RF Power Amplifiers1 מגיש: יניב מרוז. RF Power Amplifiers2 Introduction With the...

RF Power AmplifiersRF Power Amplifiers 11

מגיש: יניב מרוז


IntroductionIntroduction

With the explosive growth of RF portable With the explosive growth of RF portable devices and their increasing functional densities devices and their increasing functional densities (data, voice, video), efficient power-saving (data, voice, video), efficient power-saving techniques are intrinsic in prolonging battery techniques are intrinsic in prolonging battery lifetime.lifetime.

Consequently, energy-efficient RF power Consequently, energy-efficient RF power amplifiers are key components in mobile battery-amplifiers are key components in mobile battery-operated systems. operated systems.


These communications employ digital These communications employ digital modulation such as quadrature phase shift modulation such as quadrature phase shift keying (QPSK).keying (QPSK).

The modulation format along with the baseband The modulation format along with the baseband filtering confines most of the signal energy to the filtering confines most of the signal energy to the desired transmit frequency band, thereby desired transmit frequency band, thereby allowing efficient usage of available spectrum.allowing efficient usage of available spectrum.

However, an undesirable consequence of However, an undesirable consequence of filtering the pulses to confine spectral energy, is filtering the pulses to confine spectral energy, is to impart a time varying amplitude dependence to impart a time varying amplitude dependence on the modulated RF signal.on the modulated RF signal.



Hence, the RF power amplifier contained in the Hence, the RF power amplifier contained in the transmitter must faithfully reproduce both the transmitter must faithfully reproduce both the time varying amplitude and phase characteristics time varying amplitude and phase characteristics of the signal. of the signal.

Since these applications utilize battery powered Since these applications utilize battery powered mobile radios, maximizing power amplifier mobile radios, maximizing power amplifier efficiency at all critical power levels is crucial to efficiency at all critical power levels is crucial to achieving extended battery operation.achieving extended battery operation.



General power amplifiers efficiency improves by General power amplifiers efficiency improves by operating the amplifier near gain compression.operating the amplifier near gain compression.

Doing so causes the envelope of the signal to be Doing so causes the envelope of the signal to be distorted (compressed in amplitude) which distorted (compressed in amplitude) which results in spectral regrowth.results in spectral regrowth.

There is an inherent trade-off between amplifier There is an inherent trade-off between amplifier efficiency and spectral linearity in designing efficiency and spectral linearity in designing linear power amplifiers.linear power amplifiers.



As the power amplifier is operated in back-off As the power amplifier is operated in back-off away from gain compression, the efficiency away from gain compression, the efficiency drops rapidly.drops rapidly.

There is a need for a linear power amplifier There is a need for a linear power amplifier which efficiently amplifies time varying amplitude which efficiently amplifies time varying amplitude modulated signals over wide dynamic ranges. modulated signals over wide dynamic ranges.

One promising method is the One promising method is the envelope envelope following technique.following technique.



The envelope following technique combines a The envelope following technique combines a high efficiency envelope amplifier with a highly high efficiency envelope amplifier with a highly efficient, but non-linear RF amplifier, to form a efficient, but non-linear RF amplifier, to form a highly efficient linear RF amplifier. highly efficient linear RF amplifier.

Operationally, we deploy a drain-bias voltage Operationally, we deploy a drain-bias voltage (Vdd) to the the RF amplifier, so it is in or near (Vdd) to the the RF amplifier, so it is in or near gain compression. gain compression.

which results in high efficiency operation. which results in high efficiency operation. The overall amplifier is efficient if both the The overall amplifier is efficient if both the

envelope and RF amplifiers are efficient.envelope and RF amplifiers are efficient.

Envelope Following TechniqueEnvelope Following Technique


Envelope Following TechniqueEnvelope Following Technique..

Ein(t) describes envelope properties of the modulationEin(t) describes envelope properties of the modulation θθin(t) describes phase characteristics of the signalin(t) describes phase characteristics of the signal


The class-S modulator is similar in form to a The class-S modulator is similar in form to a buck dc-dc converter.buck dc-dc converter.

where output pulses are produced where the where output pulses are produced where the width or duty cycle of the pulse is proportional to width or duty cycle of the pulse is proportional to the input voltage. the input voltage.

The low pass filter functions to produce an The low pass filter functions to produce an average value of the pulse signal. average value of the pulse signal. Since the detected envelope signal is time Since the detected envelope signal is time varying, the drain supply bias is also time varying, the drain supply bias is also time varying.varying.



