Bipolar junction transistors. Third Semester Course code : 15EECC202 Analog electronic circuits (AEC) Team: Dr. Nalini C Iyer, R.V. Hangal, Sujata N, Prashant A, Sneha Meti AEC Team, Faculty, School of Electronics Engineering, KLE Tech University, Hubballi
Lesson Schedule (07 Hrs) 1. Dependence of ic on the collector voltage-the early effect, the common emitter characteristics 2. DC load line and bias point, base-bias 3. Collector to base bias, voltage divider, comparison of bias circuit, 4. small signal models-(the transfer characteristics) of bipolar transistors, two port modeling of amplifiers. 5. AC analysis of BJT circuits-coupling and bypass capacitor, 6. Common emitter circuit analysis, CE circuit with un-bypassed emitter resistor 7.The amplifier gain, operation as a switch
3.1. Device Structure and Physical Operation Figure 3.1. shows simplified structure of BJT. Consists of three semiconductor regions: emitter region (n-type) base region (p-type) collector region (n-type) Type described above is referred to as npn. However, pnp types do exist. Microelectronic circuits, fifth edition, Adel s Sedra and Smith Chapter 3, Page NO : 160-201
3.1.1 Simplified Structure and Modes of Operation Transistor consists of two pn-junctions: emitter-base junction (EBJ) collector-base junction (CBJ) Operating mode depends on biasing. active mode used for amplification cutoff and saturation modes used for switching.
3.1.1 Simplified Structure and Modes of Operation Figure 3.1: A simplified structure of the npn transistor.
3.1.1 Simplified Structure and Modes of Operation Table : BJT modes of operation Figure 3.2: A simplified structure of the pnp transistor.
3.1.2. Operation of the npn- Transistor in the Active Mode Analog electronics circuits (15EECC2020 Active mode is most important. Two external voltage sources are required for biasing to achieve it. Refer to Figure 3.3. Figure 3.3: Current flow in an npn transistor biased to operate in the active mode. (Reverse current components due to drift of thermally generated minority carriers are not shown.)
Current Flow Forward bias on emitter-base junction will cause current to flow. This current has two components: electrons injected from emitter into base holes injected from base into emitter. It will be shown that first (of the two above) is desirable. This is achieved with heavy doping of emitter, light doping of base.
Current Flow emitter current (i E ) is current which flows across EBJ Flows out of emitter lead minority carriers in p-type region. These electrons will be injected from emitter into base. Opposite direction. Because base is thin, concentration of excess minority carriers within it will exhibit constant gradient.
Straight line represents constant gradient. n p n p0 ( x) concentration of minority carriers a position x (where 0 represents EBJ boundary) n thermal-equilibrium value of minority carrier (electron) concentration in base regionn vbe voltage applied across base-emitter junctionnp 0 V thermal voltage (constant) n T p0 vbe / VT (eq6.1) 0 n e n p p0 p0 p0
Current Flow Some diffusing electrons will combine with holes (majority carriers in base). Base is thin, however, and recombination is minimal. Recombination does, however, cause gradient to take slightly curved shape. The straight line is assumed.
The Collector Current It is observed that most diffusing electrons will reach boundary of collector-base depletion region. Because collector is more positive than base, these electrons are swept into collector. collector current (i C ) is approximately equal to I n. i C = I n Magnitude of i C is independent of v CB. As long as collector is positive, with respect to base. saturation current (I S ) is inversely proportional to W and directly proportional to area of EBJ. Typically between 10-12 and 10-18 A Also referred to as scale current. Analog electronics circuits (15EECC202
The Base Current base current (i B ) composed of two components: i b1 due to holes injected from base region into emitter. i b2 due to holes that have to be supplied by external circuit to replace those recombined. common-emitter current gain (β.) is influenced by two factors: width of base region (W) relative doping of base emitter regions (N A /N D ) High Value of β thin base (small W in nano-meters) lightly doped base / heavily doped emitter (small N A /N D )
The Emitter Current All current which enters transistor must leave. i E = i C + i B
3.1.3. Structure of Actual Transistors Figure 3.6 shows a more realistic BJT cross-section. Collector virtually surrounds entire emitter region. This makes it difficult for electrons injected into base to escape collection. Device is not symmetrical. As such, emitter and collector cannot be interchanged. Device is uni-directional. Figure 3.6: Cross-section of an npn BJT.
Two questions must be asked to determine whether BJT is in saturation mode, or not: Is the CBJ forward-biased by more than 0.4V? Is the ratio i C /i B less than β.? 3.1.4. Operation in Saturation Mode
3.2.1. Circuit Symbols and Conventions Figure 3.13: Voltage polarities and current flow in transistors biased in the active mode.
The Collector-Base Reverse Current (I CB0 ) Previously, small reverse current was ignored. This is carried by thermally-generated minority carriers. However, it does deserve to be addressed. The collector-base junction current (I CBO ) is normally in the nano-ampere range. Many times higher than its theoreticallypredicted value.
3.2.2. Graphical Representation of Transistor Characteristics Analog electronics circuits (15EECC202 Figure 3.15/16: (left) The i C -v BE characteristic for an npn transistor. (right) Effect of temperature on the i C -v BE characteristic. Voltage polarities and current flow in transistors biased in the active mode.
Analog electronics circuits (15EECC202 When operated in active region, practical BJT s show some dependence of collector current on collector voltage. As such, i C -v CE characteristic is not straight. 3.2.3. Dependence of i C on Collector Voltage The Early Effect Fig. V CE VS I C characteristics
Figure 3.18: Large-signal equivalent-circuit models of an npn BJT operating in the active mode in the common-emitter configuration with the output resistance r o included.
CE configuration Figure 3.19: Common-emitter characteristics. (a) Basic CE circuit; note that in (b) the horizontal scale is expanded around the origin to show the saturation region in some detail. A much greater expansion of the saturation region is shown in (c).
BJT as switch Case 1 : When V i < 0.6 V the transistor is off, switch is open, the transisor is operated in cut off region V 0 = V CE = V CC as I B = 0 Case 2: When V i >> 0.6 V the transistor is ON, switch is closed, the transisor is operated in saturation region V 0 = V CE sat = V CC I C R C = 0.3 V Example 3.3 (Solve)
3.4. Applying the BJT in Amplifier Design An amplifier may be designed by transistor and series resistance. However, it is necessary to model the voltage transfer characteristic (VTC) Appropriate biasing is important to ensure linear gain, and appropriate input voltage swing. Small-signal model is employed to model the amp s operation.
Fig 3.31 (a)simple BJT amplifier with input and output VCE (b) The voltage transfer characteristics(vtc) of an amplifier
Figure 3.32: Biasing the BJT amplifier at a point Q located on the active-mode segment of the VTC.
Analog electronics circuits (15EECC202 Figure 3.33 BJT amplifier biased at a point Q, with small signal voltage V BE superimposed on the dc Bias voltage V BE.
3.6.1. Three-Basic Configurations
3.6.3. The Common-Emitter (CE) Amplifier Of three configurations, the CE amplifier is most widely used. Figure 6.50(a) shows a common-emitter amplifier with biasing arrangement omitted. signal course (v sig ) source resistance (R sig ) input resistance (R in ) gain (A vo ) output resistance (R o ) transconductance (G v )
Common-Emitter Amplifier Figure 3.50: (a) Common-Emitter Amplifier fed with a signal v sig from a generator with a resistance R sig
Summary Depending on the bias condition on its two junctions, the BJT can operate in one of three possible modes: cut-off (both junctions reverse biased) active (the EBJ forward-biased and CBJ reversed) saturation (both junctions forward biased) For amplifier applications, the BJT is operated in the active mode. Switching applications make use of the cutoff and saturation modes. A BJT operating in the active mode provides a collector current i C = I S exp{v BE /V T }. The base current i B = i C /, and emitter current i E = i C + i B.
Summary To ensure operation in the active mode, the collector voltage of an npn-transistor must be kept higher than approximately 0.4V below the base voltage. For a pnp-transistor, the collector voltage must be lower than approximately 0.4V above the base voltage. Otherwise, the CBJ becomes forward-biased and the transistor will enter saturation. At a constant collector current, the magnitude of the base emitter voltage decreases by about 2mV for every 1 O C rise in temperature. The BJT will be at the edge of saturation when v CE is reduced to about 0.3V.
Summary In the active mode, i C shows a slight dependence on v CE. This phenomenon, known as the Early Effect, is modeled by ascribing a finite output resistance to the BJT: r o = V A /I C where V A is the Early Voltage and I C is the dc collector current without the Early Effect taken into account. The dc analysis of transistor circuits is generally simplified by assuming V BE = 0.7V. To operate as a linear amplifier, the BJT is biased in the active region and the signal v be is kept small (v be << V T ). Bias design seeks to establish a dc collector current that is as independent of as possible.
What is biasing Need for biasing What is Stabilization Thermal Runaway Derivation of S ICO Bisaing techniques Fixed biasing Emitter resistor bias Base emitter resistor bias Voltage divider resistor bias Approximate analysis Accurate method Exercise 4.7 and 4.8 Biasing Electronic devices and circuits theory Boylestad and Nashelsky Page No 175-78
Voltage divider Biasing Approximate analysis V B = R 2 V CC / R 1 + R2 V E = V B V BE I E = V E / R E I CQ = I E as I B = 0 V CE = V CC I C R C I E R E Where I E is emitter current I B is base current I C is collector current R C collector resistor R E emitter resistor R 1 R 2 voltage divider circuit
Voltage divider Biasing Accurate (Exact) analysis
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Two port device and hybrid model For the hybrid equivalent model, the parameters are defined at an operating point. The quantities h fe, h re, h oe, h ie are called hybrid parameters and are the components of a small signal equivalent circuit. The description of the hybrid equivalent model will begin with the general two port system. Electronic devices and circuits theory Boylestad and Nashelsky Page No 245 48, 312-133, 314-315 voltage divider circuit
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Contd Essentially, the transistor model is a three terminal two port system. The h parameters, however, will change with each configuration. To distinguish which parameter has been used or which is available, a second subscript has been added to the h parameter notation
Analog electronics circuits (15EECC202 Contd Normally h re is a relatively small quantity, its removal is approximated by hre and hre V 0 = 0 resulting in a short circuit equivalent. The resistance determined by 1/ h oe is often large enough to be ignored in comparison to a parallel load, permitting its replacement by an open circuit equivalent
Analog electronics circuits (15EECC202 Analysis of transistor amplifier using h-parameter
Contd For analysis of transistor amplifier we have to determine the following terms: Current Gain A I = I 0 / I i = I C / I B Voltage gain Av = V 0 / Vi = - h fe R L / h ie Input impedance Z i = V i / I i = h ie Output impedance Z 0 = V o / I o = 1/ hoe
Voltage divider circuit and its equivalent AC Circuit Figure: 1Voltage divider circuit Figure: 2 AC equivalent circuit
Voltage divider circuit and its equivalent Approximate h model Figure:3 Equivalent Approximate h model for voltage divider circuit Figure: 4 BJT equivalent h Model
Approximate h model Derivation of Z i Z o, A V and A I Input impedance Z i = R 1 R 2 h ie Output impedance Z o = R c Voltage Gain AV= - h fe (R c 1/ h oe ) / h ie Current gain AI = hfe (R 1 R 2 )/ (R 1 R 2 ) + h ie