240ln02 - SMALL SIGNAL ANALYSIS LN 2 With the application...

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1 SMALL SIGNAL ANALYSIS LN 2 V in V BB With the application of an input signal V in we expect the B-E voltage to change as a function of time: V BE (t) = V BB + V in (t) This will result in a change in the collector current I c (t) + I s e V BE(t)/ V T = I s e V BB/ V T e V in(t)/ V T or i c (t) = I c DC e Vin(t)/ V T For V in (t ) < V T , which defines the meaning of the term “small signal”, we obtain .] .......... 6 1 2 1 1 [ ) ( 3 2 + + + + = T in T in T in DC c c V v V v V v I t i Again, for V in < V T , higher order terms will be insignificant, resulting in I c (t) = I c DC + I c DC v in Æ i c (t) = g m v in (t) V T The factor I c DC is known as the transconductance “g m ”. The transconductance V T measures the change in I c due to a change in V BE ~ The time varying VBE(t) also gives rise to a changing minority (as well as majority) carrier concentration within the base. The changing storage charge gives rise to a capacitance and the movement of the time varying excess charge gives rise to the concept
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240ln02 - SMALL SIGNAL ANALYSIS LN 2 With the application...

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