Điện tử cơ bản - Công thức tính phân cực tranzitor P2

CHAPTER 4 BIPOLAR JUNCTION TRANSISTORS (BJTs) Chapter Outline 4.1 Device Structure and Physical Operation 4.2 Current-Voltage Characteristics 4.3 BJT Circuits at DC 4.4 Applying the BJT in Amplifier Design 4.5 Small-Signal Operation and Models 4.6 Basic BJT Amplifier Configurations 4.7 Biasing in BJT Amplifier Circuits 48 Discrete Circuit BJT Amplifiers NTUEE Electronics – L. H. Lu 4-1 4 . 8 Discrete - Circuit BJT Amplifiers 4.1 Device Structure and Physical Operation Physical structure of bipolar junction transistor (BJT) Both electrons and holes participate in the conduction process for bipolar devices. BJT consists of two pn junctions constructed in a special way and connected in series, back to back. The transistor is a three-terminal device with emitter, base and collector terminals. From the physical structure, BJTs can be divided into two groups: npn and pnp transistors. Modes of operation The two junctions of BJT can be either forward or reverse-biased. The BJT can operate in different modes depending on the junction bias. The BJT operates in active mode for amplifier circuits. Switching applications utilize both the cutoff and saturation modes. NTUEE Electronics – L. H. Lu 4-2 Mode EBJ CBJ Cutoff Reverse Reverse Active Forward Reverse Saturation Forward Forward Operation of the npn transistor in the active mode Electrons in emitter regions are injected into base due to the forward bias at EBJ. Most of the injected electrons reach the edge of CBJ before being recombined if the base is narrow. Electrons at the edge of CBJ will be swept into collector due to the reverse bias at CBJ. Emitter injection efficiency (  ) = i En / ( i En + i Ep ) Base transport factor (  T ) = i Cn / i En Common-base current gain (  ) = i Cn / i E =  T < 1 Terminal currents of BJT in active mode: i E (emitter current) = i En (electron injection from E to B) + i Ep (hole injection from B to E) i C (collector current) = i Cn (electron drift) + i CBO (CBJ reverse saturation current with emitter open) i B (base current) = i B1 (hole injection from B to E) + i B2 (recombination in base region) NTUEE Electronics – L. H. Lu 4-3 Terminal currents: Collector current: Base current:  Hole injection into emitter due to forward bias:  Eelectron-hole recombination in base:  Total base current: Emitter current: TBETBE Vv S Vv B inBE BnBEBnBECnC eIe WN nqDA WnqDAdxxdnqDAii // 2 /)0(/)(  TBE Vv pEE ipEE EpEEB e LN nqDA dxxdpqDAi / 2 1 /)(  TBE Vv SC CBCE e Ii iiii / 1        TBE Vv nB iE nBEnnB e N qWnA WnqAQi / 2 2 2 /)0( 2 1 /            C Vv nnBpEE B nB pE SBBB i e D W L W N N D D Iiii TBE  / 2 21 ) 2 1 ( NTUEE Electronics – L. H. Lu 4-4    Large-signal model and current gain for BJT in active region Common-emitter current gain: )1/() 2 1 ( 1 2      nnBpEE B nB pE B C D W L W N N D D i i Common-base current gain i B i C i E 1  (  +1) 1 Common-emitter current gain i B i E i C  (1  ) Common-base current gain: The structure of actual transistors In modern process technologies, the BJT utilizes a vertical structure. Typically,  is smaller and close to unity while  is large. NTUEE Electronics – L. H. Lu 4-5 )1/(      Operation of the npn transistor in the saturation mode Saturation mode: both EBJ and CBJ are forward biased Carrier injection from both emitter and collector into base Base minority carrier concentraiton change accordingly  leading to reduced slope as v BC increases Collector current drops from the value in active mode for negative v CB For a given v BE , i C drops sharply to zero at v CB around 0.5 V and v CE around 0.2 V. BJT in saturation: V CEsat = 0.2 V Current gain reduces from  to  forced :   saturation B C forced i i NTUEE Electronics – L. H. Lu 4-6 n p0 n p0 exp(v BE /V T ) n p0 exp(v BC /V T ) v BC