Example - 3
The circuit of fig. 2, has a source voltage of Vs = 1.1 + 0.1 sin 1000t. Find the current, iD. Assume that
nVT = 40 mV
VON = 0.7 V
Solution:
We use KVL for dc equation to yield
Vs= VON+ ID RL
Fig.2
This sets the dc operating point of the diode. We need to determine the dynamic resistance so we can establish the resistance of the forward-biased junction for the ac signal.
Assuming that the contact resistance is negligible Rf= rD Now we can replace the forward-biased diode with a 10 W resistor. Again using KVL, we have,
vs= Rf id + RL id
The diode current is given by
I = 4 + 0.91 sin 1000 t mA
Since iD is always positive, the diode is always forward-biased, and the solution is complete.
