CHAPTER 4 - Impedance Matching
154
Figure 4.2-2 Difference in phase between sinusoids: blue, dashed curve ( );
red, solid curve
( )
Now the mistake that many students make is that they see that the red curve is further along the
abscissa than the blue curve. They assume therefore that the red curve leads.. This is WRONG!
There is an easy way to work out which one leads from this graph. The first one that reaches its peak
or, equivalently, crosses the x-axis, is the one that leads. In general, you just look at equivalent
points for each sinusoid across the time period, which is the same for both, and the one that reaches
them first is the leading one. These two sinusoids may be represented in a vector form as shown in
Figure 4.2-3.
Figure 4.2-3 Phasor representation of sinusoid with identical amplitudes and a phase difference of 60ι
Let us now remind ourselves of current and voltage relationships for resistors, capacitors
and inductors. These are shown both as waveforms in the time domain and as vectors. As shown in
Figure 4.2-4, the current through a resistor and the voltage across its terminals are in phase. In the
case of capacitors, the current leads the voltage by 90ι as shown in Figure 4.2-5. Finally in the case
of inductors the voltage leads the current by 90ι(Figure 4.2-6).
0 90 180 270 360
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Conquer Radio Frequency
154 www.cadence.com/go/awr
