DYNAMIC MEASUREMENTS ON ELECTROMAONETrr- DKVKKS 1419 



leaving a net gain of about 22 db, mcluding the 3 db effect of the input 

 grid capacity. 



Of equal concern is the reduced power handling capacity, but fortu- 

 nately the energy in the frequency spectrum of the input signal dimin- 

 ishes rapidly with increasing frequency. A measure of this can be arrived 

 at by the following analysis. The output voltage change for a motion of 

 0.040" is about 0.1 volt. This total motion ordinarily never takes place 

 in less than about 0.001 second, or a maximum rate of change of 100 

 volts per second. Now for a capacitive circuit 



JTjl 



i = C — = O.l X 10"' X 100 = 10 microamperes, (17) 



which is extremely small compared to the 12.5 milliamperes quiescent 

 cathode current. Thus in choosing a vacuum tube to provide the desired 

 amplifier output impedance characteristics, all other considerations have 

 been cared for. 



Amplifier-Differentiator. A schematic of the amplifier-differentiator is 

 shown as Fig. 18. It has three stages, is direct coupled, and the quiescent 

 output voltage is zero, set by the zero adjustment which controls the 

 plate voltage of the inner tube of the input stage. The first stage is a 

 conventional parallel stabilized circuit. It uses a high common cathode 

 resistor to return the twin plate currents to the —250- volt supply. The 

 first stage output is fractionated by an L pad to provide the proper do 

 bias for the second pentode stage. The output of the second stage is 

 likewise fractionated for the dc bias of the final stage. 



The third stage is a twin triode designed with an inside positive feed- 

 back loop. The two tubes have a common cathode resistor. After arbi- 

 trarily choosing one of the three grid resistors of the inside tube, the other 

 two can be determined so that the proper quiescent bias and voltage 

 changes result to make the plate current of the inner tube complement 

 the plate current of the output triode itself. This maintains the total 

 cathode current constant and estabfishes the cathodes as a virtual zero 

 impedance to ground point. This realizes full gain from the output stage 

 without another power supply. It also protects the triode from destruc- 

 tion if an accidental ground connection is made to the output. 



The feedback circuit permits a choice of three values of resistances. 

 The external amplification can be set at 10, 20, or 50. The maximum 

 value of feedback resistance which can be used is determined by the 

 value of the differentiating capacitor. It is set by the requirement that 

 at least 10 db of loop gain remain at 10 kc. 



It is now appropriate to discuss the scale factor for the differentiator 



