Radio Frequency Integrated Circuit Design - Webs

362 Radio Frequency Integrated Circuit Design
At very low conduction angles, the current ‘‘pulse’’ is very narrow
approaching the form of an impulse in which all harmonic components are of
equal amplitude. Here efficiency can be high, but output power is lower.
10.5.2 Class B Push-Pull Arrangements
In the push-pull arrangement shown in Figure 10.15 with transformers or in
Figure 10.16 with power combiners, each transistor is on for half the time.
Thus, the two are on for the full time, resulting in the possibility of low
distortion, yet with class B efficiency, with a theoretical maximum of 78%. The
total output power is twice that of each individual transistor.
Mathematically, each transistor current waveform as shown in Figure 10.16
is described by the fundamental and the even harmonics as shown in (10.19).
iA = IP IP 2IP 2IP
+ cos �t + cos 2�t − cos 4�t + . . . (10.19)
� 2 3� 15�
Figure 10.15 Push-pull class B amplifier.
Figure 10.16 Class B amplifier with combiner.