____________________________________________________ Video and Radio Frequency Amplifiers
6-60. The tuned circuits formed by the transformer and capacitors may not have the
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.
6-61. 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.
6-62. 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 coupled in the
Loosely coupled (having little transfer of energy).
Optimum coupled (just the right amount of energy transferred).
Over-coupled (to the point that the flux lines of primary and secondary
windings interfere with each other).
6-63. Figure 6-15 shows the effect of coupling on frequency response when parallel LC
circuits are made from the primary and secondary windings of transformers. In view (A),
the transformer is loosely coupled (the frequency-response curve shows a narrow
and the curve is relatively flat). In view (C) the transformer is over coupled (the frequency-
response curve shows a broad bandpass, however the curve "dips" in the middle showing
that these frequencies are not developed as well as others in the bandwidth).
Figure 6-15. Effect of Coupling on Frequency Response
23 June 2005