IEEE Solid-State Circuits Magazine - Fall 2015 - 78
Operation above this threshold warrants its own
class designation: I choose to call this class-RS,
for Really Stupid, because there is a lot of
current flow for very little signal output.
so that the amplifier transistor is
in compression for part of the input
cycle and in cutoff for other parts of
that same input cycle.
VIN
VOUT
Figure 7: The generic architecture for a
dual-path amplifier.
One convenient use of the
graphical description in Figure 3
is to illustrate the relationships
among the operating classes as
the bias supply value varies, as
shown in Figure 4. According to the
class definitions, class-A requires
that the circle stay within the two
boundary lines of cutoff and compression. For class-B, the circle
must always be half in cutoff and
half in the active region. For classC, there is a range of low input signal magnitudes where the amplifier
Doherty Peak-Assist
VIN
+
Splitter
VOUT
Main Amplifier
Signal Envelope
Peaking
Amplifier
(a)
(b)
Figure 8: (a) The Doherty technique allows the main amplifier to saturate, with the peak
amplifier adding its output power to that from the saturated amplifier, thus providing (b)
more output power than the main amplifier is capable of producing.
Main
Amplifier
Peaking
Amplifier
Large Input Signal
(a)
Class AB
Class A
Class C
Off
Small Input Signal
(b)
Figure 9: In the Doherty system, (a) the main amplifier is often biased in class-AB for large
signals, while the peaking amplifier is biased in class-C. When the input signal is small, as
in (b), the main amplifier operates in class-A, and the peaking amplifier remains in class-C;
with insufficient input signal to draw any load current, it thus remains off.
78
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IEEE SOLID-STATE CIRCUITS MAGAZINE
will not operate at all, and at any
larger input magnitude the current
tracks with the input signal magnitude variations. (Yes, it is true:
class-C does have linear operation aspects; these are used in the
Doherty structure discussed later.)
The signal waveforms corresponding to these class settings
from a field-effect transistor (FET)
amplifier are shown in Figure 5,
where the cutoff and compression
boundaries can be readily observed.
In these curves, the bias supply settings are changed among all of the
points illustrated in Figure 2. One
rarely hears of a bias setting higher
than the traditional class-A design
goal, i.e., midway between the cutoff
and compression boundaries. Operation above this is certainly possible
and so warrants its own class designation: I choose to call this class-RS,
for Really Stupid, because there is a
lot of current flow for very little signal output. Unfortunately, class-RS
is increasingly used as an operating
class (a topic that is covered in Part 6
of this article series).
A plot of the output signal
magnitude for the waveforms in
Figure 5 across the various bias settings is shown in Figure 6. Because
the input signal magnitude is the
same for all of the bias settings, this
graph corresponds to a map of amplifier gain across bias setting variations. Gain is maximum at class-A,
and falls off in either direction. Further, in any amplifier that operates in
various classes (which includes most
amplifiers) the gain will vary along
with the input signal magnitude.
This is called gain expansion, a distortion mechanism that is undesired
and very difficult-and expensive-
to design around.
Most first-order descriptions of
transistor action model the transfer
function of Figure 2 with three line
segments: one at zero current in the
cut-off region, one at maximum current within the compression region,
and a straight line segment connecting these other two through the
transistor active region. While this
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