TRANSFORMER COUPLING CIRCUITS 



609 



output terminals of the tube. Where the amplification factor and 

 plate circuit terminating impedance are low, as in the case of tubes oper- 

 ating as demodulators, the effective input impedance of the tube is 

 comparatively high and depends mainly on the electrode capacities. 

 The input impedance of a shielded-grid tube is likewise comparatively 

 high and depends mainly on the capacity between the grid and filament 

 terminals. The input impedance of a tube decreases with increase 

 in frequency and at very high frequencies it becomes so low that 

 difficulty is experienced in obtaining any increase in amplification in 

 the coupling circuit. In the case of the single tuned transformer, 

 previously referred to, the effective input capacity of the tube may 

 act as part of the tuning condenser with the result that at the resonance 

 frequency a substantial amplification can usually be obtained. How- 

 ever, when it is desired to transmit a broad band of frequencies the 

 magnitude of the effective input capacity becomes a very important 

 controlling factor. For a definite band width the possible voltage 

 amplification varies inversely as the value of the effective capacity 

 across the secondary winding. 



A study of the various factors entering into the use of a suitable 

 coupling circuit for successive stages of a high frequency amplifier 

 has indicated that best results are obtained if the design of the trans- 

 former is based upon the principles of the broad-band filter. Definite 

 relations are obtained between the constants of the transformer 

 windings, the tuning capacities, and the impedances of the circuits 

 between which it operates. The structure is thus essentially a band- 

 pass filter and has all of its elements properly proportioned to provide 

 the desired band selectivity, but it still retains the form and the 

 functions of a transformer. 



II. Theory and Method 

 Fig. 1 shows a simple transformer type of coupling circuit connected 

 between the plate circuit of an amplifying tube and the input circuit 

 of a second tube. The primary and secondary circuits of the trans- 



Fig. 1 — Circuit schematic of an amplifier using a transformer type of 

 coupling circuit. 



