TWTA vs SSPA: Understanding the Differences

This article delves into the comparison between TWTA (Travelling Wave Tube Amplifier) and SSPA (Solid State Power Amplifier).

As we know, an amplifier is a device that increases the current, voltage, or power of an input signal. Amplifiers are crucial in various applications, including wireless communication, broadcasting, and audio amplification. Microwave amplifiers, specifically, operate within the frequency range of 0.3 GHz to 300 GHz. Both TWTA and SSPA fall under the category of microwave amplifiers.

The following sections compare TWTA and SSPA, highlighting the differences in their advantages, disadvantages, and other design and measurement considerations.

TWTA - Travelling Wave Tube Amplifier

The figure above illustrates the structure of a TWTA device. TWTA amplifies RF energy through the interaction of an electron beam with a slow-wave structure.

When the electron beam travels through the tube structure, energy exchange occurs between the particles and the RF wave, resulting in amplification of the RF signal. As shown, a TWTA consists of a cathode, electron gun, tube structure, and collector plate.

The cathode serves as the source of electrons, heated to approximately 1000 degrees Celsius. Applying a high voltage bias causes electrons to be generated and directed down the tube structure.

Since the cathode has a finite source of electrons, TWTs have a limited lifespan.

Advantages of TWTA

• Higher output power.
• More efficient in the back-off state.
• Smaller and lighter (TWTA along with its power supply module).
• Power is distributed over a larger area, making heat sinking less challenging.
• Reliable performance in spacecraft applications for many years.
• Stable performance across a wide temperature range.

Disadvantages of TWTA

• Requires a high voltage of approximately 10,000 volts for operation.
• Tubes have a limited lifespan, typically around 70,000 to 100,000 hours with continuous operation.
• Solid-state driver amplifiers in TWTA designs require temperature compensation.

SSPA - Solid State Power Amplifier

The figure above depicts a typical SSPA configuration, consisting of low/medium power and high power sections.

SSPA designs commonly utilize GaAs FET amplifier MMIC (Monolithic Microwave Integrated Circuit) devices.

In SSPA designs, low or medium power amplifiers are combined to achieve high power output. As shown, SSPA consists of amplifier devices, power dividers, and power combiners.

One of the advantages of SSPA is that the failure of any output stage amplifier device does not necessarily lead to the failure of the entire SSPA. However, if a driver stage amplifier fails, the output signal will be lost completely.

Advantages of SSPA

• RF modules in SSPA are smaller. However, the power supply and heat sinks make them larger in size compared to TWTA.
• SSPA does not have a limited lifespan and boasts MTBF (Mean Time Between Failures) figures of around 1 million hours.

Disadvantages of SSPA

• Requires high current, on the order of 100 Amps.
• Power FET devices used in SSPA dissipate large power at a concentrated point, making heat dissipation a significant challenge.
• Power supply reliability is a major concern.
• SSPA and driver amplifiers need to be temperature compensated.

Key Comparison Points

Here are some important comparison points between TWTA and SSPA:

• An SSPA with 133 watts will have similar linearity performance compared to a TWTA of 400 watts.
• The available maximum power of TWTA is generally greater than that of SSPA.
• The efficiency of TWTA is generally greater than that of SSPA.