8.9. IMPLICATIONS ON DEVICE TECHNOLOGY AND CIRCUITS 421
Figure 8.45: Device bias point and load-line for class-B operation.for high performance Class B amplifiers is high gain near pinch-off conditions as this is the bias
condition of the device is biased near pinch-off. this requires that the device leakage near pinch-
off be minimal. Devices that have quantum well channels, p-type buffer layers, wide bandgap
buffer layers or other means of enhanced charge control are required.
Higher Classes of Operation; Most stringent on device requirements
Class-C amplifiers are similar to the class-B tuned power amplifier with the device biased
deep into cut-off region, so that the conduction angle (the fraction of the cycle that the device
is on) for a sinusoidal waveform is less than 180 ◦. Higher efficiencies are obtained by lowering
the conduction angle, up to a theoretical limit of 100%with 0 ◦conduction angle. Class-D,E are
switched mode power amplifiers, where the device is operated as a switch. The load networks
are chosen to minimize the current and voltage waveform overlap across the device, resulting in
higher efficiency. But, again as in the case of class-B tuned power amplifiers, these classes as
well as other classes like F, G, H etc. use a resonator at the output to obtain the fundamental
power and are of no significance in broadband amplifiers. The requirements for devices operat-
ing as switches are the most extreme because since the waveforms are nearly square waves, the
device has to have have gain at least to the third harmonic of the fundamental to be able to recon-
struct the waveforms with reasonable accuracy. In addition the requirements of high gain near
pinch-off, low leakage and high subthreshold slope, similar to to the case of Class B amplifiers
are still desirable.
8.9 Implications on device technology and circuits .................
Having understood the limitations on decade bandwidth high power amplifiers, the following
implications on the choice of circuits and device technology are apparent: