Figure 2-16C.Effect of coupling on frequency response. OVER-COUPLING
In view (A) the transformer is loosely coupled; the frequency response curve shows a narrow
bandwidth. In view (B) the transformer has optimum coupling; the bandwidth is wider and the curve is
relatively flat. In view (C) the transformer is overcoupled; the frequency response curve shows a broad
bandpass, but the curve "dips" in the middle showing that these frequencies are not developed as well as
others in the bandwidth.
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
Now you have been shown the way in which an rf amplifier is configured to amplify a band of
frequencies and the way in which an rf amplifier can be "tuned" for a particular band of frequencies. You
have also seen some ways in which the bandpass of an rf amplifier can be adjusted. However, the
frequencies at which rf amplifiers operate are so high that certain problems exist.
One of these problems is the losses 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
As you recall from NEETS, Module 1, the losses in a transformer are classified as copper loss,
eddy-current loss, and hysteresis loss. Copper loss is not affected by frequency, as it depends 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. Hysteresis loss, however, increases
as frequency increases.
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 a.c. increases, the number of
shifts in the magnetic field also increases (two shifts for each cycle of a.c.); 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.
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.