increases Ebers-Moll model In EM model, the EBJ and CBJ are represented by two back to back diodes i DE and i DC . The current transported from one junction to the other is presented by  F (forward) and  R (reverse). EM model can be used to describe the BJT in any of its possible modes of operation. EM model is used for more detailed dc analysis which can not be performed by the simplified models. The diode currents: The terminal currents: Application of the EM model The forward active mode: i i CEB iii   SSCRSEF III         1 1 / V v I I )1( /  TBE Vv SEDE eIi )1( /  TBC Vv SCDC eIi DEFDCC iii     DCRDEE iii    The saturation mode: NTUEE Electronics – L. H. Lu 4-7 i DE i DC  F i DE  R i DC i C i E i B          F S Vv F S E Ie I i TBE  1 1 /           1 1 / R S V v SC I e I i T B E           RF S Vv F S B Ie I i TBE  11 / TBC TBE Vv S Vv SEE eIeIi / /  TBC TBE Vv SC Vv SC eIeIi / /  TBC TBE Vv RSC Vv FSEB eIeIi / / )1()1(   The cutoff mode  I CBO (CBJ reverse current with emitter open-circuited) I CBO = (1  R  F )I SC Both EBJ and CBJ are reverse-biased. In real case, reverse current depends on v CB .  I CEO (CBJ reverse current with base open-circuited) I CEO = I CBO /(1  F )   F is always smaller than unity such that I CEO > I CBO . CBJ current flows from (C to B) so CBJ is reverse-biased.  EBJ current flows from (E to B) so EBJ is slightly forward - biased.  EBJ current flows from (E to B) so EBJ is slightly forward biased. NTUEE Electronics – L. H. Lu 4-8  + EC B (2)  R I SC I SC (1) i C i E = 0 i B (3)  R I SC  R  F I SC (4) i C = I CBO = (1  R  F )I SC (5) i B = (  R )I SC + (  F )i DE = 0  i DE = I SC (  R ) / (  F ) (5) E C B (2)  R I SC I SC (1) i C i B = 0 i E  + (3) i DE  F i DE (4) i C = I SC +  F i DE = I SC (1  R  F )/ (1  F )  I CEO = I CBO / (  F ) (6) The pnp transistor Transistor structure:  emitter and collector are p-type  base is n-type Operation of pnp is similar to that of npn Operation of pnp in the active mode Collector current: Base current: Emitter current: L ildld tifBJTi ti i TEB Vv SC eIi /   / CB ii  BCE iii   L ar g e-s ig na l mo d e l an d curren t g a i n f or BJT i n ac ti ve re gi on NTUEE Electronics – L. H. Lu 4-9 Common-base current gain i B i C i E 1  (  +1) 1 Common-emitter current gain i B i E i C  (1  ) 4.2 Current-Voltage Characteristics Circuit symbols, voltage polarities and current flow Terminal currents are defined in the direction as current flow in active mode. Negative values of current or voltage mean in opposite polarity (direction). Summary of the BJT current-voltage relationships in the active mode The values of the terminal currents for a BJT in active mode solely depend on the junction voltage of EBJ. The ratios of the terminal currents for a BJT in active mode are constant. The current directions for npn and pnp transistors are opposite. NTUEE Electronics – L. H. Lu 4-10 TBE Vv SC eIi /  TBE Vv SC B e Ii i /   TBE Vv SC E e Ii i /   TEB Vv SC eIi /  TEB Vv SC B e Ii i /   TEB Vv SC E e Ii i /   BCE iii   1          1 pnp transistornpn transistor [...]... L H Lu 4-2 4 BJT small-signal models Two models are exchangeable and does not affect the analysis result The hybrid- model  Typically used as the emitter is grounded Neglect ro The T model  Typically used as the emitter is not grounded Neglect ro NTUEE Electronics – L H Lu 4-2 5 4.6 Basic BJT Amplifier Configuration Three basic configurations Common-Emitter (CE) Common-Base (CB) Common-Collector... short-circuit  Replace the BJT with its small-signal model for ac analysis small signal  The circuit parameters in the small-signal model are obtained based on the value of IC Complete amplifier circuit DC equivalent circuit NTUEE Electronics – L H Lu AC equivalent circuit 4-3 3 The common-emitter (CE) amplifier The common-emitter amplifier with