CHAPTER 2 - Conveying Power at Radio Frequency
98
Nevertheless, there is a relatively easy solution to this problem. We need a circuit element which has
a very high impedance at DC and appears as a short
18
at high frequency.
Well, a capacitor will do the trick beautifully since its impedance may be expressed as
|
|
Equation (2.9-3) clearly shows that, if ,
|
|
will be very high, whereas at high frequency,
provided a suitable value of capacitance is chosen,
|
|
will be small.
Figure 2.9-8 shows the same circuit as Figure 2.9-7 with the practical realisation of the high
frequency short circuit clearly shown.
Figure 2.9-8 Bias line based on quarter-wave short circuit
This technique allows us to feed a DC bias voltage to any of the terminals of an active device without
RF signals flowing back to the power supply. We will see the importance of avoiding this situation in
our amplifier section, where we will also discuss the limitations of this approach. Let us now look at
an actual, physical example of this arrangement.
The
capacitors shown in Figure 2.9-9 are very small and surface mounted so as to minimize any
impedance variation due to their physical dimensions or to parasitic elements. The value chosen for
them was 5.6 pF, which is suitable for the frequency of operation of the amplifier.
18
In practice a total short circuit is not needed for this purpose and a low impedance (<10 Ω) is usually
sufficient. We will explain this in more details in the amplifier section.
Z
0
Transmission Line
DC
V
GG
λȀͶ
lines
Ground Connected caps
GATE
DRAIN
Gate
Terminal, G
(2.9-3)
Figure 2.9-9 A FET amplifier operating at 2.8 GHz
Conquer Radio Frequency
98 www.cadence.com/go/awr
