SSPA n TWTA have been briefly explained with examples UIA Malaysia Syed absar kazmi Engrabsarkazmi@gmai.com

1. RF Power Amplifiers
Travelling Wave Tube Amplifier (TWTA)
Solid State Power Amplifier(SSPA)
International Islamic University Malaysia
Satellite Communication Systems
Prof.Rafiqul Islam
Presenter > Syed Absar Kazmi
MSEE
G1220119

3. Introduction
• What is an amplifier
An amplifier is an electronic device that increases the
voltage, current, or power of a signal. Amplifiers are
used in wireless communications and broadcasting, and
in audio equipment of all kinds. They can be categorized
as either weak-signal amplifiers or power amplifiers.

4. Introduction....
Microwave Amplifier
• Amplifies the signal with in the frequency
range from 0.3GHz to 300GHz
• Wavelength ranges (1m-1mm)

5. Avaluation Parameters of an Amplifier
• Frequency range
• Gain
• Gain flatness
• Power output
• Linearity
• Noise figure
• VSWR
• Stability

6. Types of microwave amplifier
• Klystron
• Magnetron
• Travelling wave tubes
• Crossed Field Amplifiers
• Maser
• Klystrons
• Velocity modulation
• Two cavity KlystronsT
Focussing on Travelling Wave Tube (TWTA)

7. Travelling Wave Tube....
• Traveling wave tubes (twt) plays an imperative role under
the velocity-modulated tubes. As they pertain the low-
noise,high-gain,wide-bandwidth feature therefore ,it is
enourmously implicated as an active RF amplifier element
in receivers.
• TWT is capable of gains greater than 40 dB with
bandwidths exceeding an octave. (A bandwidth of 1 octave
is one in which the upper frequency is twice the lower
frequency.)
• Traveling-wave tubes have been designed for frequencies
as low as 300 megahertz and as high as 50 gigahertz. The
twt is primarily a voltage amplifier. The wide-bandwidth and
low-noise characteristics make the twt ideal for use as an rf
amplifier in microwave equipment.

8. Travelling Wave Tube ......
• Although there are various types of TWT, two major
categories are
• Helix TWT - in which the radio waves interact with the
electron beam while traveling down a wire helix which
surrounds the beam. These have wide bandwidth, but
output power is limited to a few hundred watts.
• Coupled cavity TWT - in which the radio wave interacts
with the beam in a series of cavity resonators through
which the beam passes. These function as narrowband
power amplifiers.

9. Travelling Wave Tube.....
There are two different groups of twt:
 low-power twt for receivers
occurs as a highly sensitive, low-noise and wideband amplifier in radar
equipments
 high-power twt for transmitters
these are in use as a pre-amplifier for high-power transmitters.
4 1/3 ft
Figure 1: high-power twt VTR 572B

10. Cutaway view of a HELIX TWT
1. Electron Gun
2. RF input
3. Magnets
4. Attenuator
5. Helix Coil
6. RF Output
7. Vacuum tube
8. Collector

11. Travelling Wave Tube.....
Electron Gun: produces and then accelerates an electron beam
along the axis of the tube.
A focusing magnet/magnetic system to focus the beam of
electrons through the interaction structure
A collector to collect the electron beam after the microwave
power has been generate
An input window where the small microwave signal to be
amplified is introduced to the interaction structure

12. Travelling Wave Tube.....
An helix as interaction structure, where the electron
beam interacts with the microwave signal to be
amplified.The RF signal wave injected at the input end of
the helix travels down the helix wire at the speed of the
light but the coiled shape causes the wave to travel a much
greater distance than the electron beam.
A microwave output window, where the microwave
power is taken out of the tube
An internal attenuator, to absorb the power reflected back
into the tube from mismatches in the output transmission
line

13. Travelling Wave Tube.....
Changing the number of turns or diameter of the
turns in the helix wire, the speed at which RF signal
wave travels in the form of axial E field, can be
varied.
The helical delay structure has the added advantage
of causing a large proportion of electric fields that are
parallel to the electron beam, provides maximum
interaction between the fields and the moving
electrons to form bunching.

14. Working Principle of TWT
Velocity modulation
The electrons entering the helix at zero field are not
affected by the signal wave; those electrons
entering the helix at the accelerating field are
accelerated, and those at the retarding field are
deccelerated.
This velocity modulation causes bunching of
electrons at regular intervals of one wavelength.

