* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download base region
Integrated circuit wikipedia , lookup
Molecular scale electronics wikipedia , lookup
Regenerative circuit wikipedia , lookup
Valve RF amplifier wikipedia , lookup
Thermal runaway wikipedia , lookup
Josephson voltage standard wikipedia , lookup
Power electronics wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Schmitt trigger wikipedia , lookup
Surge protector wikipedia , lookup
Nanofluidic circuitry wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Two-port network wikipedia , lookup
Opto-isolator wikipedia , lookup
Transistor–transistor logic wikipedia , lookup
Operational amplifier wikipedia , lookup
Current source wikipedia , lookup
Rectiverter wikipedia , lookup
Wilson current mirror wikipedia , lookup
College Name : Shree Swami Atmanand Saraswati Institute Of Technology(SSASIT)(076) Year : 2nd Year(3rd Sem) EC-2015 Subject Name : Electronic Devices &Circuits(EDC) (2131006) Topic Name : TRANSISTOR FUNDAMENTALS Prepared By : Desai Jay H. (140760111008) Guided By : Prof.Avni P. Lakhlani BJT STRUCTURE Basic structure of the bipolar junction transistor (BJT) determines its operating characteristics. Constructed with 3 doped semiconductor regions called emitter, base, and collector, which separated by two pn junctions. 2 types of BJT; (1) npn: Two n regions separated by a p region (2) pnp: Two p regions separated by an n region. BIPOLAR: refers to the use of both holes & electrons as current carriers in the transistor structure. Base-emitter junction: the pn junction joining the base region & the emitter region. Base-collector junction: the pn junction joining the base region & the collector region. A wire lead connects to each of the 3 regions. These leads labeled as; E: emitter B: base C: collector BASE REGION: lightly doped, & very thin EMITTER REGION: heavily doped COLLECTOR REGION: moderately doped STANDARD BJT SYMBOLS BASIC BJT OPERATION For a BJT to operate properly as an amplifier, the two pn junctions must be correctly biased with external dc voltages. Figure: shows a bias arrangement for npn BJTs for operation as an amplifier. In both cases, BE junction is forward-biased & the BC junction is reverse-biased. called forward-reverse bias. Look at this one circuit as two separate circuits, the base-emitter(left side) circuit and the collector-emitter(right side) circuit. Note that the emitter leg serves as a conductor for both circuits. The amount of current flow in the base-emitter circuit controls the amount of current that flows in the collector circuit. Small changes in base-emitter current yields a large change in collector-current. The heavily doped n-type emitter region has a very high density of conduction-band (free) electrons. These free electrons easily diffuse through the forward-based BE junction into the lightly doped & very thin p-type base region (indicated by wide arrow). The base has a low density of holes, which are the majority carriers (represented by the white circles). A small percentage of the total number of free electrons injected into the base region recombine with holes & move as valence electrons through the base region & into the emitter region as hole current (indicated by red arrows). BJT OPERATION SHOWING ELECTRON FLOW. When the electrons that have recombined with holes as valence electrons leave the crystalline structure of the base, they become free electrons in the metallic base lead & produce the external base current. Most of the free electrons that have entered the base do not recombine with holes because the base is very thin. As the free electrons move toward the reverse-biased BC junction, they are swept across into the collector region by the attraction of the positive collector supply voltage. The free electrons move through the collector region, into the external circuit, & then return into the emitter region along with the base current. The emitter current is slightly greater than