420 CHAPTER 8. FIELD EFFECT TRANSISTORS
Class-AB and B: Requires devices with excellent pinch-off characteristics and preferably
complementary devices for push-pull architectures
In class-AB (class-B) amplifiers the device is biased close to pinch-off (at pinch-off) so that
the device operates as a amplifier for half the cycle and remains cut-off for the other half of the
cycle. In tuned class-AB/B amplifiers sinusoidal output swings are obtained using a resonator at
the output (figure 8.44) tuned to the fundamental frequency. The drain voltage and current wave-
forms are sinusoidal and half-sinusoidal respectively and the drain is biased at roughly half the
peak-peak RF output voltage swing (figure 8.45). Under these conditions the LC resonant circuit
is charged during the conducting portion of the device cycle and discharges into the load when
the device is off providing the sinusoidal outputs desired. It is also apparent from figure 8.45 that
for the same device periphery as the class-A case the optimum load is now reduced by a factor
of 2, but the net fundamental output power is the same. Since the device is off when the voltage
across it is high, lower D.C. consumption and hence higher efficiency up to 78.6%is expected.
However, this configuration is inherently narrow-band because the series (parallel) resonator that
is designed to be a short (open) at a fundamental frequency acts close to a open (short) at the
second harmonic, and so even bandwidths of 2:1 are hard to realize. This bandwidth limitation
is mitigated by employing a push-pull architecture where sinusoidal output swings are obtained
using two devices, each operating for half the cycle and combining the output currents.
Figure 8.44: Circuit schematic of a simple class-B tuned power amplifierIn push-pull class-AB/B configuration. (Figure 8.46) This configuration could be made rel-
atively broadband but requires broadband transformers or complementary devices. The most
prevalent of all complementary devices is the Si-CMOS structure which is the dominant tech-
nology in the world today. The importance of having complementary devices is apparent by
comparing Figure 8.46a and b, where figure 8.46a shows the case of a technology that does not
have a complementary architecture and figure 8.46b the Si CMOS case. The complexity and the
size penalty in the former is obvious as one has to generate out of phase signals at the input using
transformer and add the outputs also using transformers. As mentioned before these transform-
ers have typically narrow band and hence limit bandwidth. As in the tuned class-B case, higher
efficiency up to 78.6%is obtained due to reduced D.C. consumption. The device requirements