The DC power (into RF PA) is the product of the average The DC power (into RF PA) is the product of the average drain supply voltage Vdd(t) and average supply current drain supply voltage Vdd(t) and average supply current Idd(t).Idd(t).

Compared to non-drain supply modulated RF amplifiers Compared to non-drain supply modulated RF amplifiers operating at a fixed supply voltage Vdc.operating at a fixed supply voltage Vdc.

Vdd(t) can be significantly less than Vdc especially when Vdd(t) can be significantly less than Vdc especially when the modulated RF signal exhibits a high dynamic range.the modulated RF signal exhibits a high dynamic range.

Since Idd(t)*Vdd(t) < Idd(t)*Vdc, the envelope following Since Idd(t)*Vdd(t) < Idd(t)*Vdc, the envelope following amplifier is much more efficient under power back-off for amplifier is much more efficient under power back-off for the same reason. the same reason.

This can be especially important for battery life in mobile This can be especially important for battery life in mobile radios that function for significant periods of time at radios that function for significant periods of time at reduced transmit power levels.reduced transmit power levels.



To achieve spectral linearity, it is imperative that the amplifier To achieve spectral linearity, it is imperative that the amplifier faithfully reproduces at it's output the time varying amplitude faithfully reproduces at it's output the time varying amplitude and phase characteristics of the modulated RF input signal. and phase characteristics of the modulated RF input signal.

Envelope Following TechniqueEnvelope Following Technique..


bandwidth of the class-S modulator.bandwidth of the class-S modulator.

time delay differences between the RF and time delay differences between the RF and envelope signal paths (due principally to the envelope signal paths (due principally to the group delay associated with the low pass filter).group delay associated with the low pass filter).

am-am and am-pm distortions in the RF amplifier.am-am and am-pm distortions in the RF amplifier.

developing the proper functional relationship developing the proper functional relationship between Vdd(t) and Ein(t) to satisfy the gain G.between Vdd(t) and Ein(t) to satisfy the gain G.

Factors for minimizing spectral distortionFactors for minimizing spectral distortion


Using the FeedbackUsing the Feedback

An error signal is developed “e(t) = Ein(t) - Eout(t)” An error signal is developed “e(t) = Ein(t) - Eout(t)” and fed back to the class-S modulator.and fed back to the class-S modulator.


Developing the proper drain supply signal Vdd(t) is Developing the proper drain supply signal Vdd(t) is critical for minimizing spectral distortion. critical for minimizing spectral distortion.

(Note that the gain G is a function of both Vdd and Ein). (Note that the gain G is a function of both Vdd and Ein).

For example, measured For example, measured load pull data illustrating load pull data illustrating gain, input envelope gain, input envelope voltage, and supply voltage, and supply voltage for a small HEMT voltage for a small HEMT device at particular bias device at particular bias and source/load and source/load conditions.conditions.

Proper drain supplyProper drain supply


The class-S modulator is integrating the pulse The class-S modulator is integrating the pulse width modulation (PWM) circuitry along with width modulation (PWM) circuitry along with large NMOS and PMOS power FET devices for large NMOS and PMOS power FET devices for switching large currents at high switching switching large currents at high switching speeds on a single chip.speeds on a single chip.

PWM develops pulses PWM develops pulses

whose width or duty whose width or duty

cycle is proportional to cycle is proportional to

the input voltage. the input voltage.

DC-DC ConverterDC-DC Converter


PWM is accomplished by first generating a PWM is accomplished by first generating a triangle waveform and then comparing that triangle waveform and then comparing that waveform to the input envelope voltage.waveform to the input envelope voltage.

The PWM signal is amplified using a totem pole The PWM signal is amplified using a totem pole arrangement of large P- and N-MOSFET devices.arrangement of large P- and N-MOSFET devices.



DC-DC ConverterDC-DC Converter Operating the class-S modulator at high Operating the class-S modulator at high

switching speeds offers several advantages: switching speeds offers several advantages:

1.1. Increases the bandwidthIncreases the bandwidth of the class-S of the class-S modulator, thereby improving the linearity of the modulator, thereby improving the linearity of the RF amplifier.RF amplifier.

2.2. Provides Provides better suppression of switchingbetter suppression of switching frequency componentsfrequency components by the low pass filter. by the low pass filter.

3.3. The value and size of the L/C filterThe value and size of the L/C filter network are network are dramatically reduced.dramatically reduced.


RF Power Amp. Output vs. InputRF Power Amp. Output vs. Input

RF Power Amplifiers1 מגיש: יניב מרוז. RF Power Amplifiers2 Introduction With the...

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