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RF Power Amplifiers
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Introduction

With the explosive growth of RF portable
devices and their increasing functional densities
(data, voice, video), efficient power-saving
techniques are intrinsic in prolonging battery
lifetime.

Consequently, energy-efficient RF power
amplifiers are key components in mobile batteryoperated systems.
RF Power Amplifiers
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Introduction

These communications employ digital
modulation such as quadrature phase shift
keying (QPSK).
 The modulation format along with the baseband
filtering confines most of the signal energy to the
desired transmit frequency band, thereby
allowing efficient usage of available spectrum.
 However, an undesirable consequence of
filtering the pulses to confine spectral energy, is
to impart a time varying amplitude dependence
on the modulated RF signal.
RF Power Amplifiers
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Introduction

Hence, the RF power amplifier contained in the
transmitter must faithfully reproduce both the
time varying amplitude and phase
characteristics of the signal.

Since these applications utilize battery powered
mobile radios, maximizing power amplifier
efficiency at all critical power levels is crucial to
achieving extended battery operation.
RF Power Amplifiers
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Introduction

General power amplifiers efficiency improves by
operating the amplifier near gain compression.

Doing so causes the envelope of the signal to be
distorted (compressed in amplitude) which
results in spectral regrowth.

There is an inherent trade-off between amplifier
efficiency and spectral linearity in designing
linear power amplifiers.
RF Power Amplifiers
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Introduction

As the power amplifier is operated in back-off
away from gain compression, the efficiency
drops rapidly.

There is a need for a linear power amplifier
which efficiently amplifies time varying amplitude
modulated signals over wide dynamic ranges.

One promising method is the envelope
following technique.
RF Power Amplifiers
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Envelope Following Technique

The envelope following technique combines a
high efficiency envelope amplifier with a highly
efficient, but non-linear RF amplifier, to form a
highly efficient linear RF amplifier.

Operationally, we deploy a drain-bias voltage
(Vdd) to the the RF amplifier, so it is in or near
gain compression.
which results in high efficiency operation.
 The overall amplifier is efficient if both the
envelope and RF amplifiers are efficient.
RF Power Amplifiers
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Envelope Following Technique.

Ein(t) describes envelope properties of the modulation
 θin(t) describes phase characteristics of the signal
RF Power Amplifiers
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Envelope Following Technique

The class-S modulator is similar in form to a
buck dc-dc converter.
where output pulses are produced where the
width or duty cycle of the pulse is proportional to
the input voltage.

The low pass filter functions to produce an
average value of the pulse signal.
Since the detected envelope signal is time
varying, the drain supply bias is also time
varying.
RF Power Amplifiers
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Envelope Following Technique

The DC power (into RF PA) is the product of the average
drain supply voltage Vdd(t) and average supply current
Idd(t).
 Compared to non-drain supply modulated RF amplifiers
operating at a fixed supply voltage Vdc.
Vdd(t) can be significantly less than Vdc especially when
the modulated RF signal exhibits a high dynamic range.

Since Idd(t)*Vdd(t) < Idd(t)*Vdc, the envelope following
amplifier is much more efficient under power back-off for
the same reason.

This can be especially important for battery life in mobile
radios that function for significant periods of time at
reduced transmit power levels.
RF Power Amplifiers
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Envelope Following Technique.

To achieve spectral linearity, it is imperative that the amplifier
faithfully reproduces at it's output the time varying amplitude
and phase characteristics of the modulated RF input signal.
RF Power Amplifiers
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Factors for minimizing spectral distortion

bandwidth of the class-S modulator.

time delay differences between the RF and
envelope signal paths (due principally to the
group delay associated with the low pass filter).

am-am and am-pm distortions in the RF amplifier.

developing the proper functional relationship
between Vdd(t) and Ein(t) to satisfy the gain G.
RF Power Amplifiers
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Using the Feedback

An error signal is developed “e(t) = Ein(t) - Eout(t)”
and fed back to the class-S modulator.
RF Power Amplifiers
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Proper drain supply

Developing the proper drain supply signal Vdd(t) is
critical for minimizing spectral distortion.
(Note that the gain G is a function of both Vdd and Ein).

For example, measured
load pull data illustrating
gain, input envelope
voltage, and supply
voltage for a small HEMT
device at particular bias
and source/load
conditions.
RF Power Amplifiers
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DC-DC Converter

The class-S modulator is integrating the pulse
width modulation (PWM) circuitry along with
large NMOS and PMOS power FET devices for
switching large currents at high switching
speeds on a single chip.

PWM develops pulses
whose width or duty
cycle is proportional to
the input voltage.
RF Power Amplifiers
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DC-DC Converter

PWM is accomplished by first generating a
triangle waveform and then comparing that
waveform to the input envelope voltage.

The PWM signal is amplified using a totem pole
arrangement of large P- and N-MOSFET devices.
RF Power Amplifiers
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DC-DC Converter

Operating the class-S modulator at high
switching speeds offers several advantages:
1.
Increases the bandwidth of the class-S
modulator, thereby improving the linearity of the
RF amplifier.
Provides better suppression of switching
frequency components by the low pass filter.
The value and size of the L/C filter network are
dramatically reduced.
2.
3.
RF Power Amplifiers
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DC-DC Converter
RF Power Amplifiers
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RF Power Amp. Output vs. Input
RF Power Amplifiers
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RF Power Amplifiers
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