hw4_sol - ECE 442 HW4 Solutions 1 a In the forward-active...

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ECE 442 HW4 Solutions 1. a) In the forward-active region, the following relation holds: I C = I S e V BE V T ( 1 + V CE V A ) Given that the Early voltage value is very large, the I C value is independent of the V CE voltage, therefore the V CE voltage value can not be calculated solely as a function of transistor voltages and currents, since it is set by external circuitry. Lowering V CE voltage would eventually move the BJT from the FA mode to saturation, therefore the lower bound can be found from: V BC < 0 . 4 V V BE - V CE < 0 . 4 V V CE V BE - 0 . 4 = V B - 0 . 4 V b) We are given: I E = 1 mA , i e = 0 . 1 mA . Using the small-signal model, we can ﬁnd the current and voltage changes as follows: i c = α i e = 0 . 099 mA i b = i c β = 0 . 99 μ A r π = β I C / V T = β V T α I E = 100 × 25 m 0 . 99 × 1 m = 2 . 53 k Ω v be = i b r π = 0 . 99 μ · 2 . 53 k = 2 . 5 mV Alternate way of expressing v be : g m = I C V T = α I E V T = 0 . 99 × 1 m 25 m = 39 . 6 mA V i c = g m v be v be

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This note was uploaded on 02/21/2010 for the course ECE 442 taught by Professor Staff during the Spring '08 term at University of Illinois, Urbana Champaign.

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hw4_sol - ECE 442 HW4 Solutions 1 a In the forward-active...

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