Efficiency of microwave devices

Updated November 2, 2007

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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.... In 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, that means a 10 megawatt power plant 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 supply gigawatts). 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.

Maximum 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.

First and second laws of thermodynamics

All energy-transforming processes must follow the First and Second Laws of Thermodynamics, which are summarized here, with pomp and circumstance of capitalized bold letters in a font that adds considerable gravitas:

The First Law of Thermodynamics:
Energy and matter are never created, they just move about. You never get something for nothing.

The Second Law of Thermodynamics
Energy is always somewhat wasted when it is transformed from one type to another, you can't even break even. Energy conversion is like playing cards in a casino, and entropy is the house handle, you dig? It's depressing when you think about it.

We'll leave the mathematical proofs of these Important Laws up to your friendly college professors.

Therefore, efficiency of 100% is never possible. No matter how loud that idiot coach screams, nothing ever "gives 110%". Next time you hear him make this remark, inform him that his poor understanding of basic tenets of physics is having a detrimental effect on the team. He'll thank you!

Three measures of efficiency

In microwave engineering, amplifier efficiency is calculated in three ways, which is downright confusing.

Drain efficiency

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 marketing types.

Power-added efficiency (PAE)

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

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):

Total efficiency 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 harmonics

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!

Some 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.

L-band
Silicon LDMOS devices are offered that achieve

X-band
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.

Ku-band

Wideband
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.

Ka-band
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.