Tài liệu EIE209 Basic Electronics Diode Circuits pdf

EIE209 Basic Electronics Diode circuits Contents • Diode models • Typical diode circuits & applications • Load line concepts for nonlinear load analysis Prof C.K Tse: Diode circuits What is a diode? Simplest view (no physics): — a unidirectional device that allows current to flow in one direction but not the other Ideally, we regard a diode as short circuit when voltage applied to it in the forward manner is positive Prof C.K Tse: Diode circuits Ideal characteristic Bias conditions Forward bias : vd > — current can flow and id > Reverse bias : vd < — current cannot flow and id = id id Ideal diode with finite forward drop (more realistic) vd Prof C.K Tse: Diode circuits 0.7V vd Real characteristic Take a closer look at the characteristic around the turning point The i-v characteristic is an exponential function id From physics, we have id = I ss (e q vd /kT Also, the diode can only stand the negative voltage up to a certain threshold VBD , beyond which the diode conducts reverse current (breakdown) - 1) –VBD breakdown Prof C.K Tse: Diode circuits vd 0.7V Which model to use? The choice depends on the external voltage magnitudes Ideal model: id = 100/100 = A With 0.7V drop: id = (100–0.7)/100 = 0.997 A Ideal model: id = 10/100 = 100 mA With 0.7V drop: id = (10–0.7)/100 = 93 mA Prof C.K Tse: Diode circuits Ideal model: id = 2/100 = 20 mA With 0.7V drop: id = (2–0.7)/100 = 13 mA Example: rectifier circuit The ideal model is valid if the external voltages are well above 0.7V What is the magnitude of vr ? Crude — 50 V Better — 50 – 1.4 = 48.6 V Prof C.K Tse: Diode circuits Application examples clamper dc restorer half-wave doubler Prof C.K Tse: Diode circuits full-wave doubler A nonlinear circuit problem Suppose we wish to find vd , given that the external voltage Vs is not large enough to validate the use of the ideal diode model ?? Prof C.K Tse: Diode circuits Step : locating the operating points Recall: The characteristic curve/line for a device actually defines where the point (v, i) can lie We know the operating point (vd, id) is somewhere on the diode characteristic curve the operating point (vR, iR) is somewhere on the resistor characteristic curve Prof C.K Tse: Diode circuits Step : KVL & KCL constraints We also know from KCL : id = iR AND from KVL : vd + vR = Vs = Vs O + = Prof C.K Tse: Diode circuits 10 Step : enforcing KVL & KCL Method: flip the resistor curve horizontally; and push the two curves together horizontally until the y-axes are Vs apart push push add to Vs Prof C.K Tse: Diode circuits 11 Solution : load line The flipped resistor line is called the LOAD LINE Prof C.K Tse: Diode circuits 12 General problem + vR – How to find vd and id ? iR id R :) Vs + _ id + vd – nonlinear device vd Prof C.K Tse: Diode circuits 13 Basic load line construction + vR – iR i R :) Vs + _ id + vd – slope = –1/R id device characteristic nonlinear device vd Prof C.K Tse: Diode circuits v Vs 14 Alternative construction + vR – iR i R :) Vs + _ id + vd – id when device is short-circuit nonlinear device device characteristic load line v vd when device is opencircuit Prof C.K Tse: Diode circuits 15 Tutorial problem Find the operating point Prof C.K Tse: Diode circuits 16 ... vd Prof C.K Tse: Diode circuits 13 Basic load line construction + vR – iR i R :) Vs + _ id + vd – slope = –1/R id device characteristic nonlinear device vd Prof C.K Tse: Diode circuits v Vs 14... — 50 V Better — 50 – 1.4 = 48.6 V Prof C.K Tse: Diode circuits Application examples clamper dc restorer half-wave doubler Prof C.K Tse: Diode circuits full-wave doubler A nonlinear circuit problem... Vs apart push push add to Vs Prof C.K Tse: Diode circuits 11 Solution : load line The flipped resistor line is called the LOAD LINE Prof C.K Tse: Diode circuits 12 General problem + vR – How to