Anritsu HFE0703 User Manual

22 High Frequency Electronics

High Frequency Design

RF POWER AMPLIFIERS

RF and Microwave Power
Amplifier and Transmitter
Technologies —

Part 2

By Frederick H. Raab, Peter Asbeck, Steve Cripps, Peter B. Kenington,
Zoya B. Popovich, Nick Pothecary, John F. Sevic and Nathan O. Sokal

P

art 1 of this series
introduced basic

concepts, discussed
the characteristics of sig­nals to be amplified, and
gave background infor­mation on RF power
devices. Part 2 reviews
the basic techniques, rat­ings, and implementation

methods for power amplifiers operating at HF
through microwave frequencies.

6a. BASIC TECHNIQUES FOR

RF POWER AMPLIFICATION

RF power amplifiers are commonly desig­nated as classes A, B, C, D, E, and F [19]. All
but class A employ various nonlinear, switch­ing, and wave-shaping techniques. Classes of
operation differ not in only the method of
operation and efficiency, but also in their
power-output capability. The power-output
capability (“transistor utilization factor”) is
defined as output power per transistor nor­malized for peak drain voltage and current of
1 V and 1 A, respectively. The basic topologies
(Figures 7, 8 and 9) are single-ended, trans­former-coupled, and complementary. The
drain voltage and current waveforms of select­ed ideal PAs are shown in Figure 10.

Class A

In class A, the quiescent current is large
enough that the transistor remains at all
times in the active region and acts as a cur­rent source, controlled by the drive.

Consequently, the drain voltage and current
waveforms are (ideally) both sinusoidal. The
power output of an ideal class-A PA is

P

o

= V

om

2

/ 2R (5)

where output voltage V

om

on load R cannot

exceed supply voltage V

DD

. The DC-power
input is constant and the efficiency of an ideal
PA is 50 percent at PEP. Consequently, the
instantaneous efficiency is proportional to the
power output and the average efficiency is
inversely proportional to the peak-to-average
ratio (e.g., 5 percent for x = 10 dB). The uti­lization factor is 1/8.

For amplification of amplitude-modulated
signals, the quiescent current can be varied in
proportion to the instantaneous signal enve­lope. While the efficiency at PEP is
unchanged, the efficiency for lower ampli-

Our multi-part series on

power amplifier tech-

nologies and applications

continues with a review of

amplifier configurations,

classes of operation,

device characterization

and example applications

This series of articles is an expanded version of the paper, “Power Amplifiers and Transmitters for RF and
Microwave” by the same authors, which appeared in the the 50th anniversary issue of the IEEE Transactions on
Microwave Theory and Techniques, March 2002. © 2002 IEEE. Reprinted with permission.

Figure 7 · A single-ended power amplifier.

From May 2003 High Frequency Electronics

Copyright © 2003 Summit Technical Media, LLC

24 High Frequency Electronics

High Frequency Design

RF POWER AMPLIFIERS

tudes is considerably improved. In an
FET PA, the implementation
requires little more than variation of
the gate-bias voltage.

The amplification process in class
A is inherently linear, hence increas­ing the quiescent current or decreas­ing the signal level monotonically
decreases IMD and harmonic levels.
Since both positive and negative
excursions of the drive affect the
drain current, it has the highest gain
of any PA. The absence of harmonics
in the amplification process allows
class A to be used at frequencies close
to the maximum capability (fmax) of
the transistor. However, the efficiency
is low. Class-A PAs are therefore typ­ically used in applications requiring
low power, high linearity, high gain,
broadband operation, or high-fre­quency operation.

The efficiency of real class-A PAs
is degraded by the on-state resistance

or saturation voltage of the transis­tor. It is also degraded by the pres­ence of load reactance, which in
essence requires the PA to generate
more output voltage or current to
deliver the same power to the load.

Class B

The gate bias in a class-B PA is
set at the threshold of conduction so
that (ideally) the quiescent drain cur­rent is zero. As a result, the transis­tor is active half of the time and the
drain current is a half sinusoid.
Since the amplitude of the drain cur­rent is proportional to drive ampli­tude and the shape of the drain-cur­rent waveform is fixed, class-B pro­vides linear amplification.

The power output of a class-B PA
is controlled by the drive level and
varies as given by eq. (5). The DC­input current is, however, proportion­al to the drain current which is in

turn proportional to the RF-output
current. Consequently, the instanta­neous efficiency of a class-B PA
varies with the output voltage and
for an ideal PA reaches π/4 (78.5 per­cent) at PEP. For low-level signals,
class B is significantly more efficient
than class A, and its average efficien­cy can be several times that of class A
at high peak-to-average ratios (e.g.,
28 vs. 5 percent for ξ = 10 dB). The
utilization factor is the same 0.125 of
class A.

In practice, the quiescent current
is on the order of 10 percent of the
peak drain current and adjusted to
minimize crossover distortion caused
by transistor nonlinearities at low
outputs. Class B is generally used in
a push-pull configuration so that the
two drain-currents add together to
produce a sine-wave output. At HF
and VHF, the transformer-coupled
push-pull topology (Figure 8) is gen­erally used to allow broadband oper­ation with minimum filtering. The
use of the complementary topology
(Figure 9) has generally been limited
to audio, LF, and MF applications by
the lack of suitable p-channel tran­sistors. However, this topology is
attractive for IC implementation and
has recently been investigated for
low-power applications at frequen­cies to 1 GHz [20].

