RF Power Amplifiers

Travelling Wave Tube Amplifier (TWTA) Solid State Power Amplifier(SSPA)

International Islamic University MalaysiaSatellite Communication Systems

Prof.Rafiqul Islam Presenter > Syed Absar Kazmi

MSEE G1220119

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.

Introduction....

Microwave Amplifier

• Amplifies the signal with in the frequency range from 0.3GHz to 300GHz

• Wavelength ranges (1m-1mm)

Avaluation Parameters of an Amplifier

• Frequency range• Gain• Gain flatness• Power output• Linearity• Noise figure• VSWR • Stability

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)

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.

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.

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

Cutaway view of a HELIX TWT1. Electron Gun

2. RF input

3. Magnets

4. Attenuator

5. Helix Coil

6. RF Output

7. Vacuum tube

8. Collector

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 structureA collector to collect the electron beam after the microwave power has been generateAn input window where the small microwave signal to be amplified is introduced to the interaction structure

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 tubeAn internal attenuator, to absorb the power reflected back into the tube from mismatches in the output transmission line

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.

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.

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.

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.

SpecificationsFrequency 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

Solid State Power Amplifier(SSPA)

Solid state power amplifiers are usually divided into following types w.r.t powerlow 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;

Solid State Power Amplifier (SSPA)....

SSPA comprise of following elements Amplifier Power Divider Power combiner

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.

SSPA specification

TWTA and SSPA

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

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

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 megawattsMedium – power satellite Higher – power satellite transponder output.

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.

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.

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. MangusIEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 52, NO. 5, MAY 2005