Lecture 9 - Review on Diode Diode Convention Lecture 09...

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1 ECTE290 Spring 2007 Lecture 09 2 Review on Diode Diode Convention In solving diode problems, first guess and then test the following: a) Guess diode ON, set v D = 0, test if i D > 0 b) Guess diode OFF, set i D = 0, test if v D < 0 Diode State: a) ON Æ v D = 0, i D >0 b) OFF Æ i D = 0, v D <0 3 Outline: BJT (Bipolar Junction Transistor) Three Terminal Devices: Transistors ± BJT Fundamental Equations ± Three Working Regions ± DC Analysis– Common Emitter Circuit ± Transistor as an Amplifier or a Switch ± AC Analysis- Small Signal Model 4 The BJT (Bipolar Junction Transistor) ± Widely used in amplifiers and switching circuits. ± The BJT is a 3-terminal device, where a current or voltage on one terminal (the BASE) has a proportional influence on the current or voltage on another terminal (the COLLECTOR). Simply: Transistor
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2 5 BJT Construction Diode BJT An NPN transistor can be considered as two diodes with a shared anode region. 6 Two Types of BJT ± There are two types of BJT transistor. The npn and the pnp. The circuit symbols for the npn and pnp BJT’s are: npn collector emitter base I B I E I C small current large current + V BE - Our focus We will concentrate only on npn transistor 7 How BJT Works? npn collector emitter base I B I E I C small current large current + V BE - base 8 Illustrative Example: Common Emitter Amplifier mA I I B C 68 . 7 = = β DC equivalent circuit ± This configuration is also called the COMMON EMITTER Amplifier because the Emitter terminal of the BJT is common to both the input and the output port of the amplifier. ± Find I B , I C , V CE ? This equation is true only when the BJT operates in the active region Now if V BB > V BE ie V BB > 0.7 then A R V R V V I B BB B BE BB B µ 8 . 76 7 . 0 = = = β =100 v R I V V C C CC CE 32 . 7 = =
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3 9 Basic Equations for NPN BJT’s ± Collector Current The Collector current is given by the following equation: II e I e CS V V S V V BE T T =− 1 where, I S = Saturation current, Constant for a particular BJT, typically in the range 10 -12 to 10 -18 Amps. V T = Thermal Voltage, typically 25mV at room temperature (20 degrees celsius). V BE = Voltage between the BASE and EMITTER 10 Basic Equations for NPN BJT’s ± Base Current The Base current is a fraction of the collector current, but directly proportional, in the active region, ie: BC ∴= I I B C β the constant of proportionality is 1/ β or I I e B S V V T = β C = β B β is called the COMMON EMITTER CURRENT GAIN, typically 50-200, but can be as high as 1000 for special units 11 Basic Equations for NPN BJT’s ± Emitter Current III EB C =+ β C B I I = since = I CE β β+1 α E α is called the COMMON BASE CURRENT GAIN I B I C I E Output circuit Input circuit C C C E I I I I + = + = 1 12 Basic Equations for NPN BJT’s as β >> 1, α ≅ 1 for example if β= 100, α=0.99, hence, Thus β= α 1 − α E C I I Finally, the voltage drop across the forward biased BASE EMITTER junction, V BE , is between 0.5 and 0.7V (for small current devices made of silicon). The normal assumption is to assume: VV = 07 .
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This note was uploaded on 10/27/2009 for the course ECTE 290 taught by Professor Zhengli during the Three '09 term at University of Wollongong, Australia.

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Lecture 9 - Review on Diode Diode Convention Lecture 09...

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