Data Acquisition Design Laboratory
Data Acquisition Design Laboratory 051 080
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This 6 page Class Notes was uploaded by Russel Osinski V on Friday October 23, 2015. The Class Notes belongs to 051 080 at University of Iowa taught by Edwin Dove in Fall. Since its upload, it has received 29 views. For similar materials see /class/228030/051-080-university-of-iowa in Biomedical Engineering at University of Iowa.
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Date Created: 10/23/15
051 080 Bioelectrical Design Spring 2006 The Bipolar Junction Transistor Topics Bipolar junction transistor BJT BJT types operating modes of the BJT BJT con gurations analysis of internal currents the EbersMoll model the BJT as an ampli er operational quiescent point analysis Transistors Before the invention of transistors electrical signals were ampli ed using vacuum tubes which are large heavy and expensive components Electronic instruments and appliances were simple and large and their use was limited Transistors were invented in the 1940 s Their rst use was in ampli er circuits for analog signals where they replaced vacuum tubes The rst widespread electronic device employing transistors was the transistor radio Transistors are small and versatile and they are inexpensive to manufacture They rapidly made their way into existing electronic systems and many new systems were designed to take advantage of their properties The introduction of integrated circuit IC fabrication techniques further increased the signi cance of transistors because with these techniques thousands of transistors can be implemented on a single semiconductor chip They remained a key component in the design of analog circuits but more importantly they became the building block of all digital circuits Today transistors are in all electronic systems from home appliances to computers The Bipolar Junction Transistor BJT The BJT is the most common transistor It consists of three sections of semiconductors an emitter a base and a collector Figure 31 In an npn type BJT the emitter and the collector are made of ntype semiconductors and the base is made of a p Emitter Base Collector E C E C E i B B a npn type BJT Emitter Base Collector E C E n C 5 4 B b imptype BJT Figure 31 Schematic Diagram and Circuit Symbol ofthe BJT Re vised 020 72006 0 Poroy 051 080 Bioelectrical Design Spring 2006 type semiconductor In a pnp type BIT it is the other way round The three sections of a BIT form two pn junctions the emitter base junction and the collector base junction Individually these junctions are not different from the pn junction in a diode The unique characteristics of the BIT originate from an interaction between these two junctions Operating modes of the BIT The operating mode of a BIT depends on how its junctions are biased Table 31 The BIT is biased to operate in the active mode in applications where it is used as an ampli er In the cut o and saturation modes the BIT behaves like an open and closed switch respectively Most BITs in digital circuits logic gates memory operate in these two modes The reverse active mode is rarely used and is listed here for reference I CVCI S C I CVCI SC fCVCfS C RCVCI SC I CVCI SC Table 31 Operating Modes ofthe BIT BIT con gurations In a typical transistor circuit the transistor is connected to an input circuit and an output circuit or load Figure 32 Additional components are often necessary to bias the BIT One of the terminals of the BIT E B or C is connected to both the input and the output circuit The con guration of a BIT in a circuit is named after this common terminal Thus we speak of i i itt i i 1 and i i i Z t configurations Input circuit Output circuit and biasing and biasing a Model representing all h BIT in commonbase BIT con gurations con guration Figure 32 BIT Con gurations Re vised 020 72006 0 Poroy 051 080 Bioelectrical Design Spring 2006 Analysis of internal currents The internal currents in an npntype BJT biased to operate in the active mode are shown in Figure 33 The emitterbase junction is forward biased by the voltage V1 In the emitter the electron current resulting from this bias is larger than the hole current indicated by the thickness of the arrows representing the currents because in a BJT the emitter is more heavily doped than the base The hole current follows the path that it would follow in a forward biased diode The electron current however behaves differently than it would in a forward biased diode In a diode all of the electrons entering the pside would recombine with the holes there In Figure 33 it is indicated that only a fraction of the electrons from the emitter recombine with the holes in the base Most of the electrons from the emitter travel through the base and reach the collector There are two reasons for that 1 As mentioned above the base is more lightly doped than the emitter Thus the number of holes in the base is not enough to accept all the electrons diffusing from the emitter 2 The base in a BJT is so narrow that its width is comparable to the di usion length of the electrons from the emitter The diffusion length is the average distance that charge carriers travel after they diffuse across a pn junction and before they recombine with the charge carriers on the opposite side Hence most of the electrons from the emitter reach the basecollector junction before they have a chance to recombine