2.9 Transmission Lines Applied to High Frequency Circuits
95
Secondly, since the wavelength changes with frequency, our circuit would only behave as expected
at a specific frequency and perhaps over a narrow band around it which is often undesirable. Thirdly,
when non-deal lines are employed, losses are incurred which are proportional to length. Lastly,
often enough, the load impedance will be some complex quantity and you may want to use equation
(2.9-1) to your advantage to turn the impedance seen by the generator into a more suitable value,
usually the same as the generator's internal impedance. As you may recall, the maximum power
transfer theorem, which largely applies at Radio Frequency, states that the power delivered to the
load is maximized when R
L
=R
S
. Figure 2.9-3 and Figure 2.9-4 demonstrate this result at DC and AC
respectively.
Figure 2.9-3 Maximum power transfer theorem simulation (DC)
Figure 2.9-4 Maximum Power Transfer Theorem simulation (AC)
Additional details about how to setup the simulations shown in Figure 2.9-3 and Figure 2.9-4 are
given in the videos below
video 2.3 (DC)
video 2.4 (AC)
RES
ID=R
R=RL Ohm
RES
ID=RS
R=50 Ohm
DCVS
ID=V1
V=2 V
0 20 40 60 80 100
0
5
10
15
20
Power
(mW)
Load Resistance ()
DC Power Transfer
ACVS
ID=V1
Mag=2 V
Ang=0 Deg
Offset=0 V
DCVal=0 V
RES
ID=RS1
R=50 Ohm
RES
ID=R1
R=RL Ohm
0 20 40 60 80 100
0
2
4
6
8
10
Power
(mW)
Load Resistance ()
AC Power transfer (50Hz)
Conquer Radio Frequency
95 www.cadence.com/go/awr
