Radio Frequency Integrated Circuit Design - Webs

Power Amplifiers
sinusoid), this peak does not occur. However, overall, Figure 10.13 is a fairly
good description of real life performance.
As an example, if VCC = 3V, then Vmax can go to 6V. If Imax is 1A, then
maximum output power is given by Pout,max = P out,norm = (V max � I max)/8 =
0.75W at � = 90° and at � = 180°.
An expression that is valid for n = 2 or higher can be found for in , the
peak current of the nth harmonic:
361
in = 2ICC cos � sin n� − n sin � cos n�
� � n(n 2 � (10.18)
− 1)
Figure 10.14 shows the current components normalized to the maximum
current excursions in the transistor. The dc component is found from (10.8),
the fundamental component is found from (10.10), and the other components
from (10.18), with normalization done using (10.15).
We note that for class A (� = 180° or conduction angle is 360°) the
collector current is perfectly sinusoidal and there are no harmonics. At lower
conduction angles, the collector current is rich in harmonics. However, the
tuned circuit load will filter out most of these, leaving only the fundamental
to make it through to the output.
Figure 10.14 Fourier coefficients for constant transconductance.