Amplifier Transformer Design

In amplifiers which operate at a single frequency, transformers are similar in design to rectifier plate transformers. Size of core is determined by the required value of OCL. See Section 65. If the winding carries unbalanced direct current, an air gap must be provided to keep B_m within the limits discussed in Reactors with Large A-C Flux. Winding resistances are limited by permissible loss in output, or in larger units by heating.

If the amplifier operates over a frequency range, the start of the design is with OCL to insure proper low-frequency performance. After ample core area and turns have been chosen, attention must be given to the winding configuration. Leakage inductance and winding capacitance are calculated and, from them, f_r and B. If the high-frequency response does not meet the requirements, measures must be taken to increase f_r or change B to a value nearer optimum. Sometimes these considerations increase size appreciably.

Below frequency f_r, the leakage inductance per turn is constant and equal to the total coil inductance divided by the number of turns. Capacitance per turn is constant and may be large because of the close turn-to-turn spacing. But the LC product per turn is smaller than the LC product per layer, because the layer effective capacitance is greater. Therefore the frequency at which the turns become resonant is higher than that at which the layers become resonant. Likewise, if there is appreciable coil-to-coil capacitance, the layer resonant frequency is higher than the coil resonant frequency f_r. If the coil design is such that resonance of part of a coil occurs at a lower frequency than f_r, the transformer frequency response is limited by the partial resonance. This condition is especially undesirable in wide range designs, but with reasonable care it can be avoided.

It is helpful in amplifier transformer design work to use a reactance chart, especially at the higher frequencies where resonance frequency f_r must be known in order to determine high-frequency properties. Several reactance charts have appeared in the literature.⁽¹⁾

Two examples of audio transformer design are given here to illustrate low- and high-frequency response calculations.

(1)	See "Reactance Chart," by H. A. Wheeler, Proc. I.R.E., 38, 1395 (December, 1950).