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hw04sol - ECE 3150 Homework 4 Solution Spring 2009...

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ECE 3150 Homework 4 Solution Spring 2009 1. (Bipolar transistor operating regions) For the following silicon BJT conditions, state the operating region (notice the voltage convention, e.x., V BE =V B – V E = - V EB . The “common” node below is the same reference node for both input and output, as illustrated in Figs. 1.17 and 1.18 in the class notes.) (a) NPN with V BE = 0.7V and V CE = 0.0V in common-emitter notation. (2 pts) Notice that “common” means that the other two terminals use it as a voltage reference (often ground). You need to translate to the junction voltage to find the operating region. This problem helps you get familiar with the different BJT circuit configurations. V BE = 0.7 V forward-biased emitter-base junction V CE = 0 V V BC = V BE – V CE = 0.7 V forward-biased collector-base junction SATURATION operation (b) NPN with V BE = 0.9V and V CE = 2.0V in common-emitter notation. (2 pts) V BE = 0.9 V forward-biased emitter-base junction V CE = 2.0 V V BC = -1.1 V reverse-biased collector-base junction FORWARD ACTIVE operation (c) NPN with V BC = 0.7V and V EC = - 2.0V in common-collector notation. (2 pts) V BC = 0.7 V forward-biased collector-base junction V EC = -2.0 V V BE = V BC – V EC = 2.7 V forward-biased emitter-base junction SATURATION operation (d) NPN with V EB = 0.3V and V CB = 2.0V in common-base notation. (2 pts) V EB = 0.3 V reverse-biased emitter-base junction V CB = 2.0 V reverse-biased collector-base junction CUTOFF operation (e) PNP with V BE = - 0.9V and V CE = - 2.0V in common-emitter notation. (2 pts) V BE = -0.9 V forward-biased emitter-base junction V CE = -2.0 V V BC =1.1 V reverse-biased collector-base junction FORWARD ACTIVE operation (f) PNP with V EB = 0.9V and V CB = - 2.0V in common-base notation. (2 pts) V EB = 0.9 V forward-biased emitter-base junction V CB = -2.0 V reverse-biased emitter-base junction FORWARD ACTIVE operation
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2. (Large-signal and small-signal BJT models) For an ideal NPN BJT at 300K, (a) Draw the large-signal Ebers-Moll model with the current gain parameters of α F and α R (pay attention to the direction of the dependent current source) (6 pts) (b) If we have designed the BJT with a large current gain by N E >>N B >>N C and W B <<L n , denote the main current components (in terms of electron-hole drift-diffusion- recombination terms) in the forward active mode for I E , I B and I C . (6 pts) From the electron-hole drift-diffusion-recombination point of view: For NPN BJT in forward active mode: I E is dominated by the injection of electrons from the emitter into the base, which is equivalent to the minority electron diffusion current at the base edge of the emitter-base junction I En = (qD n n Bpo A E /W B ) × [exp{qV BE /kT} – 1] This can be obtained by assuming W B << L n , N B >> N C linearly decreasing base minority electron density from n po exp (qV BE /kT) at the base edge of emitter-base junction to n po exp (qV BC /kT) at the base edge of collector-base junction; the latter term is assumed to be pratically 0 due to reverse-bias V BC .
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