8.8. SMALL AND LARGE SIGNAL ISSUES 417
Figure 8.42: Circuit schematic of a simple class-A power amplifier.respect to efficiency and bandwidth and associated device requirements. We will conclude the
section by referring to power amplifiers where the devices is used as a switch. These classes
offer the highest efficiency of operation but are the most stringent on device requirements.
Class-A : Least restrictive on device characteristics
Figure 8.42 shows the circuit schematic of a simple class-A power amplifier. This class of
amplifiers is used for highly linear applications and can be used for both narrow and large band-
width applications. For narrow band applications, a tuning network might be added at the output
to terminate the harmonics created due to the variation in device transconductance. In this class
of power amplifiers the device is biased normally-on, at about half the peak-peak output current
and half the peak-peak output voltage (figure 8.43).
The load-line in class-A operation is linear at low frequencies and primarily determined by the
load resistance (RL). To obtain the maximum power from the device, the load-line is chosen so
that the device operates between the maximum allowed drain to source voltage (the breakdown
voltage,Vbr) at one extreme and the maximum allowed drain current (the saturation current,
IDSS) at the other extreme. This requires that the optimum load resistanceRL,optbe,
RL,opt=(Vbr−Vk)
IDSS(8.8.19)
This ensures that device provides the maximum output power obtainable, given by
Pout,max=1
2
·
1
2
(Vbr−Vk)·1
2
IDSS
where the first term of 1 / 2 comes from time averaging. Therefore,
Pout,max≤(Vbr−Vk)IDSS
8≡
(Vbr−Vk)^2
8 RL,opt