the collector current because of the small base current that splits off from the total current injected into the base region from the emitter. TRANSISTOR CURRENTS The directions of the currents in both npn and pnp transistors and their schematic symbol are shown in Figure (a) and (b). Arrow on the emitter of the transistor symbols points in the direction of conventional current. These diagrams show that the emitter current (IE) is the sum of the collector current (IC) and the base current (IB), expressed as follows: IE = I C + I B BJT CHARACTERISTICS & PARAMETERS Figure shows the proper bias arrangement for npn transistor for active operation as an amplifier. Notice that the base-emitter (BE) junction is forwardbiased by VBB and the basecollector (BC) junction is reverse-biased by VCC. The dc current gain of a transistor is the ratio of the dc collector current (IC) to the dc base current (IB), and called dc beta (DC). DC = IC/IB The ratio of the dc collector current (IC) to the dc emitter current (IE) is the dc alpha. α DC = IC/IE Analysis of this transistor circuit to predict the dc voltages and currents requires use of Ohm’s law, Kirchhoff’s voltage law and the beta for the transistor; Application of these laws begins with the base circuit to determine the amount of base current. Using Kichhoff’s voltage law, subtract the VBE =0.7 V, and the remaining voltage is dropped across RB . Thus, VRB = VBB - VBE. Determining the current for the base with this information is a matter of applying of Ohm’s law. VRB/RB = IB The collector current is determined by multiplying the base current by beta. Thus, IC= βDC * IB What we ultimately determine by use of Kirchhoff’s voltage law for series circuits is that, in the base circuit, VBB is distributed across the base-emitter junction and RB in the base circuit. In the collector circuit we determine that VCC is distributed proportionally across RC and the transistor(VCE). BJT CIRCUIT ANALYSIS There are three key dc voltages and three key dc currents to be considered. Note that these measurements are important for troubleshooting. IB: dc base current IE: dc emitter current IC: dc collector current VBE: dc voltage across base-emitter junction VCB: dc voltage across collector-base junction VCE: dc voltage from collector to emitter When the base-emitter junction is forward-biased, VBE ≅ 0.7 V VRB = IBRB : by Ohm’s law IBRB = VBB – VBE : substituting for VRB IB = (VBB – VBE) / RB : solving for IB VCE = VCC – VRc : voltage at the collector with respect to the grounded emitter VRc = ICRC VCE = VCC – ICRC : voltage at the collector with respect to the emitter The voltage across the reverse-biased collector-base junction VCB = VCE – VBE where IC = βDCIB COLLECTOR CHARACTERISTIC CURVES Gives a graphical illustration of the relationship of collector current and VCE with specified amounts of base current. With greater increases of VCC , VCE continues to increase until it reaches breakdown, but the current remains about the same in the linear region from 0.7V to the breakdown voltage. Sketch an ideal family of collector curves for the circuit in Figure for IB = 5 μA to 25 μA in 5 μA increment. Assume βDC = 100 and that VCE does not exceed breakdown. SKETCH AN IDEAL FAMILY OF COLLECTOR CURVES FOR THE CIRCUIT IN FIGURE FOR IB = 5 ΜA TO 25 ΜA IN 5 ΜA INCREMENT. ASSUME ΒDC = 100 AND THAT VCE DOES NOT EXCEED BREAKDOWN. IC = βDC IB IB 5 μA 10 μA 15 μA 20 μA 25 μA IC 0.5 mA 1.0 mA 1.5 mA 2.0 mA 2.5 mA CUTOFF With no IB , the transistor is in the cutoff region and just as the name implies there is practically no current flow in the collector part of the circuit. With the transistor in a cutoff state, the full VCC can be measured across the collector and emitter(VCE). Cutoff: Collector leakage current (ICEO) is extremely small and is usually neglected. Base-emitter and base-collector junctions are reverse-biased. SATURATION Once VCE reaches its maximum value, the transistor is said to be in