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Drain Efficiency vs. Power-added Efficiency (PAE) and the Efficiency Assessment of RF Devices

RF amplifier circuit boards


When I think of ‘versus’ I think of head-to-head matchups, athletes at their respective peaks striving to out-do each other on their fields of choice. Or I picture iconic images of rams butting into each other as they struggle over their chosen area of dominance, and bears wrestling and trying to topple each other. 

The other side, and equally as dramatic, are choices like which side should the toilet paper roll be placed in. Of course we all recognize it should be placed so the sheets are under the roll, but some value the chaos that an over-the-roll approach brings more. When it’s all said and done, any versus situation is a choice made where a typically individual effort or action is then placed into an arena with another. With electronics, you’re rarely going to have a single component though. 

There are many choices a designer must take into consideration before ultimately deciding on a design. This includes the approach one takes on measuring efficiency. When us design engineers are asked to consider the differences between drain efficiency and power-added efficiency (PAE), well, this is where opinions start to come into play. 

What is Power-Added Efficiency?

By definition, the term efficiency is a measure of how well a device converts one energy source into another. However, according to the laws of Thermodynamics, 100% efficiency is unobtainable. Therefore, the side effect of this inefficiency is the generation of heat. As I am sure you are aware, in PCB design, heat is generally an unfavorable property.

Also, in terms of measuring efficiency, there is a method called Power-added efficiency (PAE). This method is a type of RF Power Amplifier efficiency that is also similar to another type of drain efficiency that I will discuss shortly. However, PAE uses a method of assessing efficiency that takes into account the RF power that is added to the device at its input, to complete the efficiency assessment. Also, PAE is the more acceptable figure-of-merit to use in the comparison of single devices. The formula for PAE is as follows:

PAE =100 × (Pout - Pin)/PDC

In summary, PAE is a measure of the efficiency by which a device converts DC or RF input power to a higher RF output. Generally, the PAE of a device varies between 10% and 80%, and this is while the device is in operation at frequencies between a few GHz and several tens of GHz.

What is Drain Efficiency?

Now, as I alluded to earlier, there is another method of assessing efficiency in the field of electronics, it is called drain efficiency. Drain efficiency gets its name from FET devices, in which the primary terminal where DC power is supplied is the drain. Drain efficiency is the ratio of output RF power to the input DC power. The formula is as follows:

Drain Efficiency (η) = Pout /PDC

In regards to amplifiers, efficiency is the ratio between the output power to the DC input power, and thus referred to as drain efficiency. However, this type of (drain) efficiency can be misleading to Engineers and designers because it does not consider how much power is used by the amplifier.

Also, an amplifier can have a high drain efficiency and at the same time, have a meager gain. So, this is why most Engineers, especially RF Engineers, use PAE instead because, with PAE, the input power is a consideration.

Note: In most cases, devices with a high PAE are preferable to a device with a high drain efficiency.

Drain Efficiency vs. Power-added Efficiency (PAE)

An amplifier has the ability to amplify an input signal because of the added power by its DC source. Power-added efficiency is a convenient parameter that is used to determine how much the DC input power contributes to the amplification of an input signal. It also provides a more accurate account of the efficiency of a device, including amplifiers.

In today’s increasingly portable world, finding the optimal balance between RF power output and efficiency is becoming more complicated and more critical. The need to optimize the cost and operating time for mobile electronic systems running on batteries often comes down to one crucial system-level parameter: efficiency.

Therefore, it is more important now than ever that designers properly, design, assess (efficiency), and produce devices with the highest level of efficiency. Therefore, those working with designs of RF devices can obtain a more accurate assessment of the efficiency of their designs using PAE.

Mixed-bag of components on a printed circuit board

Optimizing power for RF devices is of vital importance for design engineers.


The Overall Effect of Efficiency on Design and Functionality

The acceptable definition of efficiency in the field of electronics is a measure of how well a device or system does with the power made available to it. In regards to battery-operated devices, it is a straightforward approach to determining efficiency. In summary, products with higher levels of efficiency will simply operate longer, with the same amount of power, than devices with lower levels of efficiency.

This demand for increased efficiency is due primarily to the world’s electronics landscape moving steadily towards the portability of smartphones and the transition to 5G cellular. With the newest standard (5G), we also see cellular network’s increasing demands for higher efficiency mobile radios and a desire for increased talk times on a single (smartphone) charge.

Let's face it, tomorrow is now, and high efficiency is not exclusive to only power amplifiers within a system. High efficiency isn’t confined to just power amplifiers (PAs) within the system, though; it involves all of the components that contribute to minimizing signal losses and power.

What Affects Efficiency in Electronic Devices?

In design, there are usually multiple parameters required to make the necessary calculations to make an informed design decision. The same is true for even a design decision that requires, for example, cubic dimensions. Moreover, you cannot resolve this particular calculation without, for example, the depth parameter. Furthermore, these parameters must also be accurate, as well.

In addition, this includes efficiency. One must assess the efficiency of the components in your design, and that assessment must be as accurate as possible. Take, for example, the efficiency of an RF/microwave system. In its unadorned form, you can describe it as the amount of RF/microwave output power that’s produced, such as from a power amplifier, from a given amount of dc input power, such as a power supply or a battery. Although the concept is simple, efficiency may not be easy to determine for an application.

The reason for the assessment difficulty is because it depends on several component factors. This includes the type of power amplifier in use, the operating frequency, the active device or devices, the drive level, the impedance of the load, and the temperature.

Transistors densely packed on a circuit

Even transistor-based devices will still require an understanding of power efficiency.


Other Types of RF Device Efficiency

Understandably, achieving high-efficiency in one’s designs is a shared goal among all designers. The task of doing so can be difficult since their designs include multiple components. The less apparent reason for the difficulty is knowing which type of efficiency assessment will grant the necessary evaluation to make an accurate design decision. So, the following are the other types of efficiency assessment.

Total Efficiency: 

Many in the field of electronics refer to it as overall efficiency. This particular type of efficiency assessment provides a more comprehensive view of the ratio of output power to both DC input power and RF input power.

Total (η) = Pout / PDC + Pin

Amplifier Efficiency: 

It is the ratio of RF output power to DC input power, and is the preferable figure-of-merit for a multiple stage amplifier.

Amplifier (η) = PRFout / PDC

Wall-Plug Efficiency: 

As its name implies, it is a comparison of the average RF output power to the exact measured power that is consumed by the device-under-test from the AC outlet (wall plug), while ignoring the effects of RF input power.

Wall-plug (η) = PRFout-average / Total AC Power

Efficiency is the merit by which all designs are evaluated. The demand for increased efficiency, by all accounts, will only increase as we continue to advance our technology and designs of our electronic devices. Therefore, it is imperative that as designers you choose the most accurate method to assess your designs. In turn, it will afford more accurate design decisions and facilitate more reliable advancement of the design process. 

Assess efficiency methodology in your PCB and circuit designs with Cadence’s suite of design and analysis tools. For accurate efficiency assessment in your circuit designs, you can begin with Allegro PCB Designer as the layout stage for any of your design processes. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts