Guest Column | May 1, 2017

What's Better – TWTAs Or SSPAs?

By Mike Lee, dB Control

db-HPA

By Mike Lee, dB Control

The short answer is: it depends on the application.

First off, what is an amplifier? The amplifier is an electronic device that increases or “amplifies” the voltage, current, or power of a signal. Amplifiers in the context of this paper are used in wireless communication, broadcasting, airborne datalink, RADAR, electronic warfare (EW), and electronic countermeasure (ECM) applications. Traveling wave tube amplifiers (TWTAs) and solid state power amplifiers (SSPAs) are the subjects of the comparison in this paper. 

Some critical parameters of an amplifier include:

  • Frequency Range
  • Gain
  • Gain Flatness
  • Power Output – Pulsed or Continuous Wave (CW)
  • Linearity
  • Noise Figure, or Noise Power Density
  • Voltage Standing Wave Ratio (VSWR)
  • Power Supply Requirements
  • Prime Power – Operating Voltages and Power Consumption
  • Mean Time Between Failures (MTBF) and Reliability
  • Dissipation and Thermal Design
  • Overall Size
  • Weight
  • Operating Environment – Temperature, Altitude, Vibration, Shock, etc.

Depending on the specific application and end-customer requirements, other parameters may be critical to the amplifier design. The ultimate goal is to create an amplifier that is specification compliant, reliable, low-cost, and easy to maintain.

TWTAs

The typical TWTA consists of a traveling wave tube, an RF input section, a power supply & logic interface section, and an RF output section (Fig. 1). For high-power TWTAs in commercial and military applications, operating frequencies range from 300 MHz up to 50 GHz. TWTAs have demonstrated performance at much higher frequencies, as well, up to 650 GHz.

Fig. 1 — A Typical TWTA Block Diagram

 

The TWT, which was developed in the 1930s, is one of two critical components in a TWTA and is the amplifying portion of the high-power amplifier (HPA). The other critical component is the high-voltage power supply, which powers parts of the TWT to enable amplification of the input signal. The TWT shown in Fig. 2 is a vacuum tube with internal components, where an input RF signal directed into the input window will travel through the TWT structure, resulting in a higher RF signal at the output window.   

Fig. 2 — A Helix TWT

 

Fig. 2 depicts a Helix TWT. These account for more than half of all types of microwave vacuum tubes due to their advantages in being able to operate over a wider bandwidth, having low noise and high gain, and being able to operate over a wide range of frequencies. 

SSPA

The solid state amplifier module, depicted as triangles in Fig. 3, consists of either GaAs (Gallium Arsenide) or GaN (Gallium Nitride) field effect transistor (FET) devices. The low-power driver and medium-power stages typically are GaAs, and the higher-power stages are GaN. The SSPA comprises the amplifying devices, power dividers, and power combiners. The required output power is achieved by adding more stages of combining. 

Fig. 3 — A Typical Solid State Amplifier Block Diagram

 

Fig. 3 depicts a four-stage combiner. The power combiners, especially at higher frequencies, have considerable loss, which can severely degrade the amplifier’s efficiency. In theory, combining four amplifiers should result in an output power four times the input power, but because of the combiner or coupler losses, typical output in such a setup is only about three times the input power. As you try to increase the combiner stages, the losses increase, resulting in lower efficiencies. This is one of the limitations of the SSPA that is not shared by the TWTA. Another SSPA limitation is its inability to operate at the broadband frequencies of which the TWTA is capable, or to operate at the higher frequencies enabled by TWTA use.

The good news is that, as GaN technology advances and matures, higher power SSPA FETs and modules will become available, and the SSPA will require fewer stages of combining to achieve higher power requirements. GaN also has superior thermal handling capabilities versus GaAs, which helps it better achieve heatsinking sizes on par with comparable TWTAs. Innovative combining techniques, such as spatial power combining technology, also can reduce combining losses. Finally, GaN devices eventually will operate at higher frequencies, though currently they are limited to Ka-Band at low power.

Thus, for low frequencies up to Ku-Band, GaN SSPAs are a viable alternative to TWTAs for high-power amplification.

Conclusion

TWTAs sometimes are subject to negative perceptions and opinions because they’re considered “old technology.”  Although they’ve been around since the 1930s, TWTAs have benefited from innovations and remain relatively unmatched in power. Additionally, due to their superior efficiency, wide bandwidth, high operational frequency, and high power output, they still are preferred for use in aerospace applications. In fact, many modern UAVs (such as General Atomics’ MQ-9 Reaper) use synthetic aperture radar (SAR) systems powered by TWTAs

TWTAs also are used for major airborne datalink, RADAR, EW, and ECM applications. That said, there are applications where SSPAs have the advantage, like in some lower-power electronic countermeasures.   That said, maturing GaN SSPA technology and innovative combining techniques soon will complicate the choice between using a TWTA or an SSPA, and the decision to use either solution will come down to using good old Ben Franklin’s pro/con rules of decision-making.

dB Control has created this guide (Table 1) to help you select the amplifier that best fits your application. For more insight, contact the author at mlee@dbcontrol.com, or visit www.dbcontrol.com

Table 1 - TWTA vs. SSPA: Advantages and Disadvantages

 

About the Author

Mike Lee is the Director of Sales and Marketing at dB Control. He has more than 25 years of RF and microwave experience developing new relationships with companies whose specs require high-power TWTAs, MPMs, power supplies, contract manufacturing services or repair depot services. Prior to joining dB Control, Lee worked at Comtech Xicom Technology and IBM – where he received a U.S. Patent for his electrical circuit design. He earned a Bachelor’s of Engineering in Electrical Engineering from The City College of New York.