Junction Bipolar Transistor. Characteristics Models Datasheet

Transcription

1 Junction Bipolar Transistor Characteristics Models Datasheet

2 Characteristics (1) The BJT is a threeterminal device, terminals are named emitter, base and collector. Small signals, applied to the base, determine strong variations of the current flowing through the device (colectore emitter). The properties indicated above makes this essential element to achieve switching circuits, control circuits of current or voltage, amplifiers, oscillators. Physically, the bipolar transistor is constituted by two pn junctions connected in series, consequently two types of transistors can be realized. Transistor npn Transistor pnp Collector Base Emitter Collector Base Emitter

3 Characteristics (2) Circuit symbols and conventions considered for the currents and bias voltages. I B Base npn V BE I C I E Collector V CE Emitter I B Base pnp V BE I C I E Collector V CE Emitter Depending on the bias of the junctions the following operation modes of the BJT are obtained. Junc. BE Junc. CB Operation. Properties Forw. Bias Rev. Bias Active Region DI C ~ DI B Forw. Bias Forw. Bias Saturation V CE = V Rev. Bias Rev. Bias Cutoff I E = 0 A Signs of the currents and voltages when BJT works in the active region. I C I B I E V BE V CE npn >0 >0 <0 >0 >0 pnp <0 <0 >0 <0 <0

4 Characteristics (3) From figure, it is clear that the input of the transistor is similar to a forwardbiased pn junction and the input characteristics are similar to the characteristics of a semiconductor diode. Input characteristics of a typical silicon device. V BE I B (V BE,V CE ) I B is function of V BE and V CE, as V CE increases, the value of V BE required to produce a given base current also increases. On the other hand V BE can be considered function of I B and V CE ; V BE (I B,V CE ).

5 Characteristics (4) Active region Figure shows the relationship between the collector current IC and collector voltage VCE, for a npn device for various values of the base current I B. In the active region the transistor output can be described by controlled current source I C (I B,V CE ) I C (I B,V CE ) V CE

6 Characteristics (5) Depending on the constructor and application type for which transistor is made, it may present different shapes and sizes. C E B C B E

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8 Models (1) To describe the behavior of these devices several equivalent circuits can be used, suitable for: different operating conditions (continuous, large signal and small signal) and different configurations (CB, CE and CC). Equivalent circuits can be simplified considering the desired degree of approximation. For DC (or for large lowfrequency signal by replacing h FE with b) the following model can be considered : V I V hfe h FE 0 1 CE CE B I I h I h V BE I C h FE I B V CE In this model the Early effect is considered Where I C is function of two independent variables I B and V CE. C B FE B FE 0 1 I C V CE 0 V A I C V A Taking into account that I B is a function of V BE V A V CE (dependence on V CE is neglected): I B I h S FE0 e V V BE T V I 1 CE C IS e VA V V BE T

9 Models (2) Assuming constant h FE, you get a simplification where the effect Early is neglected and characteristics (in the active region) are horizontal therefore independent from V CE. Therefore, only the independent variable I B remains. It determines the distribution of characteristics. I C V BE I B I C h FE I B I I h ; h h C B FE FE FE0 I B I h S FE e V V BE T I B V CE Further simplification, V BE is assumed constant and equal to V BEon, consequently the characteristics are uniformly distributed. This model will be used for the synthesis of the proposed stages. I C I B V BEon h FE I B h FE h FE0 I I h C B FE I B V CE

10 Models (3) For small signal the device can be described by a linear model, in which its elements have constant values depending on the bias point (V CE,V BE, I C ). The most utilized model is the Hparameter model, which is obtained through linear approximation (first order) of electric quantities which characterize its behaviour as a two port network. In particular, considering the common emitter configuration, i b and v ce are selected as independent variables and i c and v be are the dependent variables. If the condition of small variation around the quiescent point is verified. The functions i c (i b, v ce ) and v be (i b, v ce ) can be developed in the Taylor series assuming only the terms of first order : v h i h v be ie b re ce i h i h v h fe i b c fe b oe ce v be i b h ie h re v ce h oe v ce

11 Models (4) CEC h parameters I B I B DV CE V CEQ DI B DV BE h ie DV D I BE B V V CE CEQ I BQ V CE1 V CE2 DV BE h re DV D V BE CE I B I BQ V BE V BE I C DIC I B2 I B1 h fe DI B DI D I C B V CE V CEQ DI C I C I BQ I B h oe DI D V C CE I B I BQ V CEQ V CE DV CE V CE

12 Models (5) For small signal the following models are available: v be i b h ie h fe i b h oe h rev ce h parameter model v ce v be i b h ie h fe i b h oe v ce R i b v be h ie h fe i b vce Simplified model Se 1/h oe >10R C b c r bb r b c r bb BB v be v b e C b e g m v b e r b e r ce v ce v be v b e g m v b e r b e r ce v ce v b e g m v b e r b e r ce Hybridp model Simplified hybridp model h parameter model of CBC where h rb is neglected. E i e v eb h fb i e v eb h ib h o B b C E i e h ib B h fb i e C

13 Models (6) Relations between the h parameters of the CEC and the hybrid modelp parameters [1]. v ic ic h r r h r ' r ' g i v be ie bb' b' e ib v 0 ce fe b e b e m b v 0 b' e v 0 ce ce h oe i c c ce i 0 ce v 0 ce b i v v r b ' e 1 h parameters of the CBC. h ib v i eb c cb rb ' e hfb ' ob ie i ic v e ie 0 cb0 vcb0 h v r c Parameters in terms of bias current [1]. g m ic Ic 1 v V r b' e v 0 T e ce i 1 C VCE c 0 ce vce v 0 A A CEQ be ' I I CQ r v V V V r be ' V I T E [1] Electronics: a systems approach by N. Storey

14 Datasheet (1) The BJT operation is described by a number of parameters collected and supplied by producers in datasheets. Datasheets contain several descriptive sections including: General informations: which describe the essential characteristics of the component. Pin connection: that specify the connections of the connectors Absolute maximum ratings: operating limits beyond which the IC may be damaged. Thermal data: thermal resistance. Electrical charatteristics: electrical properties of the device allow to predict the operation of the device.

15 Datasheet (2) Absolute maximum ratings Thermal data

16 Datasheet (3) Electrical charatteristics (1)

17 Datasheet (4) Sometimes a graph of V BEON values function of the current I C is given.

18 Datasheet (3) Electrical charatteristics (1)

19 Datasheet (4) Typically, for a given biasing (I C, V CE ) in the datasheet the following values of h FE may be supplied : 1. h FEmin the minimum possible value of h FE 2. h FEtyp typical value of h FE 3. h FEmax the maximum possible value of h FE During the analysis or synthesis the typical value of h FE should be used. If this value is not provided it can be estimated by the geometric mean of h FEmin and h FEmax : h h h FE FE min FE max h FE depends on the V CE (for Early effect) and by I C. The relationship between I C and h FE, in the datasheet, is described by the Normalized DC Current Gain (h FEN ) shown in the figure. h FEN hfe h ( I 2 ma) FE C h h ( I 2 ma) h FE FE C FEN

20 Datasheet (6) Sometimes a graph of the h FE values function of the current is provided.

21 Datasheet (4) Electrical charatteristics (2) If the typical values of the h parameters are not provided in the datasheet, they can be estimated by geometric mean between the minimum and maximum values. In addition, using the minimum or maximum values of the h parameters the worst or highest performance can be estimated.

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