Several factors determine the linearity of an HF so-called
"linear" amplifier :
Changes in device power gain (ratio of output to drive
power) over the range from zero to full output
collector/emitter (BJT transistor) or drain/source (MOSFET
transistor) peak RF voltage excursion
The regulation (stiffness) of the collector (+Vcc) or
drain (+Vdd) supply voltage
The standing (idle) current and the
onset of saturation in the RF transformers.
limiting factor is the constancy of power gain over the entire
power-output excursion. To visualise this, one can inspect the curve
of output vs. drive power in the transistor data sheet. Generally,
MOSFETs will exhibit superior linearity as compared to bipolar
junction transistors (BJTs). MOSFETs such as the Motorola MRF150 began to displace BJTs (e.g.
in the late 1990’s.
higher the voltage excursion, the longer the linear portion of the
output/drive power curve. For this reason, amplifiers powered
between 40-50V DC exhibit considerably better linearity, and thus lower
IMD, than 13.8V units. Typical 3rd-order IMD (IMD3) values relative
to PEP are –32 dB for 50V and –24 dB for 13.8V. Additionally, it
is more difficult to design a power supply for a 13.8 V
amplifier due to high current requirements (e.g. typically 80A peak for
a 500W PEP amplifier like Ameritron
ALS-500MX). So usually a tube amplifier is more
linear than any solid-state model (typically IMD3 < -40 dB
relative to PEP).
collector-to-collector (or drain-to-drain) load impedance must be
maintained as close to resistive as possible. This in turn requires
optimising the wideband output transformer(s), combiner (if used),
and low-pass filter passband VSWRs to as low values as possible. The
load presented to the low-pass filter output must also be as close
as possible to 50 ohms resistive.
amplification class defines the functioning mode of the
electronic circuit. It has been divided in several classes :
A, B, AB, AB1, AB2, C, D, E, G, H. Class A is the most
appreciated by fans of high fidelity audio and amateur radio
because this is the mode in which the signal yield the best
performances, in both musicality (for audio applications)
and output. However this class has some drawbacks
and therefore hams prefer using power devices in Class AB,
so many classes that we
are going to review.
work in Class A, typically the signal to amplify has a
working point (quiescent point) that normally lies in the
middle of the active region of the load line, as displayed
at left. At rest, without input signal, the cathode current
flows contineously in the device (tube or transistor),
consuming at lot of power, proportional to VceIc.
One says that this mode offers a conducting angle of 360°.
avoid distorsions (saturation or blocking) the peak values
for the output current and voltage must be always smaller
than the working point voltage and current. The maximum
signal power of this device is limited by the relationships
Class A the output signal is directly proportional to the input; hence the name
of linear amplifier.
mode is ideal if you are looking for high quality signals but it is
paid by the consumption of much energy. Its efficiency is at best
50%. That means that more than a half of the power is lost in heat,
and indeed these amplifiers heat much and request an oversized
radiator to dissipate the lost power...
A is the widely used amplification mode in low frequencies and in
the first HF stages of radio equipments. And as you probably know if
you have a Hi-Fi system, in audio it is the privileged amplification class for the purists.
Class A or
Class B amplifiers are used to increase the power
level of weak AM or SSB signals. But Class A amplification yield a
low output due to much power dissipation. The best should be to
suppress the polarization current in quiescent mode. This is
achieved in Class B amplifiers that are biased at cutoff (blocking
point) so that the collector current flows
of the input cycle.
In other words, as soon as there is an input signal, the device
unlocks during half a cycle and conducts.
we suspect that this tube or transistor will yield a better output
as it does no more consume power at rest (quiescent mode), the output signal does
only return one alternation of two and we will guess that this mode
is not satisfying. Then, as the device will only work in the
presence of an input signal, for weak inputs it will not work in the
linear region of its characteristics and thus it will not be linear.
left when a Class B amplifier works close to cut off,
its transistors or tubes can only amplify one half of
the signal. Therefore class B amplifiers
will always consist of two transistors or tubes
(push-pull circuit), where
each component will supply one half wave of the
output signal. At right the control characteristics of
two transistors of a Class B amplifier. The
non-linearity of the characteristic near zero-current
produces cross-over distortions. One correct this
effect with Class AB.
amplify the same way both alternations, Class B amplifiers are usually connected in a push-pull circuit where
one transistor amplifies each half of the input signal; there are
thus 2 transitors or tubes, each of them amplifying either the
positive or the negative alternation. To create the opposite wave
for each component, a phase splitter or phase inverter task is
inserted in the circuit. But here also there is a problem in the cross-over region that creates a
distortion at the time of alternation is changing. One
correct this effect in increasing the conducting angle so that the
phase is greater than 180°. This lead to the Class AB
summary, Class B yield a good output with an efficiency of 60%.
Drawbacks are the cross-over distortion, unsuited for amateur
emissions and the necessity to install transistors or tubes
controlled in anti-phase to get a symmetrical set.
AB, C, ...