth t ti t d V ith lidbi i ltith lidbi i lt th e sa t ura ti on curren t an d Vth e sa t ura ti on curren t an d V DD i s th e app li e d bi as i ng vo lt age. i s th e app li e d bi as i ng vo lt age. •• VV TT is the thermal equivalent voltage and is approximately 26 mV at room is the thermal equivalent voltage and is approximately 26 mV at room temperature. The equation to find Vtemperature. The equation to find V TT at various temperatures is:at various temperatures is: VV TT = = kTkT qq k = 138x10k = 138x10 2323 J/K T = temperature in Kelvin q = 16x10J/K T = temperature in Kelvin q = 16x10 1919 CC k 1 . 38 x 10k 1 . 38 x 10 J/K T temperature in Kelvin q 1 . 6 x 10J/K T temperature in Kelvin q 1 . 6 x 10 CC •• ηη is the emission coefficient for the diode. It is determined by the way is the emission coefficient for the diode. It is determined by the way the diode is constructed. It somewhat varies with diode current. For a the diode is constructed. It somewhat varies with diode current. For a silicon diodesilicon diode is aro nd 2 for lo c rrents and goes do n to abo t 1 atis aro nd 2 for lo c rrents and goes do n to abo t 1 at Kristin Ackerson, Virginia Tech EEKristin Ackerson, Virginia Tech EE Spring 2002Spring 2002 silicon diode silicon diode ηη is aro u nd 2 for lo w c u rrents and goes do w n to abo u t 1 at is aro u nd 2 for lo w c u rrents and goes do w n to abo u t 1 at higher currentshigher currents Diode Circuit ModelsDiode Circuit Models The Ideal DiodeThe Ideal Diode The diode is designed to allow current to flow inThe diode is designed to allow current to flow in The Ideal Diode The Ideal Diode ModelModel The diode is designed to allow current to flow in The diode is designed to allow current to flow in only one direction. The perfect diode would be a only one direction. The perfect diode would be a perfect conductor in one direction (forward bias) perfect conductor in one direction (forward bias) and a perfect insulator in the other directionand a perfect insulator in the other direction and a perfect insulator in the other direction and a perfect insulator in the other direction (reverse bias). In many situations, using the ideal (reverse bias). In many situations, using the ideal diode approximation is acceptable.diode approximation is acceptable. Example: Assume the diode in the circuit below is ideal. Determine the Example: Assume the diode in the circuit below is ideal. Determine the value of Ivalue of I DD if a) Vif a) V AA = 5 volts (forward bias) and b) V= 5 volts (forward bias) and b) V AA = = 5 volts (reverse 5 volts (reverse bias)bias) II RR S S = 50 = 50 ΩΩ a) With Va) With V AA > 0 the diode is in forward bias > 0 the diode is in forward bias and is acting like a perfect conductor so:and is acting like a perfect conductor so: II =V=V /R/R =5V/50=5V/50 ΩΩ = 100 mA= 100 mA ++ VV AA II DD II DD = V= V AA /R/R SS = 5 V / 50 = 5 V / 50 ΩΩ = 100 mA= 100 mA b) With Vb) With V AA < 0 the diode is in reverse bias < 0 the diode is in reverse bias and is actin g like a perfect insulator, and is actin g like a perfect insulator, Kristin Ackerson, Virginia Tech EEKristin Ackerson, Virginia Tech EE Spring 2002Spring 2002 __ gg therefore no current can flow and therefore no current can flow and II DD = 0.= 0. Diode Circuit ModelsDiode Circuit Models The Ideal Diode withThe Ideal Diode with This model is more accurate than the simpleThis model is more accurate than the simple The Ideal Diode with The Ideal Diode with Barrier PotentialBarrier Potential This model is more accurate than the simple This model is more accurate than the simple ideal diode model because it includes the ideal diode model because it includes the approximate barrier potential voltage. approximate barrier potential voltage. Remember the barrier potential voltage is theRemember the barrier potential voltage is the ++ Remember the barrier potential voltage is the Remember the barrier potential voltage is the voltage at which appreciable current starts to voltage at which appreciable current starts to flow.flow. Example: To be more accurate than just using the ideal diode model Example: To be more accurate than just using the ideal diode model VV φφ ++ include the barrier potential. Assume Vinclude the barrier potential. Assume V φφ = 0.3 volts (typical for a = 0.3 volts (typical for a germanium diode) Determine the value of Igermanium diode) Determine the value of I DD if Vif V AA = 5 volts (forward bias).= 5 volts (forward bias). II DD RR S S = 50 = 50 ΩΩ With VWith V A A > 0 the diode is in forward bias > 0 the diode is in forward bias and is acting like a perfect conductor and is acting like a perfect conductor so write a KVL equation to find Iso write a KVL equation to find I :: ++ __ VV AA II DD so write a KVL equation to find Iso write a KVL equation to find I DD :: 0 = V0 = V AA ––II DD RR SS VV φφ II DD = V= V AA VV φφ = 4.7 V = 4.7 V = 94 mA = 94 mA VV ++ Kristin Ackerson, Virginia Tech EEKristin Ackerson, Virginia Tech EE Spring 2002Spring 2002 φφ RR SS 50 50 ΩΩ VV φφ Diode Circuit ModelsDiode Circuit Models The Ideal DiodeThe Ideal Diode This model is the most accurate of the three It includes aThis model is the most accurate of the three It includes a The Ideal Diode The Ideal Diode with Barrier with Barrier Potential and Potential and Li F dLi F d This model is the most