with the holes in the base Once they reach the basecollector junction they are accelerated into the collector because of the charge buildup on both sides of the junction The basecollector junction is reverse biased by the voltage V2 Therefore there is a negative charge buildup on the base side which pushes the emitter electrons into the collector and there is a positive charge buildup on the collector side which pulls them in Emitter Base Collector 11 p n l t d39ff 39 1C IE 3 Li 1 1quot F ng ICI emitter electrons 2 IE total emitter current rang e slum FeaChing the conecmr 8 base junction 3 recombination recombination Icu reverse holes diffusing through saturation current the emitterbasejunction T 1B V Figure 33 Internal Currents in an npntype BIT in the Active Mode Re vised 020 72006 0 Poroy 051 080 Bioelectrical Design Spring 2006 The interaction between the emitter and the collector is basic principle behind the operation of the BJT A BIT in which the base is as wide as and as heavily doped as the emitter would not work because the electrons from the emitter would never reach the collector This kind of a BJT would be nothing but two diodes put together back to back The fraction of the total emitter current that reaches the collector is known as the large signal forward current gain ocF Thus 101 F 39 IE The collector current Ic has two components ICI and Ice the reverse saturation current Since the reverse saturation current is much smaller than the forward currents we can write C C1C0 z Cl C m 01F 15 Applying KCL to the BJT in Figure 33 yields 8 E CzIE aFJE B z 1ap39E BJTs are manufactured such that ocF is very close to unity Its value is typically larger than 0900 and can be as high as 0997 Thus in an npntype BJT most of the current owing out of the emitter comes from the collector with only a small contribution from the base Similarly in a pnptype BJT most of the current entering at the emitter leaves through the collector with only a small portion eXiting through the base The Ebers Moll Model The above discussion of the internal currents in a BJT is valid only for the active mode The EbersMoll model Figure 34 was developed to represent the currents in a BJT in all operational modes According to this model the emitter and the collector currents have two components the diode current IDE and IDC and the interaction current represented by the dependent current sources The diode current is the current that would ow through each junction if it were a single diode This current is determined by the biasing of the junction and OCRIDC OCFIDE IDE IDc Figure 34 The EbersMoll Model of an npntype BJT Revised 020 72006 0 Poroy 051 080 Bioelectrical Design Spring 2006 does not take the interaction between the junctions into account The currents resulting from the interaction between the emitter and the collector are represented by the dependent current sources cm is the largesignal reverse current gain It has an effect only in the reverse active mode In the other three modes the collectorbase is reverse biased and IDC is very small Therefore the collector cannot have an effect on the current owing through the emitter BJTs are not designed to operate in the reverse active mode Unlike the forward current gain the value of the reverse current gain is much less than unity In the cutoff mode both junctions in the BJT are reverse biased and both diode currents equal to the reverse saturation current According to the model this also reduces the interaction currents to the level of reverse saturation current Since the emitter and collector currents are the sum of their respective diode and interaction currents they are also at the level of reverse saturation current No signi cant currents ow through the BJT it is cutoff In the saturation mode both junctions are forward biased and both diode currents are large Consequently the interaction currents are also large Large currents ow both through the emitter and the collector the BJT is saturated Beta analysis of the operating point If we assume that the BJT in Figure 38 is operating in the active mode we can write Ic ocFIE Applying KCL to the transistor yields IE Ic IE I Substituting for IE C 10 IE F l aF CB l a aFF39ICIB a I F J C 1aF B aF Wede ne l aF Then Ic 13 IE 139IB Revised 020 72006 0 Poroy 051 080 Bioelectrical Design Spring 2006 The variable 3 is another way of expressing the large signal forward current gain In commonemitter con gurations it is more convenient to use than ocF The following examples show how beta can be used to calculate the operating point of a BJT Example 31 Find Ic and VCB in the circuit shown in Figure 38 Ignore reverse saturation currents and let VI 5V V2 10V and 100 Assuming that the BJT is in the active mode Figure 38 npntype BJT in we can set VBE 07 V We have to verify Comm0nemmer Con guration this assumption later 5V 07V 43V 3 200m 200m From the input circuit I 215JA 1C 13 2150Lamp4215mA IE ICIB 215mA215uA218mA From the output circuit V2 10 3kQ VCR VBE Solving for VCB VCR V2 1C 3kQ VBE 10V 215mA3k 2 07V VCR 10V 645V 07V VCR 285V VCB is positive The collectorbase junction is reverse biased The assumption was correct Example 32 Find Ic and VCB in the circuit shown in Figure 39 Ignore reverse saturation currents and let V15V V2 10V and 100 Figure 39 npntype BIT in Commonemitter Configuration with Emitter Resistance Revised 020 72006 0 Poroy
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