The Design of Transformers for Audio-Frequency Amplifiers with Preassigned Characteristics

The requirements of an ideal transformer are stated, and the difficulties encountered in attempting to build transformers for interstage coupling units which will meet these requirements are pointed out. The aim in design for audio amplifiers is stated to be a reasonable voltage amplification for one tube and one transformer which is independent of the frequency over a range necessary for broadcast reception. The equivalent a.c. circuit for some types of transformers with tube source and tube load is set up and solved. The impedance and voltage amplification characteristics are explained from the solution of the equivalent circuit. Expressions for calculating the constants of the transformer are given. A study of the design relations and voltage amplification characteristics reveals difficulties which are encountered in design and some methods for overcoming these difficulties. The effect of the continuous flux in the core of the transformer is illustrated and a scheme is given for balancing out the continuous flux. A rather universal type of bridge for making the necessary impedance measurements in connection with transformer studies is shown. This bridge is adapted to measuring iron core coils, which carry both alternating and continuous currents, when either inductive or capacitive reactive.