DYNAMIC MEASUREMENTS ON ELECTROMAGNETK l)l.\ h i > 1 I |7 



to pro\ade the output voltage and the other as a driver for the inMilairc! 

 inner shield to reduce the effective capacity to ground of the intersta^ir 

 cable connection. The cathode potential of the output tube is operated 

 at exactly +125 volts, set by means of the zero adjustment previously 

 described. 



The input networks of the succeeding amplifier-differentiator are 

 shown as part of this circuit as they provide the path for the output tube 

 current and enter directly into the fre(nu'ii< \ response and loop gain 

 cutoff design of this feedback amplifier. The dc grid voltage of the suc- 

 ceeding amplifier is 0, and this potential is found one-third the way down 

 the cathode resistor which connects to the —250 volts. This point then 

 provides the input to the following grid for displacement measurements 

 and the input resistance for gain computations of the following amplifier 

 is only this one-third part of the total cathode resistance or 10,000 ohms. 



For velocity measurements, the succeeding grid is switched to the 

 polystyrene differentiating capacitor whose grid side also is kept in 

 readiness at voltage by a grounded 1 -megohm resistor. 



For a change in current from the photocell due to a change in light, 

 the feedback acts to supply an equal but opposing current from the 

 output to keep the input grid nearly at its virtual ground potential. 

 Thus the output voltage change E, is given to a good approximation by 

 the simple relation 



E, = I^^-I,Rf. (16) 



1 - /ijS 



where Is = change in photocell current, Rf = feedback resistance, and 

 MiS = loop gain. 



A word about the differentiator connection is in order here. For this 

 use, the cathode load approaches 160 ohms at high fre(iuencies because 

 the input grid of the next amplifier is a virtual ground point, and the 

 load becomes equal to the phase shift controlling resist. )i- in scries with 

 the 0.1 mf capacitor, the value of which will he (hs.ussed later. The 

 impedance of the cathode follower without feedback is 250 ohms but 

 this does not mean that it can be connected to a 250 ohm load and op- 

 erated at the normal power output rating. The basic limitation for any 

 tube is the allowable plate current change, regardless of the extenial 

 load, consistent with never drawing grid current or l)eing cutoff. For 

 instance, in a cathode follower, if a load equal to l/Clm is used, the small 

 signal output voltage is only half of the applied grid voltage, a (i db loss. 

 Lower load resistances result in corresponding greater losses. In the 

 present circuit, about an 8 db reduction in loop gain occurs at 10 kc 



