8.8. SMALL AND LARGE SIGNAL ISSUES 419
research. Another extremely important figure of merit for power transistors after output power
capability is efficiency. Why is this important? Power amplifiers transmit power. Depending on
the application these powers can be large. 120W per amplifier and over 1kW for a base station is
typical for cellular phone applications. The requirements for RADAR are even larger. Imagine
an amplifier that has an efficiency of say 50%(the best one can do in class A operation). Then
approximately 1 kW is wasted as heat for 1 kW of transmitted power. This is not only is wasteful
but poses a severe challenge in the packaging of the device as the heat has to be removed from the
chip. If the temperature of the chip rises then the mobility of the materials drop as discussed in
chapter 3 and the resistances rise which in turn heat up the chip even more. To prevent this from
resulting in a catastrophic failure of the device, adequate thermal management (cooling) has to
be in place. The average efficiency of an amplifier operating under GSM modulation schemes
(a popular scheme in wireless transmission the world over) is closer to 18%. It should not be a
surprse to the reader that forced air cooling is required for many base stations. The efficiency of
amplifiers is therefore becoming as important if not more than the requirement for high power.
There are two definitions of efficiency; the Drain Efficiency (DE) and the power added efficiency
(PAE) and are explained below.
1.Drainefficiency (DE%) or D.C. to RF conversion efficiency is defined as the ratio of R.F.
output power (Pout) to the D.C. power drawn from the drain supply (PDC,D) expressed as
a percentage ;
DE%=Pout
PDC,D·100%. (8.8.21)
Drain efficiency represents what fraction of the D.C. power is converted into R.F. output
power.2.PowerAddedEfficiency (PAE%) is defined as the ratio of the difference in the R.F. output
to input power, to the total D.C. power drawn from all bias supplies (PDC).PAE%=
Pout−Pin
PDC·100%≡
Pout
PDC(
1 −
1
G
)
·100%, (8.8.22)
wherePinis the RF input power, andGis the power gain. AsPAEalso accounts for
the input R.F. drive power required for the amplifier it is representative of how the power
amplifier output stage is going to impact the overall system efficiency.ThePAEis the more important of these figures because it includes the amount of input
power required to achieve the desirable output power. In many instances the amount of gain and
output power simultaneously required for the system may not be achievable in a single amplifier
stage. An example is the emerging need for mm-wave imaging (cameras operating at 94 GHz).
Here gain of over 30 dB is required which requires multi-stage amplifiers. In this instance a
high input drive (low power gain) would imply that the efficiency of the driver stage is also
going to significantly affect the overall efficiency. So at least a power gain of 10 (10 dB) is
required to obtain highPAE(sayupto45%for the class-A case). This ensures that the system
efficiency is primarily determined by the efficiency of the output stage and the driver stages do
not significantly affect the overall efficiency.