6
Power Amplifiers
6.1 Introduction
Power amplifiers consist of an active device, biasing networks and input and
output reactive filtering and transforming networks. These networks are effectively
bandpass filters offering the required impedance transformation. They are also
designed to offer some frequency shaping to compensate for the roll-off in the
active device frequency response if broadband operation is required. However the
function of each network is quite different. The input circuit usually provides
impedance matching to achieve low input return loss and good power transfer.
However, the output network is defined as the ‘Load Network’ and effectively
provides a load to the device which is chosen to obtain the required operating
conditions such as gain, efficiency and stability. For example if high efficiency is
required the output network should not match the device to the load as match
causes at least 50% of the power to be lost in the active device. Note that
maximum power transfer using matching causes 50% of the power to be lost in the
source impedance.
For high efficiency one usually requires two major factors: optimum waveform
shape and a device output impedance which is significantly lower than the input
impedance to the load network.
This chapter will provide a brief introduction to power amplifier design and
will cover:
1.

Load pull measurement and design techniques.
2.

A design example of a broadband efficient amplifier operating from 130
to 180 MHz.
3.


to design an amplifier as illustrated in the design example.
6.2 Load Pull Techniques
A system for making load pull measurements is shown in Figure 6.1 and offers
both CW and pulsed measurements. The system consists of a signal generator and
250 Fundamentals of RF Circuit Design
a directional coupler on the input side of the device jig. The directional coupler
measures both the incident and reflected power with a typical coupling coefficient
of -around -20dB dB. Note the signal generator often incorporates an isolator to
prevent source instability and power variation as the load is varied.
Figure 6.1
Large signal measurement set-up
The signals are then applied to a three stage jig capable of being split into three
parts as shown in Figure 6.2. This jig includes an input matching network, a device
holder and an output matching network and can be split after the measurement.
The matching networks could include microstrip matching networks,
LC
matching
networks and transmission line tuneable stub matching networks. The printed
matching networks are varied using silver paint.
Matching networks
AB
MICROSTRIP LINE
Figure 6.2
Three piece large signal jig
The output of the amplifier jig is connected to a power detector. In fact the
power detector for either the input or output of the directional coupler could
Power Amplifiers 251
consist of a modern spectrum analyser. Most modern analysers are capable of both
CW and pulsed power measurements.
The procedure is as follows:

Now redesign the input and output matching networks to obtain all the
required components centred on reasonable values and then test the
amplifier again. Also remove any external stubs that were used by
incorporating their operation into the amplifier matching networks. Note
that the losses in the initial matching network may have been significant
so the second iteration may produce slightly different results.
6.

Test for stability by varying the bias and supply voltage over the full
operating range and by applying a variable load network capable of
varying the impedance seen by the amplifier.
Note that it is also possible to use a similar test jig with 50
Ω
lines to measure
the small signal
S
parameters while varying the bias conditions and thereby deduce
a large signal equivalent circuit model. This is used in the design example given to
252 Fundamentals of RF Circuit Design
produce an efficient broadband power amplifier. Although the techniques are
applied to a Class E amplifier they are equally applicable to any of the amplifier
classes.
6.3 Design Example
The aim here is to design a power amplifier covering the frequency range 130
MHz to 180 MHz.
6.3.1 Introduction
This section describes techniques whereby broadband power-efficient Class E
amplifiers, with a passband ripple of less than 1dB, can be designed and built. The
amplifiers are capable of operating over 35% fractional bandwidths with
efficiencies approaching 100%. As an example, a 130 to 180 MHz Class E

The losses caused by the on-resistance can be reduced by ensuring that the on-
resistance is considerably less than the load resistance presented to the switching
device.
The switching transition losses can be reduced by choosing a device with a fast
switching time. Efficiency can be further increased if the overlap of the voltage
and current waveforms can be reduced to minimise the power losses during the
switching transitions. The loss of the energy stored in the shunt capacitance at
switch-on (½CV
2
) can be reduced by choosing a device with a low parasitic shunt
capacitance. At VHF even a small parasitic shunt capacitance can result in large
losses of energy. The requirement to discharge this capacitor at switch-on also
imposes secondary stress on the switching device.
6.3.3 ClassE Amplifiers
The Class E amplifier proposed by the Sokals [7] [8] [9] and further analysed by
Raab [10] [11] is designed to avoid discharging the shunt capacitance of the
switching device and to reduce power loss during the switching transitions. This is
achieved by designing a load network for the amplifier, which determines the
voltage across the switching device when it is off, to ensure minimum losses.
Typical Class E amplifier waveforms are shown in Figure 6.3.
Figure 6.3 Class E amplifier voltage and current waveforms
254 Fundamentals of RF Circuit Design
The design criteria for the voltage are that it:
1.

Rises slowly at switch-off.
2.

Falls to zero by the end of the half cycle.
3.

L
1
) is sufficiently large to provide a constant input from the
power supply. The series
LC
circuit (
L
2
,
C
2
) is tuned to a frequency lower than the
operating frequency and can be considered, at the operating frequency, as a series
tuned circuit in series with an extra inductive reactance. The tuned circuit ensures a
substantially sinusoidal load current (Figure 6.5) and the inductive reactance
Power Amplifiers 255
causes a phase shift between this current and the fundamental component of the
applied voltage. The difference between the constant input current and the
sinusoidal load current flows through the switching device when it is on and
through the shunt capacitor (
C
1
) when it is off. The capacitor/switch current is
therefore also sinusoidal; however, it is now 180° out of phase with respect to the
load current and contains a DC offset to allow for the current flowing through the
RF choke (
L
1
).
Figure 6.5

C
1
), and as the AC component of this current also
flows through the series
LC
circuit (
L
2
,
C
2
) when the switch is off, then the load
angle of the series tuned circuit defines the optimum angle for producing the
correct voltage waveform. This load angle defines the phase shift between the
fundamental components of the voltage across the switch and the current flowing
through the series tuned circuit (
L
2
,
C
2
). In the basic Class E amplifier circuit the
harmonic impedance of the series tuned circuit is assumed to be high because of its
Q
. The value of the shunt capacitor (
C
1
) must also be correct to produce the correct
voltage when the switch is off and to satisfy the steady state conditions. The load
angle of the total network is also therefore important.