# Math Help - q1 fizics

1. ## q1 fizics

For the diode that conducts 2mA at a forward voltage drop of 0.3v and whose n=2. Find the equation of the straight line tangent at ID=2mA

2. Originally Posted by TAHIRI
For the diode that conducts 2mA at a forward voltage drop of 0.3v and whose n=2. Find the equation of the straight line tangent at ID=2mA
$
I_D = I_S \left[ e^{\frac{V_D}{nV_T}}-1 \right]
$

Expand the RHS as a Taylor series about $V_D=0.3$:

$
I_D = I_S \left[ \left(e^{\frac{0.3}{nV_T}}+(V_D-0.3)\left. \frac{d }{dV_D} e^{\frac{V_D} {nV_T}}\right|_{V_D=0.3}+ \cdots\right)-1\right]
$

Truncate the series after the first term to get:

$
I_D = I_S \left[ \left(e^{\frac{0.3}{nV_T}}+(V_D-0.3)\left. \frac{d }{dV_D} e^{\frac{V_D} {nV_T}}\right|_{V_D=0.3}\right)-1\right]
$

When you simplify this using the given data you will find that the $I_S$ can be eliminated but you will be left with $V_T$ in the equation.

RonL