With so much attention on IBOC, it is appropriate to step back and
review the basic principles of RF amplifiers.
The radio transmitter is a collection of stages. Each stage modifies
the signal in some way to produce the desired output. In the first
stage, an oscillator or exciter generates the desired operating
frequency. The output from this section is then raised to the specified
transmitter output value. This power increase may be by means of
successively larger amplifying stages or in some cases, where the
exciter output is sufficient, directly to the final power amplifier
(PA) of the transmitter.
The RF signal transmitted must carry some information. In
broadcasting, the information transmitted takes the form of speech or
music and is called modulation. With amplitude modulation (AM), the RF
carrier is varied in strength (amplitude) at a rate depending on the
frequency of the sound.
Figure 1. In a Class A amplifier, no grid current flows until the grid
goes positive. Nonlinear operation occurs when the grid current stops
tracking the plate current.
Regardless of where modulation of the carrier takes place, it is
essential that the amplifying stage produces a clean, linearly
From the beginning
The earliest transmitters used amplitude modulation and this has
continued in one form or another for about 100 years. It is probably
the simplest method of modulation, requiring only the ability to vary
the power output of an RF stage by varying the input audio signal.
In the 1930s frequency modulation (FM) was developed. It is
accomplished by varying the frequency of the transmitted RF signal
instead of the amplitude. Various methods of producing frequency
modulation have been developed, including common mechanical and phase
changing systems. Phase modulation produces the same effect in an FM
receiver as frequency modulation.
The final stage of the transmitter may be directly modulated (in
AM), or it receives an already modulated RF signal (FM). Many modern
broadcast transmitters use solid-state modules in their power amplifier
stages, however, there are still a considerable number of transmitters
that continue to use vacuum tubes in their final stages. Solid-state
devices provide considerable reduction in operating costs and their use
provides the ability, in most cases, to change a faulty module on an
operating transmitter without having to shut down.
Know the A, B, Cs
The most important characteristic of an amplifier is linearity. That
is the ability of the stage to amplify all parts by the same amount so
that all signals are amplified equally.
In a class A amplifier, current flows constantly and is not cut off
during any part of the cycle. In a tube design, this is achieved by
supplying sufficient negative bias voltage to the control grid to
ensure that it never goes positive above 0V at any time in the
This means that no grid current flows and the source is not required
to produce any drive power. For example, if the input signal has a 30V
swing and the bias is -30V, the grid voltage would swing between -60V
and 0V and no plate current would flow.
Figure 2. When a Class B amplifier is heavily cutoff, the positive
peaks cause grid current and plate current flow in a series of
Because class A amplifiers are inherently inefficient in terms of
required voltage and current, they are not generally used today in
commercial broadcast transmitters. Instead, class B and class C
amplifiers are common or variations of class B and class C circuits,
such as a class AB amplifier.
With the introduction of pulse-duration modulation and digital
operation systems, amplifiers have changed considerably, but the basic
facts still apply.
The principles of amplification remain the same regardless of
whether it is a tube or a solid-state amplifier. Because of the
proliferation of high-power transmitters still using tubes, consider
the control characteristics of a vacuum tube amplifier.
Figure 1 shows the dynamic characteristics of a triode tube
amplifier. The solid line represents the plate current. The
intersection of this line and the negative grid voltage axis shows the
cut-off point at which the tube is so heavily negatively biased that no
plate current flows. As the negative bias is decreased and passes
through zero into the positive region, the plate current increases. The
more steeply the plate current rises as the grid voltage becomes
positive, the greater the transconductance of the tube. This controls
the amplification factor. As the superimposed RF voltage is applied to
the control grid, the bias becomes more negative on negative peaks and
less negative on positive peaks. However the grid will never become
positive so that no grid current will flow.
Differences in options
The major difference between the various classes of amplifiers in
tube designs is the level of voltage applied to the power amplifier
control grid. In class A, because the plate current is never cut
completely off, the efficiency of a class A amplifier is low, about 30
percent, and so is the power output. Class AB operation is achieved by
allowing a small amount of grid current to flow as required.
In class B operation, the control grid bias is increased so that the
plate current is just at cut-off. The positive portion of the applied
signal will cause plate current to flow immediately. No matter how far
negative the grid goes, plate current will never flow. This type of
operation requires sufficient signal voltage to drive the grid
positive. The peak plate current is raised and sometimes the average
plate current uses two tubes in push-pull operation. Figure 2 shows the
operating characteristics. The output is a series of half waves with an
efficiency of about 65 percent.
Class C operation is similar except that the control grid is biased
far past cut off. Plate current only flows with high excitation and can
reach saturation. Efficiency is high, around 90 percent. However, the
waveform can be badly distorted in class B and C operation. Because of
this, the correct load impedance must contain a resistive component to
develop the required power. This is usually the input resistance of the
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