Radio Frequency Integrated Circuit Design - Webs

364 Radio Frequency Integrated Circuit Design
Figure 10.17 Power transistor f T versus current.
or
We can also determine the current.
ip = 2Po
vp
= 2 � 0.200
Po = v p � i p
2
3
= 0.133 or 133 mA
Thus, for class A, the nominal current should be about 133 mA with
peak excursion from about 0 to 266 mA. Thus, with the transistor as given,
with peak f T at about 15 mA, and recognizing that we will have extra losses,
let us choose to use 10 of these units. Then with operation close to the peak
f T , the resulting simulated collector current is 147 mA, and with a dc current
gain of about 85, the input current is set to about 1.75 mA.
It was noted that the circuit had a potential stability problem for lowimpedance
inputs. This problem was minimized with a 5� resistor in series
with the input. The input impedance was then seen to be about 5.55 − j3.14.
Through simulations, the input was matched with 3 nH in series and 9 pF in
parallel. Note that the finite Q of the input series inductor will help with