Choosing the Right Power Amplifier Helps Minimize Harmonics in
2.4GHz Range Extension Circuits
By John Allan, RF Business Development Manager, California Eastern Laboratories

To achieve any kind of real range, 2.4GHz ISM band wireless applications need real output. ZigBee® networks are a good example. Where up to 100mW output might be required to achieve the range some networks demand, most ZigBee/802.15.4 transceiver ICs deliver only 5 to 10dBm output. To boost power, designers are turning to MMIC Power Amplifiers (PAs). With the addition of PAs and other range extension components like LNAs, RFIC switches and filters, designers are able to achieve up to +20dBm POUT from their transceiver circuits.

The use of individual components provides flexibility, along with the opportunity to more finely tune and optimize a design’s performance. But the choice of these devices is critical, especially when it comes to the PA. A number of parameters must be taken into consideration, not the least of which is harmonics.

In power amplifiers, an increase in input is matched by a proportional increase in output — up to a point. Once the input level is driven beyond the amplifier’s linear range, the transfer function is no longer linear. Gain is compressed and harmonic distortion occurs.

Different designers have different approaches to managing harmonics. Some like to crank up the power, then add lots of filtering to block the out-of-band signals that result. Others prefer to back off a bit on efficiency in order to reduce the amount of filtering required. The key is to choose a PA that best serves your approach. CEL offers a number of choices. A look at their characteristics at +20dBm output will help to illustrate how they can be tuned to address the specific needs of a design.

NEC’s UPG2314T5N is a 2-stage InGaP HBT MMIC PA. Operating at 3.3V, it can deliver +20dBm output, but it does so at saturation. At the same time, it draws just 65mA at +20dBm, making it ideal for designs in which low current consumption and long battery life offset the need for filtering that operating at saturation may require.

The UPG2301TQ is a higher power device. Also a 2-stage InGaP HPT MMIC, it delivers +23dBm POUT at saturation. To deliver our target +20dBm output, the RF drive from the transceiver can be backed off, resulting in better linearity and a significant reduction in harmonics. The PA draws 105mA from a 3.3V power supply and, while efficiency is reduced from PAE at saturation, the improved harmonics could make it the right choice for your design.

The uPG2250T5N is a very different device. A 3-stage GaAs MMIC, it delivers over +25dBm output at 3.0V with 60% PAE. However, it’s also designed to operate at +1.8V and, at near-saturation, can deliver +20dBm POUT at this voltage. At 1.8V, +20dBm, it draws just 110mA while achieving 51% PAE. Because it’s operating at near-saturation, its harmonic levels will not be as low as the UPG2301TQ, but its ability to deliver high output at 1.8V makes it ideal for new, low-voltage designs.

In our examples, +20dBm has been the target output. In reality, circuit and filter losses before the antenna may require increasing the output power above this level. Since neither the UPG2301TQ or UPG2250T5N are operating at saturation at +20dBm, a slight increase in the PA’s input drive from the transceiver IC can increase output to the level desired.

In order to meet FCC compliance, filters must be used after these PA devices (Figure 1). But, the amount of filtering can be reduced by careful selection of the PA and other range extension components. When the filtering is reduced, so is the loss before the antenna. Less filtering also means a reduced component count, a more compact design, and an overall lower cost.

Combine the right PA on the output path with the right LNA on the input path, add filtering and proper RFIC switching and you’ll get the kind of flexibility and range extension performance only individual components can deliver.

CEL
www.cel.com
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