Analog EC01 PDF

 Electronic Devices And Circuits Theory 10th Edition (English, Paperback, Robert L. Boylestad, Louis Nashelsky) Design of Analog CMOS Integrated Circuits by Behzad Razavi, Bipolar Junction Transistors  Simplified Structure and Modes of Operation of BJT A terminal is connected t...

 Electronic Devices And Circuits Theory 10th Edition (English, Paperback, Robert L. Boylestad, Louis Nashelsky) Design of Analog CMOS Integrated Circuits by Behzad Razavi, Bipolar Junction Transistors  Simplified Structure and Modes of Operation of BJT A terminal is connected to each of the three semiconductor regions of the transistor, with the terminals labeled emitter (E), base (B), and collector (C). The transistor consists of two pn junctions, the emitter-base junction (EBJ) and the collector-base junction (CBJ). Depending on the bias condition (forward or reverse) of each of these junctions, different modes of operation of the BJT are obtained, as shown in Table 5.1. Physical Structure of NPN transistor Symbol of a transistor BJT Modes of Operation BJT in active mode Current flow in an NPN transistor  Operation of the npn Transistor in the Active Mode The forward bias on the emitter-base junction will cause current to flow across this junction. Current will consist of two components: electrons injected from the emitter into the base, and holes injected from the base into the emitter. It is highly desirable to have the first component (electrons from emitter to base) at a much higher level than the second component (holes from base to emitter). This can be accomplished by fabricating the device with a heavily doped emitter and a lightly doped base. The direction of iE is "out of" the emitter lead, which is in the direction of the hole current and opposite to the direction of the electron current, with the emitter current iE being equal to the sum of these two components. Since the electron component is much larger than the hole component, the emitter current will be dominated by the electron component.  Operation of the npn Transistor in the Active Mode Let us now consider the electrons injected from the emitter into the base. These electrons will be minority carriers in the p-type base region. Because their concentration will be highest at the emitter side of the base , so the injected electrons will diffuse through the base region towards the collector. through this journey some of the electrons will combine with hole, however, since base is lightly dopped the electron that are “lost” though the recombination process will be quite low. thus most of the diffusing electrons will reach the boundary of the collector –base depletion region. As collector is more positive under reverse bias voltage, the successful electrons will be swept across the CBJ depletion region into the collector to contribute the collector current iC Collector current An important observation to make here is that the magnitude of ic is independent of vCB. That is, as long as the collector is positive with respect to the base, the electrons that reach the collector side of the base region will be swept into the collector and register as collector current.  Base Current For moder npn transistors, β is in the range 50 to 200, but it can be as high as 1000 for special devices. the parameter β is called the common-emitter current gain.  Emitter Current The emitter current iE is equal to the sum of the collector current ic and the base current iB  It can be seen from Eq. (5.17) that α is a constant (for a particular transistor) that is less than but very close to unity. For instance, if β = 100, then α ≈ 0.99 Small changes in a correspond to very large changes in β. α is called the common-base current gain. Equation (5.12) indicates that the value of β is highly influenced by two factors: the width of the base region, W, and the relative dopings of the base region and the emitter region, (NA /ND). To obtain a high β(which is highly desirable since β represents a gain parameter) the base should be thin (W small) and lightly doped and the emitter heavily doped (making NA /ND small).  Basically, the forward-bias voltage vBE causes an exponentially related current ic to flow in the collector terminal. The collector current ic is independent of the value of the collector voltage as long as the collector-base junction remains reverse-biased; that is, vCB ≥0. Thus in the active mode the collector terminal behaves as an ideal constant-current source where the value of the current is determined by vBE. The base current iB is a factor 1/ β of the collector current, and the emitter current is equal. Since iB is much smaller than ic (i.e., βF > 1), iE ≈ ic. More precisely, the collector current is a fraction αF of the emitter current, with αF smaller than, but close to, unity.  Equivalent Circuit model for NPN transistor Here diode DE has a scale current ISE equal to (Is /αF) and thus provides a current iE related to vBE according to Eq. (5.18). The current of the controlled source, which is equal to the collector current, is controlled by vBE according to the exponential relationship indicated, a restatement of Eq. (5.3). This model is in essence a non-linear voltage-controlled current source. It can be converted to the current-controlled current-source mode l shown in Fig. 5.5(b) by expressing the current of the controlled source as αF iE. Equivalent Circuit model for NPN transistor A simple Circuit (active mode) A simple Circuit Ebers Moll Model of BJT Ebers Moll Model for a PNP transistor Ebers Moll Model Ebers Moll Model for active mode BJT I-V characteristics BJT I-V characteristics BJT I-V characteristics BJT I-V characteristics