Page 106 - Complete Wireless Design

Amplifier Design

Amplifier Design 105

Figure 3.10 The frequency response of the pi network
with resonant peak.

channel interference, as well as a faulty, distorted replica of the original base-
band signal. This increases the BER, as will any noise, which can be con-
tributed to the system from almost any internal or external source.
Distortion. Distortion can form frequency intermodulation products by the
internal nonlinear mixing of any single signal with one or more other fre-
quencies, or create harmonic distortion products when only one frequency is
present. Distortion may have different causes, but comparable results: a mod-
ulated or unmodulated waveform that is altered in shape or amplitude from
the original signal due to improper circuit response.
Frequency distortion will result from circuits that increase or decrease the
amplitude of different frequencies better than others. This is normally only a
serious problem in IF or RF amplifiers if they are pushed to their extreme fre-
quency limits, since the active device in such a circuit imposes its upper—and
many times its lower—frequency limits. Thus, by the inability of a transistor to
function properly at higher frequencies (its gain decreases), induced by transit
time problems and the negative effects of junction capacitance, the frequency
distortion will be exacerbated. But since RF transistor circuits in general are
matched, filtered, coupled, and decoupled by reactive components—which are
frequency dependent—such a reactive circuit will act as a bandpass, bandstop,
high-pass, or low-pass filter, thus altering the expected frequency response of
the system if improperly designed or tuned.
Amplitude distortion, a form of nonlinear distortion, can be produced by
unsuitable biasing of an amplifier, causing either saturation or cutoff of the
transistor. This is extremely nonlinear behavior, and generates harmonics and
intermodulation distortion (IMD) products. Overdriving the input of any
amplifier (overload distortion) will create the same effect, called flattopping,
whether the bias is correct or not; creating both saturation and cutoff condi-
tions. The harmonics and IMD so generated will create interference to other
services and/or to adjacent channels and increase the system BER in a digital
data radio, while a voiceband device will have an output signal with a harsh,
coarse output.

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