Considerations When Choosing An RF Power Amplifier

In this article, we’ll talk about the important operating parameters to consider when choosing an amplifier, as well as the various types of amplifiers offered by Quantic Corry.

Quantic Corry provides power amplifiers that can output tens to hundreds of watts, and low-noise amplifiers with output in the hundreds of milliwatts. The amps we offer can be used with basic continuous wave signals or complex signals, like LTE or WCDMA.

When choosing an amplifier the basic specs you’ll want to consider are:

  • Gain
  • Operating Frequency
  • Output Power Level
  • Efficiency
  • Linearity
  • Mismatch Tolerance
  • Noise Level

Gain is a measure of how much the amplifier increases the signal level, and is measured in dB as the ratio of the signal power at the amplifier output to the signal power at the amplifier input. Quantic Corry amplifiers have gains in the range of 30 dB to 50 dB. This high gain allows you to use a signal source like a software-defined radio or a standard, off-the-shelf signal generator that has an output of milliwatts to get 10s or 100s of watts out of the amplifier. In the case of a low-noise amplifier, the high gain will contribute to lowering your overall system noise figure.

Operating frequency is a straightforward parameter and provides the lowest and highest frequencies wherein an amplifier meets stated specifications. Some amplifiers are broadband and offer a wide operating frequency range, while others have a narrower operating band. The narrow band amplifiers will offer better performance, since they can be optimized for operation over a smaller set of frequencies than the broadband amps. Quantic Corry supplies both broadband and narrow band amplifiers.

Output power level is the amount of RF power that the amplifier can deliver at its output. While this parameter is simple in concept, you need to understand that it can be specified many different ways. It could represent the amplifier’s saturated output power — the absolute power that the amp can provide. When saturated, an amplifier will not provide increased output when the input level is increased. Instead, when saturated, the amplifier output remains constant, even when the input level is increased. This means the measured gain of the amplifier decreases.

Power at the 1 dB compression point is another way of rating an amplifier’s output power. The 1 dB compression point is the output power level at which an amplifier’s gain has decreased by 1 dB from gain measured when the output power is low. For example, if an amplifier has 10 dB of gain at 1W output power, and 9 dB of gain at 10W output power, then 10W is the amplifier’s output level at 1dB compression.

Yet another way to state output power is as a power level for a specific type of signal, with certain operating parameters met. For example, an amplifier could be specified as 10W with an LTE signal at 10 percent error vector magnitude (EVM). This means the amplifier could output an LTE signal at up to 10W average, with the EVM measuring 10 percent or better. Most likely, the amplifier output level can go higher, but the EVM would get worse.

Quantic Corry uses all three methods to specify amplifiers; the method used is determined by the amplifier’s intended application.

Efficiency is the ratio of the amplifier’s power consumption to the amplifier’s output power. Obviously, the closer this parameter is to 100 percent, the better. If 100 percent efficiency was possible, it would mean that all the power consumed by the amplifier was delivered as RF to the load. Efficiencies below 100percent represent power lost in the amplifier (which generates heat within the amplifier). To maintain proper operation, enough of this heat must be removed to maintain a safe operating temperature.

Quantic Corry amplifier modules typically require that the case temperature not exceed 85 degrees C (185 degrees F). Heatsinks and fans are the most common methods used to cool power amplifiers.

Quantic Corry is experienced with cooling amplifiers and can assist you with your system cooling needs. We can package the amplifier module in an enclosure, complete with heat sinks and fans, or help you select the proper heat sinks and fans to package in-house.

Linearity could be critical, or something you may not care about, depending on your specific application. For example, if you need a power amplifier to output a constant amplitude, signal linearity shouldn’t be a concern. If you’re amplifying an amplitude or vector modulated signal, then linearity can be very important. An amplifier with inadequate linearity will distort and add noise to the output signal that will degrade system bit error rate. You can use the amplifier’s 1 dB compression point as a reference to know what power level the amplifier is becoming nonlinear. Using this knowledge, plus your required output power and your signal characteristics, you can size the amplifier accordingly.

Linearity also can be described as the third order intercept point. This number can be used to determine third order intermodulation product levels that are generated by the amp. These intermodulation products act as added noise to your output signal. If they are too high, the output can’t be decoded properly and bit errors are introduced.

If you’re amplifying a vector modulated signal — like LTE or WCDMA — then the error vector magnitude measurement is the best indicator of linearity. Vector modulated signals can have very high peak-to-average-power ratios, some greater than 10d B. These types of signals require that the amp produce peak power levels much higher (10 times or more) than the average output of the amplifier. The best way to handle these types of signals is with special amplifier architectures, like Doherty, and to incorporate linearity correction. These architectures can maintain amplifier efficiency and economics.

Using an amplifier that can deliver much more average power than you require is an option, too, but this would be inefficient and costly. It’s best to know your amplifier’s linearity performance with your signal type. If that information is not available on the data sheet, but 1dB compression point, saturated output power, or third order intercept is, you can use that information to gauge what type of power the amplifier could provide you’re your specific signal, and then follow up by testing the amplifier with your signal.

Quantic Corry has the capability to measure amplifier linearity, including measuring EVM for advanced signals, like LTE and WCDMA. We also are happy to help you determine our amplifier performance with your signal type, and we can customize amplifiers to your specifications.

Mismatch tolerance provides information on an amplifier’s ruggedness. For example, some applications may require the amplifier to tolerate a fault on its output, like an open or short circuit. A condition like this could damage an amplifier, that hasn’t been designed to handle it. If your application requires a rugged amplifier, then you’d want to look for something that can handle high reflected power. Quantic Corry amps are ruggedized and can tolerate full reflections over all phases.

Quantic Corry also offers amplifiers with advanced monitoring and control functions. This includes automatic level control (ALC), forward and reflected output power monitoring, temperature monitoring, over-temperature shut down, and power supply current monitoring. ALC allows the amplifier to regulate the output power to a level set by the user.

Finally, noise figure is gaiged in dB and represents the increase in noise level at the amplifier output. At the output of any amplifier, the noise level will be higher than the input noise level, and will be increased by more than just the gain of the amplifier; it will increase by the gain of the amp plus the noise figure value.

Noise figure can be used to determine the noise level out of the amplifier, as well. For power amplifiers, noise figure typically is not critical, but it can be an issue if the PA output can leak into a high-sensitivity receiver. Noise figure is very important for LNAs; since they are amplifying low level signals that can be close to the noise floor, any added noise to these signals will reduce receiver sensitivity. The lower the noise figure of the LNA, the more sensitive your receiver can be. Power amplifiers typically have noise figures of 10 dB or higher. LNAs have noise figures of a few dB to below 1 dB.

Quantic Corry offers Pas and LNAs to fit numerous applications, supplied as modules or as integrated assemblies for incorporation into your system. When purchasing a module, the user must supply the necessary cooling. When purchasing a system, we’ll integrate the amplifier into a rack mount or bench top enclosure, along with the required cooling. We can also integrate our amplifiers with our other components — including filters, diplexers, and switches — into a complete assembly or sub-assembly designed to your requirements.