Announcements Assignment 1 solutions posted Assignment 2 due Thursday First mid-term Thursday...

download Announcements Assignment 1 solutions posted Assignment 2 due Thursday First mid-term Thursday October 27 th (?)

of 18

  • date post

  • Category


  • view

  • download


Embed Size (px)

Transcript of Announcements Assignment 1 solutions posted Assignment 2 due Thursday First mid-term Thursday...

  • AnnouncementsAssignment 1 solutions postedAssignment 2 due ThursdayFirst mid-term Thursday October 27th (?)

  • Lecture 8 OverviewInductors in transient circuitsSemiconductorsDiodesRectifying circuitsOther Diode ApplicationsTransistors

  • Time response of InductorsSwitch to position a:Integrate and apply boundary condition t=0, i=0Time constant =L/R. Switch to position b:Talk about "Charging a capacitor""Current build-up" in an inductor

  • Time response of InductorsA battery is connected to an inductor. When the switch is opened does the light bulb:

    1.Remain off2.Go off3. Slowly Dim out4. Keep burning as brightly as it did before the switch was opened5. Flare up brightly, then dim and go outAnswer 5

  • SemiconductorsSimple review of basic concepts: What is a semiconductor?(for more detail see e.g. Simpson Ch. 4)Elements such as Silicon and Germanium have 4 valence electrons in their outer shellThey form covalent bonds with neighbouring atoms to form strong crystal lattice structures.In pure silicon, all valence electrons are bound in the lattice structure

  • SemiconductorsThe addition of impurities ("doping"), such as Sb(Antimony) with 5 valence electrons, leaves one electron unbound and free to move and create a current flow (n-type semiconductor). Alternatively, an impurity with 3 valence electrons can be used to create positive "holes".When a p-type and an n-type are joined (p-n junction), mobile electrons diffuse from the n-type to the p-type, forming positive and negative ions at fixed positions in a state of equilibrium which inhibit further transfer of electrons (depletion region)+-

  • What happens when you apply a voltage?Reverse bias: Apply an electric field in this direction, mobile electrons are driven away from the junction (unlike fixed charged ions). Mobile holes are also driven away in the opposite direction. Depletion region acts like an insulating slab - No current flowsForward bias: Helps electrons overcome the depletion region. Current flows easilyFor the device to conduct, electrons from the n-type region must cross the junctionapplied E-fieldapplied E-fieldDepletion region E-fieldDepletion region E-field

  • Ideal diode A diode is a non-linear circuit element Only passes current in one direction Constructed from a p-n semiconductor junction

  • Real diodeDiode law:IS = reverse-leakage currentv = voltage across the diodekB = Boltzmann's constante- = electron chargeT = Temperature (K)Strong dependence on TIS is small ~ 10-6A (Ge), ~10-8(Si)

  • Diode Circuit Diodes are non-linear; how do we calculate the operating conditions? (Cant easily use V=IR) Consider the simplest diode circuitMust satisfy both equations: Operating point can be calculated by seeing when diode law line intersects load lineKVL:When ID=0; VD=VDDWhen VD=0; ID=VDD/R

  • Rectifying CircuitOne of the most important applications of a diode is in rectifying circuits: used to convert an AC signal into the DC voltage required by most electronicsIdeal transformer: VS/VP=NS/NPReal transformers are ~98% efficient

  • Half-wave rectifier Only lets through positive voltages. Rejects negative voltages

  • Full-wave rectifier To use both halves of the input sinusoid,can use a centre-tapped transformer:e.g. Battery Charger

  • or use a Bridge rectifier

    Does not require centre-tapped transformer Requires 2 diodes in each direction cheap, but voltage drop is double

  • Bridge rectifier Current flow in the bridgevOvO++--

  • Peak rectifier Most devices need steady DC To smooth out the peaks and obtain a DC voltageWhen source voltage < capacitor voltageDiode is reversed biasedCapacitor discharges through resistor

  • Another diode application: Voltage doubler High Voltage transformers are expensive and impractical at voltages above a few thousand Volts. How do we get higher?C2 charges to VsecC1 charges to 2Vsec

  • Voltage doubler Can extend this circuit to produce extremely high voltages (~750kV).Voltage Quadrupler Cockroft-Walton voltage multiplier 1932, Cavendish Labs reached 250 kV Accelerated protons onto a Lithium target Split the atom!