linearization techniques in power amplifiers

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There exists a great demand for wireless devices that achieve a high data transmission rate

for cellular phones and wireless internet services.

The power consumption in such devices has to be low to ensure a longer battery life.

The number of wireless users is increasing exponentially, and the limited frequency

spectrum must be used in an efficient way.

One of the techniques that have been used to service multiple channels in a limited

frequency bandwidth is to place the channels close to each other.

The PA designs that do not have considerable amount of first and third order harmonic

distortion are not very power efficient.

There is a need for PAs with better characteristics that have low distortion and high power

efficiency.

Various linearization techniques are investigated to improve the linearity and facilitate

operation with less back off and high efficiency.


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Three well-known transmitters are considered to

determine the performance using the linearization

techniques., namely

Single step transmitter,

Two step transmitter

PLL based transmitter


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The base-band signal frequency is 50 MHz and the frequency of the voltage controlled

oscillator (VCO) is 1.95 GHz.

The output carrier frequency is approximately equal to local oscillator (LO) frequency.

The input signal is split into an in-phase signal and a quadrature signal.

These signals are fed to the mixer, whose other input comes from the VCO.

The two paths are added in the analog adder before entering the RF PA.

This output signal contains one frequency, which is the VCO frequency minus the input

signal frequency.

The simplicity of the topology makes it attractive for higher level of integration

Single step transmitters have a serious drawback, caused by the disturbance (noise) of the

local oscillator by the power amplifier output.

The feedback from the PA can perturb the VCO because they are at approximately the same

frequency


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Removes harmonicsRemoves unwanted sidebands


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The base-band signal is upconverted to carrier frequency in two or more steps so

that the PA output spectrum is far from the oscillator frequency.

The base-band signals I and Q undergo quadrature modulation at a lower

frequency 1, and is up-converted to 1+2 by mixing and band-pass filtering.

The first band pass filter removes the harmonics of the IF signal and the

second removes the unwanted sideband centered around 12 .

Advantages are

Quadrature modulation is performed at lower frequencies.

Less cross talk between the two input bit streams.

Lower frequency results in better performance of operational amplifier.

Disadvantage

requires filters that may be difficult to implement in integrated form.


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In PLL transmitter ,the output carrier frequency goes through PLL before

entering the PA.Huge reductions of VCO distortion by PA can be achieved if a PLL is used

instead of VCO operating directly at the required RF frequency.


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Supports better functionality to improve the switch voltage and current

waveforms.

The output tank is tuned to resonate at the carrier frequency

In comparison to class E PA, class F stage provides high impedance at

the third harmonic

Hence efficiencies are greater than 85%.

Resonates atcarrier freq


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The input signal Vdrive toggles the switch periodically with approximately 50% dutycycle.

When the switch is ON, a linearly increasing current is built up through the inductor.

When the switch is OFF, this current is steered into the capacitor, causing the voltage

across the switch VS to rise.

The band pass filters then selectively passes the fundamental component of VS to the

load, creating a sinusoidal output that is synchronized in phase and frequency with the

input.


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A linearization technique used for the third order harmonic of a power amplifier is

used to generate a signal, which compensates the non-linear component at the

fundamental frequency of the power amplifier output.

The above method is used along with a phase-locked loop (PLL) transmitter toimprove the system linearity and to reduce first order and third order harmonics.

The linearization techniques applied to the above mentioned transmitter

architectures are

Cartesian feedback,

Feed-forward

Ying linearization


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In Cartesian feedback, the output of the PA is down-converted in a demodulator

circuit, and is compared with the original low frequency signal in a negative feedback

loop

The input signal is at 900 MHz and the oscillator frequency is 1GHz.

The base-band input signals are generated from the quadrature modulator circuit.

The quadrature modulator circuit generates two signals that are 90 degrees out of phase

with each other.

These signals are fed into the differential subtractors in order to generate the error

signals.

The outputs of the differential subtractors are up-converted to RF signals by a VCO.

The resultant RF signals from the two up-converting signals are then added in analogadder to produce a complex RF output signal.

The RF output signal then goes through a PLL loop.


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The power amplifier amplifies the low power RF signal before feeding to the antenna.

The part of the output signal from the power amplifier is attenuated and fed to the down

conversion mixers at a suitable level.

These mixers are supplied with exactly the same oscillator signals because the up-

conversion and down-conversion processes are coherent.

Then the down converted output signals forms the feedback path to the differential

subtractors completing the two feedback loops.

The phase shifter can be used to align the phase of up and down conversion processes

to ensure negative feedback is created and the phase margin of the system is optimized.

PLL transmitter with Cartesian feedback conversion transmitter suffers from the

disturbance of the local oscillator by the power amplifier output.


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The non-linear PA output contains an error (distortion) with

respect to the input signal.

This signal is attenuated, and an error is computed with respect tothe input signal to the PA.

This error signal is subtracted with a proper scale, from the output

signal of the PA to form the final output.

Advantages are:

It has superior performance

Inherent stability

give high inter-modulation distortion (IMD) suppression

Disadvantages are:

its complexity and cost is high

high sensitivity to environmental variations.


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Rf i/p signal


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The RF input signal, running at 1.9 GHz, is amplified by the power

amplifier, which introduces harmonics.

A passive, band pass filter, with a center frequency f0 of 5.7 GHz, filtersnoise outside the third harmonic of the output.

A class F driver with class E load two stage power amplifier was

designed for improved linearity.

A differential subtractor for the subtraction of the synthesized signal

from the PA is employed next to acquire desired output.

A high gain amplifier (HGA) is added to improve the stability of the

feed-forward loop.

Ying linearization technique has achieved a high 1 dB compression point

of 24.51 dBm.


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The 1 dB compression point of PLL transmitter with Cartesian feedback was

21.34 dBm, whereas that with Ying linearization was 24.51 dBm.


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Single step, two step and PLL transmitters are designed in 0.18 m CMOS technology

using Cadence Tools.

The linearity of these transmitters was checked using Cartesian and Ying feedback

techniques.

The class F driver with class E output stage gives better linearity

In PLL transmitter with Cartesian feedback technique, a 1 dB compression point of

21.34 dBm was achieved.

PLL based transmitter with Ying linearization technique achieved a 1 dB compression

point of 24.51 dBm.

This led to an increase of 6dbm in linearity in Ying linearization technique as

compared to Cartesian technique.

Hence Ying linearization is best among other techniques


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Khalid H. Abed, Marian K. Kazimierczuk Shailesh B. Nerurkar Melaka P.

Senadeera, Linearization Techniques In Power Amplifiers For 1.9 Ghz

Wireless Transmitters, 0-7803-9197-7/05/$20.00 2005 IEEE.


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linearization techniques in power amplifiers

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