Wide-Band Transformers

Untuned transformers operate in all frequency ranges from 0 to VHF. The lowest operating frequency is a fraction of 1 cycle. The highest frequency is in the VHF band, somewhere around 150 megacycles. No known transformer covers this whole range at present. Television coaxial-line terminating transformers have been made to cover the frequency range of 50 cycles to 6,000,000 cycles, or a ratio of highest to lowest frequency of over 100,000:1. This is an exceptionally wide band. More common wideband transformers are those in the audio band of 20 to 20,000 cycles, or 10 to 30,000 cycles, that is, with about a 3,000:1 frequency ratio. Often, transformers used at frequencies on the order of 100 megacycles are for relatively narrow bands, say 20 to 60 megacycles wide.

In low-impedance circuits, it is leakage inductance that determines transformer behavior, whereas at high impedance it is winding capacitance. In most audio transformers the coupling coefficient is 0.9995 or higher. With bifilar windings, this figure may increase to 0.999995.⁽¹⁾ Such a high coefficient of coupling requires the use of good core materials. For a given source impedance and transformer core material, the product of turns ratio and band width is a rough indication of size. Quite generally, for low power the widest-band transformers are made of Permalloy or Supermalloy.

In high-impedance circuits the matter of size is not merely one of space for mounting; it also has a direct bearing on the upper frequency limit, since transformer capacitance is roughly proportional to size. If capacitance is low, the band-width ratio (highest/lowest frequency) is approximately equal to the ratio of OCL/leakage inductance. This may be verified by comparing Figs. 108 and 109. It is most nearly true for low-impedance transformers. With given primary impedance, core size, and material, there is a limit to the step-up turns ratio possible for any specified frequency response.

(1)	See "New 50-Watt Amplifier Circuit," by F. H. McIntosh and Gordon J. Gow, Audio Engineering, December, 1949, p. 9.