The slope the versus curve the point

218	C H A P T E R	F O U R	o f		n o n l i n e a r
218	C H A P T E R	F O U R	o f		iD
FIGURE 4.38 Graphical			∆	iD		point VD, ID given by
						ID
						vTH
interpretation of operating point						vTH

and incremental signals.

third term	1	1	�v.	(4.66)
second term	=	VTH

The relationship in Equation 4.67 is itself derived from the Taylor series �iD = dvD��VD �vD (4.67) df expansion as follows. Recalling Equation 4.55,
i f (v) f (V)	df		(v V)1	d2f
D =D =D +	dvD	��VD		dv2 D	��VD	D −D+ · · ·
D =D =D +	dvD	��VD		dv2 D	��VD	(4.68)

4.5 Incremental Analysis	C H A P T E R		219

enough that we can ignore second order terms in �vD we get In words, the incremental change in the current is equal to df/dvD evaluated �iD = dvD��VD �vD. (4.71) df

of iD for the given form of vD. We are given that the input is of the form

iD = ID + �iD.