Design of Audio-Frequency Amplifier Circuits Using Transformers

The computation of performance of a transformer at frequencies near resonance is simplified for the case in which the secondary is loaded with resistance. The information is presented in the form of a family of curves in which the generator and load resistances are the dependent and independent variables. The parameter, or third variable, held constant for a given curve, is the ratio of gain at resonance to the gain at some frequency at which resonance effects do not enter. It is referred to as the "resonance gain" and is expressed in decibels rise at resonance. Here the reference level of zero decibels is taken as either the gain at zero frequency for the equivalent circuit, which acts as a low-pass filter, or at infinite frequency for the case of a high-pass equivalent. Frequency response curves are given to illustrate the use of the design charts and to show the effect of varying the ratio of load to generator resistance, keeping the gain parameter constant. A typical curve showing the effect of the ratio of load-to-generator resistance on the phase angle is also shown. Use of the charts for design purposes is explained and numerical examples given.