418 CHAPTER 8. FIELD EFFECT TRANSISTORS
Choosing this load-line minimizes the total device periphery (and hence the die area) required
for a given RF output power. This also provides the best bandwidth. Larger device periphery
results in larger device input and output capacitances which degrade the bandwidth. Oversizing
is done if the device on-resistance in the linear region (Ron) is large (i.e.Vk/Vbris large). Then
by operating atId,max<IDSS,theId^2 Ronlosses are reduced and the efficiency is improved.
However this is achieved at the cost of reduced bandwidth.
Figure 8.43: Optimum load-line for class-A operationThe maximum output power obtainable is roughly half the DC power, which means that the
theoretical maximum drain efficiency (ratio of the output RF power to the DC power) is 50%.
Since the device is normally on, a constant DC power of nearly twice the peak RF output power
is dissipated at all times. This might degrade the performance of high power amplifiers with
time. But the advantages of class-A operation include broadband operation and high linearity.
Reflecting on the discussion above the following device requirements can be extracted. To
minimize the losses in the linear region or on-state of the device the channel conductivity or
the product of sheet charge and electron mobility should be maximized. This allows the device
periphery for a certain value of allowed on resistance to be minimized which in turn reduces
the device capacitances and hence reduces the amount of circuit inductance required to tune the
device. This device requirement is applicable to all classes of operation, a universal requirement.
It is intuitive clear that the presence of large tuning elements result in LC networks which are
inherently narrow band centered around their resonance frequency and are undesirable in broad-
band applications. The output power is a function a the product of available current and voltage.
The current is typically proportional to the channel conductivity and electron velocity. This is
compatible to the requirement of low on resistance but typically materials that have high mobil-
ity and electron velocity have low bandgaps such as, Si, GaInAs and InAs. The one remarkable
exception is GaN which has a large bandgap, high electron mobility, and high electron velocity
enabling large currents and large voltages simultaneously and is hence the subject of intensive