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across a coil to form a tuned circuit. In the same way, the secondary of T2 represents the output of this
circuit. A capacitor connected across the secondary of T2 would form a parallel LC network. This
network could act as the input-signal-developing impedance for the next stage, or the network could
represent some type of output device, such as a transmitting antenna.
The tuned circuits formed by the transformer and capacitors may not have the bandwidth required for
the amplifier. In other words, the bandwidth of the tuned circuit may be too "narrow" for the requirements
of the amplifier. (For example, the rf amplifiers used in television receivers usually require a bandwidth
of 6 MHz.)
One way of "broadening" the bandpass of a tuned circuit is to use a swamping resistor. This is
similar to the use of the swamping resistor that was shown with the series peaking coil in a video
amplifier. A swamping resistor connected in parallel with the tuned circuit will cause a much broader
bandpass. (This technique and the theory behind it are discussed in more detail in NEETS, Module 9.)
Another technique used to broaden the bandpass involves the amount of coupling in the
transformers. For transformers, the term "coupling" refers to the amount of energy transferred from the
primary to the secondary of the transformer. This depends upon the number of flux lines from the primary
that intersect, or cut, the secondary. When more flux lines cut the secondary, more energy is transferred.
Coupling is mainly a function of the space between the primary and secondary windings. A
transformer can be loosely coupled (having little transfer of energy), optimumly coupled (just the right
amount of energy transferred), or overcoupled (to the point that the flux lines of primary and secondary
windings interfere with each other).
Figure 2-16, (view A) (view B) (view C), shows the effect of coupling on frequency response when
parallel LC circuits are made from the primary and secondary windings of transformers.
Figure 2-16A.—Effect of coupling on frequency response. LOOSE COUPLING
Figure 2-16B.—Effect of coupling on frequency response. OPTIMUM COUPLING