an emitter resistance NTUEE Electronics – L H Lu 4-3 4... (large capacitance) are considered short-circuit  All internal capacitive effects (small capacitance) are considered open-circuit  Midband gain is nearly constant and is evaluated by small-signal analysis  The bandwidth is defined as BW = fH – fL  A figure-of-merit for the amplifier is its gain-bandwidth product defined as GB = |AM|BW NTUEE Electronics – L H Lu 4-3 6 ... and the iC starts to decrease for vBC < 0.4V  I-V characteristics in the saturation mode and vCEsat is considered a constant ( 0.2 V) Current gain (): large-signal   iC/iE and small-signal (incremental)   iC/iE NTUEE Electronics – L H Lu 4-1 2 Common-emitter output characteristics (I) iC versus vCE plot with various vBE as parameter Common-emitter current gain is defined as  = iC / iB The... RB /   RE (1  1 /  ) RC is chosen to ensure the BJT in active (VCE > VCEsat) A two-power-supply version of the classical bias arrangement Two power supplies are needed Similar dc analysis VEE  VBE BJT operating point: I C  RB /   RE (1  1 /  ) NTUEE Electronics – L H Lu 4-3 1 Biasing using a collector-to-base feedback resistor A single power supply is needed RB ensures the BJT in active... 100 V) Common-emitter output characteristics (II) Plot of iC versus vCE with various iB as parameter BJT in active region acts as a current source with high (but finite) output resistance The cutoff mode in common-emitter configuration is defined as iB = 0 Current gain: large-signal dc  iC/iB and ac  iC/iB NTUEE Electronics – L H Lu Early effect breakdown 4-1 3 Saturation of common-emitter configuration... Electronics – L H Lu 4-3 4 The common-base (CB) amplifier The common-collector (CC) amplifier NTUEE Electronics – L H Lu 4-3 5 The amplifier frequency response The gain falls off at low frequency band due to the effects of the coupling and by-pass capacitors The gain falls off at high frequency band due to the internal capacitive effects in the BJTs Midband:  All coupling and by-pass capacitors (large capacitance)... 4-1 1 Common-base output characteristics Early effect breakdown iC versus vCB plot with various iE as parameter is known as common-base output characteristics The slope indicates that iC depends to a small extent on vCB  Early effect iC increases rapidly at high vCB  breakdown BCJ is slightly forward-biased for 0.4V < vCB < 0 No significant change is observed in iC The BJT still exhibits I-V... small-signal or linear equivalent circuit where dc components are not included vo RL  Avo vi RL  Ro RL Overall voltage gain: Gv  vo  Rin Av  Rin Avo Rin  Rsig RL  Rso vsig Rin  Rsig Voltage gain: Av  NTUEE Electronics – L H Lu 4-2 6 The common-emitter (CE) amplifier Characteristic parameters of the CE amplifier  Input resistance: Rin  r  Output resistance: Ro  RC || ro  RC  Open-circuit... Electronics – L H Lu 4-2 8 The common-base (CB) amplifier Characteristic parameters of the CE amplifier (by neglecting ro)  Input resistance: Rin  re  Output resistance: Ro  RC  Open-circuit voltage gain: Avo  g m RC  Voltage gain: Av  g m ( RC || RL )  Overall voltage gain: Gv  re g m ( RC || RL ) re  Rsig CE amplifier can provide high voltage gain Input and output are in-phase due to p p . v BE = 0.7 V and v CE = 0.2 V  i C /i B =  forced <  NTUEE Electronics – L. H. Lu 4-16 Equivalent circuit models NTUEE Electronics – L. H. Lu 4-17 DC analysis of BJT circuits Step 1:. another assumption if necessary Example 4.4 Example 4.5 NTUEE Electronics – L. H. Lu 4-18 Example 4.9 Example 4 11 Example 4 . 11 NTUEE Electronics – L. H. Lu 4-19 4.4 Applying the BJT in Amplifier. i B (base current) = i B1 (hole injection from B to E) + i B2 (recombination in base region) NTUEE Electronics – L. H. Lu 4-3 Terminal currents: Collector current: Base current:  Hole injection