ECE3300 Lecture 7-1 Input Impedance

Uploaded by cfurse on 07.09.2009

Welcome to ECE 3300 Introduction to
Electromagnetics at the University of Utah. In lecture number seven,
we're going to be discussing the input impedance. Now we've already
looked at several kinds of impedance and let's discuss those in general.
Here's our transmission line like this and it has a
characteristic impedance Z zero. So the characteristic impedance, we
already know how that is defined. That is V zero plus divided by I
zero plus or it is V zero minus divided by the negative of I zero minus.
Z zero is given in ohms. It is a ratio. It is a ratio of voltage to current
but in this case, the characteristic impedance is always either the
ratio of voltage to current in the forward traveling wave or the ratio
of voltage to current in the negative traveling wave. It is not related
to the sum or the total field caused by these two waves.
Last time, we talked about impedance and that was
the load impedance. If we consider the load impedance ZL that was
related to VL divided by IL. Here is VL and here is IL. We can see
that VL is the sum of the incident and the reflected waves. So the
load impedance represents the ratio of a total field. These are waves
that are going in both directions. The input impedance is like the load impedance.
The input impedance, we're going to call ZN and that
is going to be right here. That's the location of ZN. That is going to
be VN divided by IN. VN is the voltage right here from this point to
this, and IN is the input current to the transmission line.
So there are two basic kinds of impedances that show
up in our model. The characteristic impedance, Z zero, is the ratio of
voltage to current for a positive traveling wave or a negative traveling
wave but not the sum of the two. The load impedance, ZL, is at the
load and that is caused by the sum of the incident reflected wave.
The input impedance is at the input or at front end of the
transmission line. That also is a ratio and that is related to a
summation of the incident and the reflected wave on the front end of
that line.