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TWTA vs SSPA | Difference between TWTA and SSPA

This page on TWTA vs SSPA provides difference between TWTA and SSPA. TWTA stands for Travelling Wave Tube Amplifier and SSPA stands for Solid State Power Amplifier. The other difference between terms are provided here.

As we know amplifier is a device which increases current, voltage or power of signal provided as its input. It is widely used for various applications in wireless communication, broadcasting and audio amplification etc.

Microwave amplifier amplifies signal in the frequency range from 0.3GHz to 300GHz. Both TWTA and SSPA falls under types of microwave amplifiers.

Following sections compares TWTA vs SSPA and mention difference between TWTA and SSPA with respect to their advantages, disadvantages and other design and measurement factors.

TWTA-Travelling Wave Tube Amplifier


The figure depicts the structure of TWTA device. TWTA amplifies RF energy through interaction of electron beam with slow wave structure as shown in the figure-1. When electron beam travels through the tube structure, energy exchange between particles and Rf wave will takes place which results into amplification of RF signal.

As shown TWTA consists of cathode, electron gun, tube structure and collector plate. Cathode is the source of electrons. This cathode is heated at approx. temperature of 1000 degree celsius. High voltage bias application results into generation of electrons down the tube structure. As cathode has finite source of electrons, TWT will have certain life time.

Advantages of TWTA

Following are the advantages of TWTA:
• Higher output power.
• More efficient in the back off state.
• TWTA along with power supply module are smaller and of less weight.
• In TWTA power is distributed over larger area and hence heat sinking is less challenging task here.
• TWTAs are used reliably in space craft since years.
• TWTs are very stable over wide temperature range.

Disadvantages of TWTA

Following are the disadvantages of TWTA:
• Needs high voltage of about 10000 volts for operation.
• Tubes will have life of about 70000 to 100000 hours with continuous operation.
• In TWTA design, solid state driver amplifiers need to be temperature compensated.

SSPA-Solid State Power Amplifier


The figure depicts SSPA configuration consisting low/medium power and high power sections. Usually in SSPA design GaAs FET amplifier MMIC devices are used. In SSPA design low or medium power amplifiers are combined to obtain high power output.

As shown SSPA consists of amplifier devices, power dividers and power combiner. One of the advantages of SSPA is that any output stage amplifier device failure does not result into failure of SSPA as a whole. But when driver stage amplifier fails the output signal will be lost completely.

Advantages of SSPA

Following are the advantages of SSPA:
• RF modules in SSPA are smaller. But power supply and heat sinks will make them larger in size compare to TWTA.
• SSPA does not have limited life and will have MTBF figures of about 10 lakh in hours.

Disadvantages of SSPA

Following are the disadvantages of SSPA:
• SSPA needs high current on the order of 100 Amps.
• Power FET devices used in SSPA dissipate large power at concentrated point. Heat dissipation here is most challenging problems.
• Power supply reliability is a major concern in SSPA.
• SSPA and driver amplifiers have to be temperature compensated.

Following are the important comparison points between the two.

• SSPA with 133 watt will have same linearity performance compare to TWTA of 400 watt. Available maximum power os TWTA is greater than SSPA.

• Efficiency of TWTA is greater than SSPA.

TWTA is rated in units of saturated output power(Psat). SSPA is rated in units of 1 dB compression point.
TWTA IP3 is 2 to 5 dB greater than Psat SSPA IP3 is 7 to 10 dB greater than 1dB compression point
Failure rate is lower. Failure rate is higher.
Heat in the TWTAs usually is less than or equal to the heat produced in SSPA devices. -

TWT and SSPA Related Links

SSPA Test Procedure   Power Amplifier Basics and SSPA design example   TWT calculator   microwave tubes    TWT Vs Klystron   

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