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Bandpass Signaling Principles and Circuits Chap. 4
input test tone. For Class B operation, the amplifier is biased so that current flows for 180° of
the applied signal cycle. Therefore, if a Class B amplifier is to be used for a baseband linear
amplifier, such as an audio power amplifier in a hi-fi system, two devices are wired in pushpull
configuration so that each one alternately conducts current over half of the input signal
cycle. In bandpass Class B linear amplification, where the bandwidth is a small percentage of
the operating frequency, only one active device is needed, since tuned circuits may be used to
supply the output current over the other half of the signal cycle. For Class C operation, the
bias is set so that (collector or plate) current flows in pulses, each one having a pulse width
that is usually much less than half of the input cycle. Unfortunately, with Class C operation, it
is not possible to have linear amplification, even if the amplifier is a bandpass RF amplifier
with tuned circuits providing current over the nonconducting portion of the cycle. If one tries
to amplify an AM signal with a Class C amplifier or other types of nonlinear amplifiers, the
AM on the output will be distorted. However, RF signals with a constant real envelope, such
as FM signals, may be amplified without distortion, because a nonlinear amplifier preserves
the zero-crossings of the input signal.
The efficiency of a Class C amplifier is determined essentially by the conduction angle
of the active device, since poor efficiency is caused by signal power being wasted in the
device itself during the conduction time. The Class C amplifier is most efficient, having an
efficiency factor of 100% in the ideal case. Class B amplifiers have an efficiency of
p>4 * 100 = 78.5% or less, and Class A amplifiers have an efficiency of 50% or less
[Krauss, Bostian, and Raab, 1980]. Because Class C amplifiers are the most efficient, they are
generally used to amplify constant envelope signals, such as FM signals used in broadcasting.
Class D, E, F, G, H, and S amplifiers usually employ switching techniques in specialized
circuits to obtain high efficiency [Krauss, Bostian, and Raab, 1980; Smith, 1998].
Many types of microwave amplifiers, such as traveling-wave tubes (TWTs), operate on
the velocity modulation principle. The input microwave signal is fed into a slow-wave structure.
Here, the velocity of propagation of the microwave signal is reduced so that it is slightly
below the velocity of the DC electron beam. This enables a transfer of kinetic energy from the
electron beam to the microwave signal, thereby amplifying the signal. In this type of amplifier,
the electron current is not turned on and off to provide the amplifying mechanism; thus, it is not
classified in terms of Class B or C operation. The TWT is a linear amplifier when operated at
the appropriate drive level. If the drive level is increased, the efficiency (RF outputDC input)
is improved, but the amplifier becomes nonlinear. In this case, constant envelope signals, such
as PSK or PM, need to be used so that the intermodulation distortion will not cause a problem.
This is often the mode of operation of satellite transponders (transmitters in communication
satellites), where solar cells are costly and have limited power output. The subject is discussed
in more detail in Chapter 8 in the section on satellite communications.
4–10 LIMITERS
A limiter is a nonlinear circuit with an output saturation characteristic. A soft saturating limiter
characteristic is shown in Fig. 4–5. Figure 4–7 shows a hard (ideal) limiter characteristic,
together with an illustration of the unfiltered output waveform obtained for an input waveform.
The ideal limiter transfer function is essentially identical to the output-to-input characteristic of