INTRODUCTION 3 



High input impedance is desirable if the output from the preceding 

 stage is furnished in the form of a voltage and if a minimum of current 

 or power is to be drawn from the source. Low input impedance is 

 preferred if the output from the preceding stage is furnished in the 

 form of a current and if a minimum of power is to be drawn from the 

 source, or if the presence of a large impedance in the input of 

 the element adversely affects the signal level or the operation of 

 the source. 



The input impedance (or the input force, energy, or power) is not 

 the only quantity which may influence the preceding stage and cause 

 a variation of the signal. Sometimes a potential difference exists 

 between the input terminals of an element and may have an 

 undesirable effect on the preceding stage. 



B. Transfer Characteristics 



1 . Transfer Function. This is the relationship between the magni- 

 tude of the input quantity Q t and the output quantity, or the result, 

 Q - 



Qo=f(Qi) (1) 



The differential quotient 



dQo q 



is the sensitivity of the element. In general, the sensitivity varies 

 with the input magnitude Q^; in the special — although frequent- 

 case of a linear transfer function the sensitivity is 



dQo = Qo = o, 



and is constant throughout the range of the element. In some appli- 

 cations the quantity S is called "gain" or "attenuation factor," also 

 "scale factor"; in others, the reciprocal value l/S is called the "scale 

 factor." The value of S is sometimes expressed in logarithmic (deci- 

 bel) notation. 



2. Instrument Error. In general an instrument will not follow Eq. 

 (1) correctly but will have an output 



Q'o=I(Q l )+F (2) 



where F is the (absolute) error of the result or of the output quantity, 

 i.e., the deviation of the observed output quantity #„ from the correct 

 value Q . 



F =Q' -Q 

 The error can be expressed either in terms of the output or input 



