Note on Radio-Frequency Transformer Coupled Circuit Theory

Equations considering the effects of output and distributed capacities and primary resistance are developed for radio-frequency transformer-coupled amplifiers using either a tuned or an untuned primary. These equations are transformed to such a form that they may be compared with the well-known equations derived for an untuned primary neglecting the output and distributed capacities. The equations for an untuned primary are verified experimentally. It is shown that the amplification obtainable with a tube and a transformer having an untuned primary may be made nearly uniform over a frequency range such as that covered by the broadcast band by adding resistance to the primary to reduce the high-frequency amplification. It is also shown that the addition of primary resistance reduces the selectivity approximately the same percentage as it reduces the amplification. The selectivity of a stage with a tuned primary is found to be approximately the square of the selectivity of a stage with an untuned primary.