186 



BELL SYSTEM TECHNICAL JOURNAL 



In order to calculate the maximum amplification from formula (6) 

 it is necessary to know the resistance R' of the circuit, the voltage 

 amplification factor ^i, and the internal plate impedance Rp of the 

 amplifier tube. R' was obtained by running resonance curves for the 

 circuit with the tubes connected up as usual, but with no filament 

 current in the amplifier tube, in which case Rp may be regarded as 

 being infinite. 



These resonance curves are shown in tJie lower jKut of I'ig. 4, and 

 the resistance is then calculated from the well-known f(]rnuiia 



R' = 2n{f,-f.^L, (7) 



in which /i and /j are the frequencies, for which £ = £tojx,' v2. 



The resistance R' may also be obtained from the amplification 

 curves as these can be regarded as resonance cur\-es for the timed 

 circuit with the resistance Rp across part of the coil, and since tills 



KiK- 4— ICx|iiiiimnl.iI Aniplifu-aliiin .ind R( sonanrc Curves of TuiumI Cirriiil .Amplifier 



part of the coil is chosen so as to give Za-b = Rp, the equi\alent 

 series resistance shoidd ha\e increased to exactly twice the \alue 

 found in formula (7). By comparing the widths of the amplification 

 cur\es in I-"ig. 4 with the widths fif the corresponding resonance curves 

 it is seen that (his actually was the case. 



The internal plate impedance Rp and the aini)lirK-,ui()n factor m 

 were obtained from the slope of the static characteristic of the ampli- 

 fier tube used (a Western Electric 215-A or "peanut" tube). 



