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# ch12 - Attia John Okyere Transistor Circuits Electronics...

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Attia, John Okyere. “Transistor Circuits.” Electronics and Circuit Analysis using MATLAB. Ed. John Okyere Attia Boca Raton: CRC Press LLC, 1999 © 1999 by CRC PRESS LLC

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CHAPTER TWELVE TRANSISTOR CIRCUITS In this chapter, MATLAB will be used to solve problems involving metal- oxide semiconductor field effect and bipolar junction transistors. The general topics to be discussed in this chapter are dc model of BJT and MOSFET, biasing of discrete and integrated circuits, and frequency response of amplifiers. 12.1 BIPOLAR JUNCTION TRANSISTORS Bipolar junction transistor (BJT) consists of two pn junctions connected back- to-back. The operation of the BJT depends on the flow of both majority and minority carriers. There are two types of BJT: npn and pnp transistors. The electronic symbols of the two types of transistors are shown in Figure 12.1 . B E C I E I C I B B C I E I C I B (a) (b) Figure 12.1 (a) NPN transistor (b) PNP Transistor The dc behavior of the BJT can be described by the Ebers-Moll Model. The equations for the model are I I V V F ES BE T =  − exp 1 (12.1) I I V V R CS BC T =  − exp 1 (12.2) © 1999 CRC Press LLC
and I I I C F F R = α (12.3) I I I E F R R = − + α (12.4) and ( ) ( ) I I I B F F R R = + 1 1 α α (12.5) where I ES and I CS are the base-emitter and base-collector saturation currents, respectively α R is large signal reverse current gain of a common-base configuration α F is large signal forward current gain of the common-base configuration. and V kT q T = (12.6) where k is the Boltzmann’s constant ( k = 1.381 x 10 -23 V.C/ o K ), T is the absolute temperature in degrees Kelvin, and q is the charge of an electron (q = 1.602 x 10 -19 C). The forward and reverse current gains are related by the expression α α R CS F ES S I I I = = (12.7) where I S is the BJT transport saturation current. The parameters α R and α F are influenced by impurity concentrations and junction depths. The saturation current, I S , can be expressed as © 1999 CRC Press LLC

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I J A S S = (12.8) where A is the area of the emitter and J S is the transport saturation current density, and it can be further expressed as J qD n Q S n i B = 2 (12.9) where D n is the average effective electron diffusion constant n i is the intrinsic carrier concentration in silicon ( n i = 1.45 x 10 10 atoms / cm 3 at 300 o K) Q B is the number of doping atoms in the base per unit area. The dc equivalent circuit of the BJT is based upon the Ebers-Moll model. The model is shown in Figure 12.2 . The current sources α R R I indicate the interaction between the base-emitter and base-collector junctions due to the narrow base region. In the case of a pnp transistor, the directions of the diodes in Figure 12.2 are reversed. In addition, the voltage polarities of Equations (12.1) and (12.2) are reversed. The resulting Ebers-Moll equations for pnp transistors are I I V V E ES EB T =  − exp 1  − α R CS CB T I V V exp 1 (12.10) I I V V C F ES EB T = −
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