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HOWARD J. CURTIS 



reason or another. By far the best method, in the author's experi- 

 ence, is to allow the potentials to cascade. If the plates of the first 

 stage are at a potential of +25 v. (with a plate supply voltage of +50 

 v.), the grids of the second stage should be connected directly to them 

 and run at +25 v., with the cathodes of the second stage slightly 

 higher, say, +26 v. Such an arrangement is illustrated in Figure 8. 

 The cathode resistor (Re) in the second stage should be variable so 

 that it can be varied until the proper potential on the cathode is se- 

 cured. The potentials can be checked by means of a commercial 

 vacuum tube voltmeter, which is an indispensable aid in the con- 

 struction of electronic equipment. When this amplifier has been 

 balanced, the deflection of the meter, M, will be proportional to the 

 differential input voltage. 



Knput 



Fig. 8. Diagram showing cascade method of interstage coupling. 



The cascade method of interstage coupling can be used for almost 

 any number of stages. It is only necessary to have a voltage increase 

 of 30 to 60 V. per stage so a four stage amplifier can be run from a 

 single 300 v. power supply. However, in using an amplifier with low 

 plate voltages, care must be taken not to allow the signal to overload 

 the last stages. 



4. Capacitor-Coupled Amplifiers 



If it is only necessary to measure relatively rapid changes in po- 

 tential, such as the action potential of nerve or muscle, capacitance 

 coupling between stages is quite satisfactory, and indeed is much 

 more convenient to use than direct coupling. Direct coupling should 

 never be used unless it is absolutely necessary. The balanced ampli- 

 fier with common mode degeneration should always be used even for 

 capacitor-coupled amplifiers. The advantages gained are worth 

 many times the slight extra cost. 



Almost the only special precaution necessary for a capacitor- 

 coupled amplifier is to be sure to use good capacitors for interstage 



