6-64. Optimum coupling will usually provide the necessary bandpass for the frequency-
determining network (and therefore the RF amplifier). For some uses, such as RF
amplifiers in a television receiver, the bandpass available from optimum coupling is not
wide enough. In these cases, a swamping resistor, as mentioned earlier, will be used with
the optimum coupling to broaden the bandpass.
COMPENSATION OF RF AMPLIFIERS
6-65. The frequencies at which RF amplifiers operate are so high that certain problems
exist. One of these problems is the loss that can occur in a transformer at these high
frequencies. Another problem is with interelectrode capacitance in the transistor. The
process of overcoming these problems is known as compensation.
Transformers in RF Amplifiers
6-66. TC 9-60 states that the losses in a transformer are classified as copper loss, eddy-
upon the resistance of the winding and the current through the winding. Similarly, eddy-
current loss is mostly a function of induced voltage rather than the frequency of that
voltage. However, hysteresis loss increases as frequency increases.
6-67. Hysteresis loss is caused by the realignment of the magnetic domains in the core of
the transformer each time the polarity of the magnetic field changes. As the frequency of
the AC increases, the number of shifts in the magnetic field also increases (two shifts for
each cycle of AC). Therefore, the "molecular friction" increases and the hysteresis loss is
greater. This increase in hysteresis loss causes the efficiency of the transformer (and
therefore the amplifier) to decrease. The energy that goes into hysteresis loss is taken away
from energy that could go into the signal.
6-68. RF transformers, specially designed for use with RF, are used to correct the
problem of excessive hysteresis loss in the transformer of an RF amplifier. The windings of
RF transformers are wound onto a tube of nonmagnetic material and the core is either
powdered iron or air. These types of cores also reduce eddy-current loss.
6-69. The problem of interelectrode capacitance in the transistor of an RF amplifier is
solved by neutralization. Neutralization is the process of counteracting or "neutralizing"
6-70. Figure 6-16 shows the effect of the base-to-collector interelectrode capacitance in
an RF amplifier. The "phantom" capacitor (CBC) represents the interelectrode capacitance
between the base and the collector of Q1. This is the interelectrode capacitance that has the
most effect in an RF amplifier. As you can see, CBC causes a degenerative (negative)
feedback that decreases the gain of the amplifier.
6-71. Remember, using positive feedback can counteract (neutralize) unwanted
degenerative feedback. This is exactly what is done to neutralize an RF amplifier. Positive
feedback is accomplished by the use of a feedback capacitor. This capacitor must feed back
a signal that is in phase with the signal on the base of Q1. Figure 6-17 shows one method
on how this is done.
23 June 2005