Class B amplifier
Class B amplifiers are much more efficient than Class A amplifiers. For medium and
high power applications, Class B is generally used. In a Class B amplifier, there are
generally two transistors, one drives the current into the load while the other draws
the current out of the load. Hence this configuration is generally called a “push-pull”
configuration. From the Q-point perspective, the two transistors are biased just at
their turn-on voltages as shown in Figure 3.6. The two transistors are driven 180? out
of phase so that each one is active for half of a cycle and cutoff during the other half
of the cycle. Moreover, as the devices are biased at the edge of turn-on, there is no
current flow with no applied signal, thus Class B attempts to maximize the efficiency.
The conduction angle of a Class B amplifier is ? .
In the ideal case, the efficiency of a Class B amplifier can approach a maximum of
78% . In practice, as the drain/collector voltage swing is less than maximum and
due to additional losses in the circuit, PAE of Class B implementations may reach as
high as 63% . However, Crossover distortion occurs when the signal is transferred
from one device to the other which degrades the linearity of the PA . Due to lack
of high-speed p-type devices in most bipolar and FET technologies prohibits the use
of push-pull configuration.