Diode Circuits-Basic Electronics-Lecture 03-Electronic and Information Engineering, Lecture notes of Basic Electronics

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EIE209 Basic Electronics

Diode circuits

Prof. C.K. Tse: 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 Real characteristic Take a closer look at the characteristic around the turning point. The i-v characteristic is an exponential function. From physics, we have Also, the diode can only stand the negative voltage up to a certain threshold VBD , beyond which the diode conducts reverse current (breakdown). vd id 0.7V id = Iss ( e q vd / k T

• 1 )
  • VBD breakdown

Prof. C.K. Tse: Diode circuits Which model to use?

The choice depends on the external voltage magnitudes.

Ideal model: id = 100/100 = 1 A With 0.7V drop: id = (100–0.7)/ = 0.997 A Ideal model: id = 10/100 = 100 mA With 0.7V drop: id = (10–0.7)/ = 93 mA Ideal model: id = 2/100 = 20 mA With 0.7V drop: id = (2–0.7)/ = 13 mA

Prof. C.K. Tse: Diode circuits Application examples clamper dc restorer half-wave doubler full-wave doubler

Prof. C.K. Tse: Diode circuits 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 2 : KVL & KCL constraints from KCL : id = iR AND from KVL : vd + vR = Vs We also know =

= Vs O

Prof. C.K. Tse: Diode circuits Step 3 : enforcing KVL & KCL Method: flip the resistor curve horizontally; and push the two curves together horizontally until the y-axes are Vs apart. add to Vs

push push

Prof. C.K. Tse: Diode circuits General problem : )

_ R

vd

• vR – iR id V s nonlinear device vd id How to find vd and id?

Prof. C.K. Tse: Diode circuits Basic load line construction : )

_ R

vd

• vR – iR id V s nonlinear device v i V s slope = – 1/ R vd id device characteristic

Prof. C.K. Tse: Diode circuits Tutorial problem

Find the operating point.