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On this page we
will define the meaning of efficiency, review some power amplifier
efficiency equations, and then speculate on what level of DC-to-RF
efficiency you can expect depending on device type, frequency and
bandwidth. Maybe we'll even put down some rules of thumb!
Overview of efficiency
Efficiency is a
measure of how well a device converts one energy source to another.
What doesn't get converted to goes into heat; heat is almost universally
a bad byproduct of energy conversion.
In different processes
that occur in every day life, you can expect different levels of
efficiency, usually based on the physics of the devices involved.
Automobile internal combustion engines are lucky to convert 20%
of your gas tank to measurable work. Of course, there is big difference
between "measurable work" and "unnecessary work"
when it comes to today's SUV-infested highways, but we'll leave
that topic alone for now....
photo-voltaic (solar) cells, we are interested in converting sunlight
to usable electric power, and 10% efficiency is a dream come true
for amorphous silicon solar cells. With solar radiation at 1 kilowatt/m2,
a 100 megawatt power plant (thanks for the correction, Rob!) will
require one square kilometer of PV solar cells, which is unaffordable
so you'll never
see this happen (but nuclear power plants have been built that
Some switching voltage regulators can convert one voltage to another
with an amazing 90% efficiency.
In microwave engineering,
we are interested in converting DC power to RF power. The higher
the power amp efficiency, the longer your cell phone can operate,
because it's primarily the transmit amplifier chain that is draining
the battery when you're yacking away. Some Class-E power amplifiers
have been known to exceed 80% efficiency.
of a microwave device is a function of frequency, temperature, input
drive level, load impedance, bias point, device geometry, and intrinsic
device characteristics. It is truly a multidimensional problem!
You can determine the maximum efficiency under different conditions
using load pull.
In engineering, Greek
lower-case letter delta ()
is often used to denote efficiency.
Three measures of efficiency
In microwave engineering, amplifier
efficiency is calculated in three ways, which is downright confusing.
The laws of thermodynamics
won't allow 100% efficiency, no matter how you calculate it.
Drain efficiency is gets its
name from FET devices, where the primary
terminal where DC power is input is the drain. Drain efficiency
is the ratio of output RF power to input DC power:
Drain efficiency is a measure
of how much DC power is converted to RF power. The problem with
this unit of measure is that it doesn't take into account the incident
RF power that goes into a device. In the case of a single-stage
RF amplifier, the RF input power can be substantial, because the
gain is low. Drain efficiency is only quoted by cheaters and other
Power added efficiency is similar
to drain efficiency, but it takes into account the RF power that
is added to the device at its input, in the numerator. Sorry,
this equation was incorrect up until May 6, 2006!
In a theoretical sense, an amplifier
with infinite gain will have power added efficiency equal to drain
efficiency. For a real amplifier, PAE will always be less than drain
efficiency, but once you get to 30 dB gain or so, the two quantities
become very close in value because input power will be less than
1/10 of 1% of output power.
The maximum possible power-added
efficiency of a device almost always decreases with frequency. This
is because of the natural tendency for maximum gain of an active
device decreases with frequency.
Total efficiency, sometimes called
overall efficiency, gives a complete picture of the ratio of output
power to both types of input power (DC and RF):
is the measure that makes the most sense from a thermodynamic point
of view. But PAE is still the most popular measure in the microwave
community. Maybe someday someone will explain that to us!
Efficiency of power amplifiers
can be improved by providing proper terminations at harmonic frequencies
for both load and source. The "low-hanging fruit" is the
second harmonic load impedance. Merely terminating this frequency
in a short circuit can be worth a few efficiency points. The third
harmonic wants to see an open circuit. Trying to deal with multiple
harmonics in a design is not usually worth the headache unless your
devices have significant gain capabilities at the third and higher
Now let's talk about a Microwave
Stupidity topic. Suppose you redesigned an amplifier, and improved
its power added efficiency from 20% to 22%. Did you improve the
efficiency by 2%? No, you improved it by "2 percentage points".
Get that straight, Einstein!
power amplifier efficiency benchmarks
These examples below are purely
hypothetical, and are based on experience but we don't offer and
references to back them up. Why's is that? Here's a better question...
why don't you offer us some references and we'll build this into
a page of content for your company's power amplifier expertise!
Always consider when someone
is talking about PAE, whether it is at the device level,
or the amplifier level. Amplifiers will always have lower
PAE than devices because matching networks and ensuring stability
always take a toll on gain.
When someone quotes a number
for PAE, remember that it only occurred at a specific drive level.
Lower or higher drive will almost assuredly mean lower efficiency!
Unless otherwise noted, assume
we are talking about solid-state devices here.
Silicon LDMOS devices are offered that achieve
MESFET amplifiers with 10% bandwidth can exceed 30% efficiency at
X-band. PHEMT amplifiers can exceed 40% PAE at X-band. TWTs routinely
deliver 60% efficiency.
The more bandwidth, the lower the efficiency, because you just
can't hit the best load over that much bandwidth. Distributed amplifiers
are notoriously inefficient, because the devices don't all get the
same voltage: some are ready to burn out and some are coasting!
A solid-state amplifier that
works from 2-18 GHz will have less than 10% PAE.
Here you might read about parts that hit 30% efficiency on a good
day, but don't expect to beat 20% .
Q and W-band
These frequencies, solid-state devices are just getting started.
Expect efficiencies in the high single digits for PHEMT and MHEMT.
For HBT and InP HEMT you can expect even lower efficiency.