15. Operation TWTA
• The helix acts as a delay line, in which the RF signal
travels at near the same speed along the tube as the
electron beam.
• The electromagnetic field due to the RF signal in the
helix interacts with the electron beam, causing bunching
of the electrons (an effect called velocity modulation),
and the electromagnetic field due to the beam current
then induces more current back into the helix (i.e. the
current builds up and thus is amplified as it passes
down).
• A second directional coupler, positioned near the
collector, receives an amplified version of the input signal
from the far end of the helix.
• An attenuator placed on the helix, usually between the
input and output helices, prevents reflected wave from
traveling back to the cathode.

16. Why attenuator?
An attenuator is placed over a part of the helix on
midway to attenuate any reflected waves generated
due to the impedance mismatch.
It is placed after sufficient length of the interaction
region so that the attenuation of the amplified
signal is insignificant compared to the
amplification.

17. Specifications
Frequency Range: 3 GHz and higher
Bandwidth: about 0.8 GHz
Efficiency: 20 to 40%
Power Output: up to 10kW average
Power gain: up to 60dB

18. Solid State Power Amplifier(SSPA)
Solid state power amplifiers are usually
divided into following types w.r.t power
low power driver sections
high power output stages.
Gallium Arsenide Field Effect Transistors
(GaAs FET) are used for power amplification.
To obtain high powers, many stages are fed in
parallel from a medium to high power amplifier
and combined at the output;

19. Solid State Power Amplifier (SSPA)....
SSPA comprise of following elements
Amplifier
Power Divider
Power combiner

20. Solid State Power Amplifier(SSPA)....
• The power combiners, especially at higher frequencies
can have considerable loss (0.5 dB to 1 dB plus the
VSWR uncertainty factor), thereby limiting the number of
parallel stages to four
• Eight parallel stages are sometimes used but the
amplifier efficiency can be severely degraded.For
instance, combining four amplifiers in parallel
theoretically increases the output power four times, but
because of the coupler losses, the output power only
increases about three times. The primary power
increases four times, lowering the overall amplifier
efficiency about 30%.
• In reality, the amplifier efficiency is even lower when
considering the fact that higher power driver amplifiers
are necessary to power multiple parallel output stages.

21. SSPA specification

22. TWTA and SSPA

23. TWTA and SSPA Comparison regarding to
Output power,Efficiency and weight

24. TWTA Versus SSPA:
A Comparison of On-Orbit Reliability Data

26. Applications of TWTA
A major advantage of the TWT over some other
microwave tubes is its ability to amplify a wide range of
frequencies, a wide bandwidth.
The bandwidth of the helix TWT can be as high as two
octaves, while the cavity versions have bandwidths of 10 -
20%
Operating frequencies range from 300 MHz to 50 GHz.
The power gain of the tube is on the order of 40 to 70
decibels and output power ranges from a few watts to
megawatts
Medium – power satellite
Higher – power satellite transponder output.

27. Applications of TWTA...
A TWTA whose output drives an antenna is a type of
transmitter.
TWTA transmitters are used extensively in radar,
particularly in airborne fire-control radar systems, and
in electronic warfare and self-protection systems.
In such applications, a control grid is typically
introduced between the TWT's electron gun and slow-
wave structure to allow pulsed operation. The circuit
that drives the control grid is usually referred to as a
grid modulator.

28. Applications of SSPA
• Impression that SSPA is superior (stable, lighter
etc) for lower power HPA's (say down to 50-
100w amplifiers)
• All these factors like efficiency,linearity, spectral
regrowth depends upon efficiency of the medium
for conversion of electrical energy to usable RF
power. are usually better with SSPA in the
operating region
• The SSPA e.g GaAs or GaN which is a new type
of material that claimed to be superior in thermal
handling can be used at higher temperature.

29. References
• www.Radar Basics - Traveling Wave Tube
“A Comparison of SSPA and TWTA Amplifier Systems / Advantech”
Application Note.
“Traveling Wave Tube vs. Solid State Amplifiers,” Stephan Van
Fleteren Communications and Power Industries (CPI, formerly
Varian Associates).
TWTA Versus SSPA: A Comparison of On-Orbit Reliability Data
Jan M. Weekley and Bill J. Mangus
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 52, NO. 5, MAY 2005