In this chapter, you will learn about: Ideal & practical diodes; terminal characteristics of junction diodes; DC load line and quiescent conditions; piecewise linear model; small signal analysis of diodes; dynamic resistance, AC resistance; capacitance and switching response.
COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012 The Diode CircuitsII Lecture No: 10 Contents: References: Ø Microelectronic Circuits: Adel S. Sedra and Kenneth C. Smith Ø Electronic Devices and Circuit Theory: Robert Boylestad & Louis Nashelsky ( Prentice Hall ) Ø Introductory Electronic Devices and Circuits: Robert T. Paynter. Ø Electronic Devices : References (Figures): Chapter 2 Diodes: Figures are redrawn (with some modifications) from Introductory Electronic Devices and Circuits By Robert T. Paynter The Diode Models 1. The Ideal Diode Model The Diode: PN Junction Diode Schematic Symbol: Anode Cathode p n Diode Circuits: anode Reversed bias + + Forward bias cathode The left hand diagram shows the reverse biased junction. No current flows flows. The other diagram shows forward biased junction A current flows ForwardBiased Diode Circuit: R R I F > 0A I F > 0A IF V IF V +V -V R R IF IF ReverseBiased Diode Circuit: R R V 0A 0A IT IT V +V R -V R Effect of VF: I VS 5V 4.3 V VD1 = 0.7V R1 1k VR1 = VS − VD1 = 5V − 0.7V = 4.3V VR1 4.3V I= = = 4.3mA R1 1kΩ D1 Value Ideal Practical VF 0 V 0.7 V VR1 5 V 4.3 V I 5 mA 4.3 mA 10 SmallSignal Analysis of Diodes: Ø Smallsignal analysis is performed at a DC bias point by perturbing the voltage by a small amount and observing the resulting linear current perturbation Ø If two points on the IV curve are very close, the curve in between these points is well approximated by a straight line: ID VD e x dI D dVD VD VD I s VD1 / VT e VT x x2 2! x3 3! I D1 VT SmallSignal Analysis of Diodes: Ø Since there is a linear relationship between the smallsignal current and smallsignal voltage of a diode, the diode can be viewed as a linear resistor when only small changes in voltage are of interest SmallSignal Resistance (or Dynamic Resistance) rd VT ID SmallSignal Analysis of Diodes: ID V I D1 VT Smallsignal analysis is performed around a bias point by perturbing the voltage by a small amount and observing the resulting linear current perturbation. 28 SmallSignal Analysis of Diodes: ID VD dI D |VD dVD Is I D1 exp VT VT I D1 VT Ø If two points on the IV curve of a diode are close enough, the trajectory connecting the first to the second point is like a line, with the slope being the proportionality factor between change in voltage and change in current. 29 VD1 Small Sinusoidal Analysis: v If a sinusoidal voltage with small amplitude is applied, the resulting current is also a small sinusoid around a value V (t ) V0 V p cos t I D (t ) I0 I p cos t I s exp 30 V0 VT VT V p cos t I0 Resistance Levels: Ø Ø Ø The operating point of a diode moves from one region to another the resistance of the diode will also change due to the nonlinear shape of the characteristic curve The type of applied voltage or signal will define the resistance level of interest Three different types of applied voltage – DC or Static Resistance – AC or Dynamic Resistance – Average AC Resistance DC or Static Resistance • • • The application of a dc voltage to a circuit containing a semiconductor diode will result in an operating point on the characteristic curve that will not change with time The resistance of the diode at the operating point can be found simply by finding the corresponding levels of VD and ID The lower current through a diode the higher the dc resistance level AC or Dynamic Resistance • The varying input will move the instantaneous operating point up and down a region of the characteristics and thus defines a specific change in current and voltage as shown in the Fig. Temperature Effects: J Js eVa J s exp( 1) kT eD p pno eDn n po ( ) Lp Ln steady state : pno ni2 , n po Nd ni2 Na Js : strong function of temperature Js i n exp( Eg kT ) Temperature Effects on Diode Operation: I F(mA) 100 C 10 25 C V2 V1 I2 I1 VR T = 25 C T = 35 C 0.2 IR = A IR = 10 A 0.4 0.6 0.8 VF(V) 1.0 10 15 T = 45 C IR = 20 A 20 IR 35 Typical Diodes Diode Maximum Ratings Rating Discussion Peak repetitve reverse voltage, VRRM Maximum allowable reverse voltage Nonrepetitive peak reverse voltage, VRSM Maximum allowable value of a single event reverse voltage (VRSM > VRRM) RMS reverse voltage, VR(rms) VR(rms) = 0.707 VRRM Average rectified forward current, I0 Maximum average diode current Nonrepetitive peak surge current, IFSM Maximum allowable value of forward current surge (30A for 1N400X) Operating and storage junction temperature, TJ or Tstg Temperature that diode can withstand 37 Diode Capacitance: Insulator Conductor Conductor n p Insulator VR 38 Application of PN Junction: BJT (Bipolar Junction Transistor) P HBT (Heterojunction Bipolar Transistor) Switching diode N Junction diode J U N Tunnel diode PN Junction diode Photodiode C T N Breakdown diode Varactor diode Solar cell Photodetector Light Emitting diode & Laser Diode JFET I O Rectifiers FET (Field Effect Transistor) MOSFET memory MESFET HEMT Semiconductor Devices Semiconductor Devices Summary: Ø Three diode models Ø Diode specifications Ø Diode Applications 40 ... X' X 100 where X = the measured value X’ = the calculated value 14 Example4 I R1 1.5 k D1 R2 1.8 k VS 10 V I ideal = VS 10V = = 3.03mA R1 + R2 1.5kΩ + 1.8kΩ I prac = VS − VD1 − VD 10V... Forward Power Dissipation PD(max): I VS 10 V D1 RL 100 Choose a diode with forward power dissipation PD(max) at least 20% greater than actual power dissipation VS − VD1 10V − 0.7V I= = = 93mA RL 100 Ω PD = VD1 I =... Practical VF 0 V 0.7 V VR1 5 V 4.3 V I 5 mA 4.3 mA 10 Example1 V I VS 6V R1 10 k VR1 = VS − VD1 = 6V − 0.7V D1 = 5.3V VR1 5.3V I= = = 530μA R1 10kΩ 11 Example2 I VS 5V R1 1.2 k D1 R2 2.2 k VS