810 



EXPLORATION GEOPHYSICS 



transient. This is usually accomplished by utilizing some form of vacuum 

 tube time delay and time averaging circuit.f 



Amplifiers. — Electrical amplification of the feeble electromotive forces 

 generated by the seismometers is usually accomplished by means of multi- 

 stage electronic amplifiers.^ Many amplifier circuits have been designed for 

 this purpose. However, the usual practice is to follow the general specifica- 

 tions necessary for a high fidelity audio-frequency transformer or resist- 

 ance-coupled amplifier. From two to five stages of vacuum tube amplifica- 

 tion are frequently employed, to have a useable gain of from 100 to 150 DB. 

 Some form of feedback is employed for stabilization. 



A majority of the amplifiers used in geophysical work are resistance 

 coupled, with a relatively narrow-band frequency response. A good descrip- 

 tion of amplifier characteristics and their design is contained in the 

 literature. § 



-^AA 1(- 



-e- 



LEVeU CONTROL 



Fig. 497. — Schematic diagram of amplifier with automatic gain control. Fi, compensating high 

 pass filter; F2, low pass filter (step construction); Cs, variable storage condenser (step construc- 

 tion). (Courtesy of D. D. Dressen, Electronic Supply C^^orp.) 



A combination type of automatic gain control and limiting amplifier is 

 shown by schematic diagram, Figure 497. Four cascade-controlled circuits 

 are shown in push-pull arrangement. A single ended system may be used. 

 However, with the development of miniature tubes, the advantages of 

 push-pull to balance out possible transients developed in gain control 



t H. E. Haynes, "Gain Control System Responsive to an Average Value," U. S. Patent 

 2,454,169, Nov. 16, 1948. 



t P. K. Chatterjea and C. T. Scully, "Thermionic Amplifier," U. S. Patent 2,431,306, Nov. 25, 

 1947. 



R. H. Park, "High Gain Amplifier," U. S. Patent 2.434,223, Jan. 6, 1948. 



§ L. G. Cowles, "The Narrow Band Resistance Coupled Amplifier," Geophysics, Vol. X, No. 3, 

 July, 1945, and "The Resistance-Coupled Amplifier," A.I.E.E. Transactions, Vol. 64, 1945, June 

 Supplement, Paper 45-69. 



Valley and Wallman, "Vacuum-Tube Amplifiers," Radiation Laboratory Series 18, McGraw- 

 Hill, New York, 1948. 