saturation. Saturation: As IB increases due to increasing VBB, IC also increases and VCE decreases due to the increased voltage drop across RC. When the transistor reaches saturation, IC can increase no further regardless of further increase in IB. Baseemitter and base-collector junctions are forward-biased. DC LOAD LINE The dc load line graphically illustrates IC(sat) and cutoff for a transistor. Active region of the transistor’s operation. DC load line on a family of collector characteristic curves illustrating the cutoff and saturation conditions. MAXIMUM TRANSISTOR RATINGS A transistor has limitations on its operation. The product of VCE and IC cannot be maximum at the same time. If VCE is maximum, IC can be calculated as IC PD (max) VCE Ex 4-5 A certain transistor is to be operated with VCE = 6 V. If its maximum power rating is 250 mW, what is the most collector current that it can handle? IC PD (max) VCE 250 mW 41.7 mA 6V THE BJT AS A SWITCH A transistor when used as a switch is simply being biased so that it is in cutoff (switched off) or saturation (switched on). Remember that the VCE in cutoff is VCC and 0V in saturation. CONDITIONS IN CUTOFF & SATURATION A transistor is in the cutoff region when the base-emitter junction is not forward-biased. All of the current are zero, and VCE is equal to VCC VCE(cutoff) = VCC When the base-emitter junction is forward-biased and there is enough base current to produce a maximum collector current, the transistor is saturated. VCC VCE ( sat ) The formula for I C ( sat ) collector saturation current is RC The minimum value of base current needed to produce saturation is I B (min) I C ( sat ) DC Transistor Construction There are two types of transistors: • pnp • npn pnp The terminals are labeled: • E - Emitter • B - Base • C - Collector npn 26 Transistor Operation With the external sources, VEE and VCC, connected as shown: • The emitter-base junction is forward biased • The base-collector junction is reverse biased 27 Currents in a Transistor Emitter current is the sum of the collector and base currents: IE IC IB The collector current is comprised of two currents: IC IC I CO majority minority 28 Operating Regions • Active – Operating range of the amplifier. • Cutoff – The amplifier is basically off. There is voltage, but little current. • Saturation – The amplifier is full on. There is current, but little voltage. 29 Approximations Emitter and collector currents: I C I E Base-emitter voltage: VBE 0.7 V (for Silicon) 30 Alpha (a) Alpha (a) is the ratio of IC to IE : αdc IC IE Ideally: a = 1 In reality: a is between 0.9 and 0.998 Alpha (a) in the AC mode: αac ΔI C ΔI E 31 Beta () represents the amplification factor of a transistor. ( is sometimes referred to as hfe, a term used in transistor modeling calculations) In DC mode: βdc IC IB In AC mode: ac IC I B 32 VCE constant Beta () Relationship between amplification factors and a α β β1 α β α 1 Relationship Between Currents I C βI B I E (β 1)I B 33 Power Dissipation Common-base: PCmax VCB I C Common-emitter: PCmax VCE I C Common-collector: PCmax VCE I E 34 LOAD LINE FOR EMITTER-BIAS CIRCUIT IC I C (sat ) IC(sat) VCC (VEE ) VCC VEE RC RE RC RE VCE (off ) VCC VEE VCC VEE VCE(off) VCE 35 FIG 7.28 EMITTER-BIAS CHARACTERISTICS. (1) +VCC IC Circuit recognition: A split (dualpolairty) power supply and the base resistor is connected to ground. RC IB Q1 Input RB RE IE -VEE Advantage: The circuit Q-point values are stable against changes in hFE. Output Disadvantage: Requires the use of dual-polarity power supply. Applications: Used primarily to bias linear amplifiers. 36 FIG 7.28 EMITTER-BIAS CHARACTERISTICS. (2) +VCC IC Load line equations: I C (sat ) RC VCC VEE RC RE VCE (off ) VCC VEE IB Q1 Input Output Q-point equations: I CQ RB RE IE -VEE VBE VEE hFE RB hFE 1 RE VCEQ VCC I CQ RC RE VEE 37 FIG 7.31 EMITTER-FEEDBACK BIAS. +VCC RB IC RC VCC VBE IB RB hFE 1 RE I CQ hFE I B I E hFE 1 I B IB VCEQ VCC I C RC I E RE IE RE VCC I CQ RC RE 38 THANK YOU