accurate of the three . It includes a This model is the most accurate of the three . It includes a linear forward resistance that is calculated from the slope of linear forward resistance that is calculated from the slope of the linear portion of the transconductance curve. However, the linear portion of the transconductance curve. However, this is usually not necessary since the Rthis is usually not necessary since the R FF (forward (forward Li near F orwar d Li near F orwar d Resistance Resistance resistance) value is pretty constant. For lowresistance) value is pretty constant. For low power power germanium and silicon diodes the Rgermanium and silicon diodes the R FF value is usually in the value is usually in the 2 to 5 ohms range, while higher power diodes have a R2 to 5 ohms range, while higher power diodes have a R FF value closer to 1 ohm.value closer to 1 ohm. value closer to 1 ohm.value closer to 1 ohm. Linear Portion of Linear Portion of transconductancetransconductance II DD ++ VV φφ RR FF transconductance transconductance curvecurve  II VV  II DD RR FF = = VV DD II DD VV DD VV DD Kristin Ackerson, Virginia Tech EEKristin Ackerson, Virginia Tech EE Spring 2002Spring 2002 Diode Circuit ModelsDiode Circuit Models The Ideal DiodeThe Ideal Diode ElA thdidilElA thdidil di ddi d The Ideal Diode The Ideal Diode with Barrier with Barrier Potential and Potential and Li F dLi F d E xamp l e: A ssume th e di o d e i s a l ow E xamp l e: A ssume th e di o d e i s a l ow power di o d e power di o d e with a forward resistance value of 5 ohms. The with a forward resistance value of 5 ohms. The barrier potential voltage is still: Vbarrier potential voltage is still: V φφ = 0.3 volts (typical = 0.3 volts (typical for a germanium diode) Determine the value of Ifor a germanium diode) Determine the value of I ifif Li near F orwar d Li near F orwar d Resistance Resistance for a germanium diode) Determine the value of Ifor a germanium diode) Determine the value of I DD if if VV AA = 5 volts.= 5 volts. RR S S = 50 = 50 ΩΩ Once again, write a KVL equationOnce again, write a KVL equation ++ VV AA II DD ++ Once again, write a KVL equation Once again, write a KVL equation for the circuit:for the circuit: 0 = V0 = V AA ––II DD RR SS VV φφ II DD RR FF __ VV φφ ++ RR FF II DD = V= V AA VV φφ = 5 = 5 –– 0.3 = 85.5 mA0.3 = 85.5 mA RR SS + R+ R FF 50 + 550 + 5 Kristin Ackerson, Virginia Tech EEKristin Ackerson, Virginia Tech EE Spring 2002Spring 2002 RR FF [...].. .Diode Circuit Models Values of ID for the Three Different Diode Circuit Models Ideal Diode Model ID Ideal Diode Model with Barrier Potential Voltage g Ideal Diode Model with Barrier Potential and Linear Forward Resistance 100 mA 94 mA 85.5 mA These are the values found in the examples... is: rF = ηVT ID The dynamic resistance is used in determining the voltage drop across the diode in the situation where a voltage source is supplying a sinusoidal signal with a dc offset The ac component of the diode voltage is found using the following equation: vF = vac rF rF + RS The voltage drop through the diode is a combination of the ac and g g dc components and is equal to: VD = Vφ + vF Kristin... 1000 Ω ID VA ID = VA – V φ + = 6V _ First t e load line is found by subst tut g in st the oad e s ou d substituting different values of Vφ into the equation for ID using the ideal diode with barrier potential model for the diode With RS at 1000 ohms the value of RF wouldn’t have much impact on the results results Vφ + RS Using V φ values of 0 volts and 1.4 volts we obtain ID values of 6 mA and 4.6 mA... 10 The transconductance curve below is for a Silicon diode The Q point in this p example is located at 0.7 V and 5.3 mA 8 Q Point: The intersection of the load line and the transconductance curve 6 5.3 53 4.6 4 2 VD (Volts) 0.2 0.4 0.6 0.8 0.7 1.0 1.2 1.4 Kristin Ackerson, Virginia Tech EE Spring 2002 Dynamic Resistance The dynamic resistance of the diode is mathematically determined y y as the inverse... value was assumed to be 5 ohms Kristin Ackerson, Virginia Tech EE Spring 2002 The Q Point The operating point or Q point of the diode is the quiescent or nonosignal condition The Q point is obtained graphically and is really only needed when the applied voltage is very close to the diode s barrier potential voltage The example 3 below that is continued on the next slide, shows how the Q point is determined... Resistance Example: Use the same circuit used for the Q point example but change E l the voltage source so it is an ac source with a dc offset The source voltage is now, vin = 6 + sin(wt) Volts It is a silicon diode so the barrier potential voltage is still 0.7 volts t ti l lt i till 0 7 lt RS = 1000 Ω ID + vin vF = vac Vφ + The DC component of the circuit is the same as the previous example and therefore ID... than 1 mA as it is in this example rF = sin(wt) V 4.9 Ω = 4.88 sin(wt) mV rF + RS 4.9 4 9 Ω + 1000 Ω Therefore, VD = 700 + 4.9 sin (wt) mV (the voltage drop across the Kristin Ackerson, Virginia Tech EE diode) Spring 2002 . 3 Lý thuyết diode Lý thuyết diode Từ Vựng (1) Từ Vựng (1) • anode anode • bulk resistance = điện trở khối th d •ca th o d e • diode • ideal diode. currentshigher currents Diode Circuit ModelsDiode Circuit Models The Ideal DiodeThe Ideal Diode The diode is designed to allow current to flow inThe diode is designed