# bjt-7 - (2.44 where VBEQ is normally referred to as the...

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1 -----(2.44) where V BEQ is normally referred to as the base emitter turn-on voltage, V BE (on). The term [ I S ] · exp (V BEQ /V T ) is the quiescent base current, so we can write -----(2.45) The base current, given in this form, is not linear and cannot be written as an ac current superimposed on a dc quiescent value. However, if v be >> V T , then we can expand the exponential term in a Taylor series, keeping only the linear term. This approximation is what is meant by small signal. We then have -----(2.46(a)) where i b is the time-varying (sinusoidal) base current given by -----(2.46(b)) The sinusoidal base current, i b , is linearly related to the sinusoidal base emitter voltage, v be . In this case, the term small-signal refers to the condition in which v be is sufficiently small for the linear relationships between i b and v be given by Equation (2.46(b)) to be valid. As a general rule, if v be is less than l0 mV, then the exponential relation given by Equation (2.45) and its linear expansion in Equation (2.46(a)) agree within approximately 10 percent. Ensuring that v b e < 10 mV is another useful rule of thumb in the design of linear bipolar transistor amplifiers. If the v be signal is assumed to be sinusoidal, but if its magnitude becomes too large, then the output signal will no longer be a pure sinusoidal voltage but will become distorted and contain harmonics.

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2 Harmonic Distortion If an input sinusoidal signal becomes too large, the output signal may no longer be a pure sinusoidal signal because of nonlinear effects. A non sinusoidal output signal may be expanded into a Fourier series and written in

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