Class C

In the classical (true) class-C PA,
the gate is biased below threshold so
that the transistor is active for less
than half of the RF cycle (Figure 10).
Linearity is lost, but efficiency is
increased. The efficiency can be
increased arbitrarily toward 100 per­cent by decreasing the conduction
angle toward zero. Unfortunately,
this causes the output power (utiliza­tion factor) to decrease toward zero
and the drive power to increase
toward infinity. A typical compromise
is a conduction angle of 150° and an
ideal efficiency of 85 percent.

The output filter of a true class-C
PA is a parallel-tuned type that

Figure 8 · Transformer-coupled
push-pull PA.

Figure 9 · Complementary PA. Figure 10 · Wavefrorms for ideal PAs.

26 High Frequency Electronics

High Frequency Design

RF POWER AMPLIFIERS

bypasses the harmonic components
of the drain current to ground with­out generating harmonic voltages.
When driven into saturation, effi­ciency is stabilized and the output
voltage locked to supply voltage,
allowing linear high-level amplitude
modulation.

Classical class C is widely used in
high-power vacuum-tube transmit­ters. It is, however, little used in
solid-state PAs because it requires
low drain resistances, making imple­mentation of parallel-tuned output
filters difficult. With BJTs, it is also
difficult to set up bias and drive to
produce a true class-C collector-cur­rent waveform. The use of a series­tuned output filter results in a
mixed-mode class-C operation that is
more like mistuned class E than true
class C.

Class D

Class-D PAs use two or more tran­sistors as switches to generate a
square drain-voltage waveform. A
series-tuned output filter passes only
the fundamental-frequency compo­nent to the load, resulting in power
outputs of (8/π

2

)V

DD

2

/R and

(2/π

2

)V

DD

2

/R for the transformer-cou­pled and complementary configura­tions, respectively. Current is drawn
only through the transistor that is
on, resulting in a 100-percent effi­ciency for an ideal PA. The utilization
factor (1/2π = 0.159) is the highest of
any PA (27 percent higher than that
of class A or B). A unique aspect of
class D (with infinitely fast switch­ing) is that efficiency is not degraded
by the presence of reactance in the
load.

Practical class-D PAs suffer from
losses due to saturation, switching
speed, and drain capacitance. Finite
switching speed causes the transis­tors to be in their active regions while
conducting current. Drain capaci­tances must be charged and dis­charged once per RF cycle. The asso­ciated power loss is proportional to
V

DD

3

/2 [21] and increases directly

with frequency.

Class-D PAs with power outputs
of 100 W to 1 kW are readily imple­mented at HF, but are seldom used
above lower VHF because of losses
associated with the drain capaci­tance. Recently, however, experimen­tal class-D PAs have been tested with
frequencies of operation as high as 1
GHz [22].

Class E

Class E employs a single transis­tor operated as a switch. The drain­voltage waveform is the result of the
sum of the DC and RF currents
charging the drain-shunt capaci­tance. In optimum class E, the drain
voltage drops to zero and has zero
slope just as the transistor turns on.
The result is an ideal efficiency of 100
percent, elimination of the losses
associated with charging the drain
capacitance in class D, reduction of
switching losses, and good tolerance
of component variation.

Optimum class-E operation
requires a drain shunt susceptance

0.1836/R and a drain series reac­tance 1.15R and delivers a power out­put of 0.577V

DD

2

/R for an ideal PA
[23]. The utilization factor is 0.098.
Variations in load impedance and
shunt susceptance cause the PA to
deviate from optimum operation [24,
25], but the degradations in perfor­mance are generally no worse than
those for class A and B.

The capability for efficient opera­tion in the presence of significant
drain capacitance makes class E use­ful in a number of applications. One
example is high-efficiency HF PAs
with power levels to 1 kW based upon
low-cost MOSFETs intended for
switching rather than RF use [26].
Another example is the switching­mode operation at frequencies as
high as K band [27]. The class-DE PA
[28] similarly uses dead-space
between the times when its two tran­sistors are on to allow the load net­work to charge/discharge the drain
capacitances.

Class F

Class F boosts both efficiency and
output by using harmonic resonators
in the output network to shape the
drain waveforms. The voltage wave­form includes one or more odd har­monics and approximates a square
wave, while the current includes even
harmonics and approximates a half
sine wave. Alternately (“inverse class
F”), the voltage can approximate a
half sine wave and the current a
square wave. As the number of har­monics increases, the efficiency of an
ideal PA increases from the 50 per­cent (class A) toward unity (class D)
and the utilization factor increases
from 1/8 (class A) toward 1/2π (class
D) [29].

The required harmonics can in
principle be produced by current­source operation of the transistor.
However, in practice the transistor is
driven into saturation during part of
the RF cycle and the harmonics are
produced by a self-regulating mecha­nism similar to that of saturating
class C. Use of a harmonic voltage
requires creating a high impedance
(3 to 10 times the load impedance) at
the drain, while use of a harmonic
current requires a low impedance
(1/3 to 1/10 of the load impedance).
While class F requires a more com­plex output filter than other PAs, the
impedances must be correct at only a
few specific frequencies. Lumped-ele­ment traps are used at lower fre­quencies and transmission lines are
used at microwave frequencies.
Typically, a shorting stub is placed a
quarter or half-wavelength away
from the drain. Since the stubs for
different harmonics interact and the
open or short must be created at a
“virtual drain” ahead of the drain
capacitance and bond-wire induc­tance, implementation of suitable
networks is a bit of an art.
Nonetheless, class-F PAs are success­fully implemented from MF through
Ka band.

A variety of modes of operation in­between class C, E, and F are possi-