Amplifiers are fundamental circuit-design elements. They
drive everything from earbuds to antennas. Placed ahead
of analog-to-digital converters (ADCs), they reshape signals
from sources as diverse as strain-gauges to ultrasound
probes. Through proper selection of feedback passives, they
can be configured into high-pass, low-pass, band-pass, and
band-elimination filters. Feed them with multiple signals, and
they produce harmonic series of all the components of those
inputs—good for some applications, a headache in others.
You’d think this venerable component had little left in the
innovation department, but it continues to evolve nonetheless.
What follows is a review of amplifier basics with a twist. It
assumes some engineering knowledge of how transistors are
configured into amplifiers, so it will focus more on what’s new
and what may not be so widely understood in a universe dominated
by digital thinking.
AMPLIFIER ABCS
These ABCs don’t get into how to bias a transistor, but rather
how the transistor is operated. It once was a matter of where
the device was operated on its transfer curve (Fig. 1). Now, the
amplifier class alphabet extends beyond the old classifications.
In class A, the active device is biased so that it operates in its
linear conduction region during the entire input cycle. In class B
amplifiers, there are two devices. The input waveform is split—
one active device conducts during half of an input cycle, the
other during the other half. Class AB amplifiers resemble class
Bs, except they bias their active devices so that both conduct
during some portion of the input cycle.
In class C, a single device is biased whereby it conducts during
only a small portion of an input cycle. Energy is driven into
a high-Q LC tank circuit that continues to ring at its resonant
frequency during the portions of the cycle when the active
device isn’t conducting. An analogy would be continuously
tapping a big bell with a small hammer at a rate equal to the
resonant frequency of the bell.
In the beginning, classification had something to say about
linearity versus efficiency. Class A amplifiers can be made very
linear, but their efficiency is limited. In theory, a class A amp
can achieve 50% efficiency with inductive output coupling or
25% with capacitive coupling. Class B amplifiers are subject
to “crossover” distortion, but their efficiency is theoretically as
high as 78.5%.
Class AB sacrifices some of that efficiency for lower distortion.
Class C amps can achieve up to 90% efficiency. The
resonance effect of the LC-tuned circuit minimizes distortion.
Class B amplifiers are sometimes called “push-pull” because
the outputs of the active devices have a 180° phase relationship.
(For a different kind of push-pull configuration, see fig. 4, as well as “Bridge-
Tied Load Amplifiers,” at www.electronicdesign.com, Drill
Deeper 18998.)
Beyond class C, the simple relationship between alphabetical
designations and the input signal’s application to the active
device’s transfer characteristic starts to break down. For example,
in class D amps there are two devices, as in a class B amp.
But they’re driven between saturation and cutoff by a square
wave with a frequency significantly higher than the highestfrequency
component of the input waveform.
The pulse width or pulse density of the square wave is
variable, and one or the other is controlled by the input signal.
At the amplifier output, the switching frequency and its
harmonics are attenuated by a low-pass filter, leaving only the
amplified input waveform.
With field-effect transistors operating most of the time in
either cutoff or saturation, losses are primarily from the transistors’
forward-voltage drops. Class D amps can achieve efficiencies
as high as 90%, with distortion levels approaching class AB. Suppressing
radiated and conducted interference from the switching
circuitry is challenging.
Classes E and F are related to class C, since they’re RF
amplifier topologies that use LC tank circuits.
Where class C amplifiers are widely
used below 100 MHz, class E amps are used
at VHF and microwave frequencies.
Class E amps differ
from class C amps in
that the active device is
used as a switch, rather
than being operated in
the linear portion of its transfer characteristic. Figure 2 is an
equivalent circuit for a class E amplifier.
The switch S represents the active device.
L and C are the series-tuned circuit, and R
is the load. CS represents all of the parasitic
capacitances across the active device, and
the choke isolates the power source.
Class F amplifiers resemble class E
amplifiers, but use a more complex load
network. In part, this network improves
the impedance match between the load
and the switch. Also in part, it’s designed
to eliminate the even harmonics of the
input signal so that the switching signal is
more nearly a square wave. This improves
efficiency by making the switch run at saturation
or cutoff longer.
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