RESISTANCE AND CAPACITANCE IN SERIES 



The equivalent circuit of the arrangement is Figure 3.41, from which it is 

 clear that we have a low-pass lilter whose turn-over frequency depends on the 

 amplitude control setting; at maximum output, Ry = 0, so the effective 



I 





A 



W\NV^ 





/ 



B 



I J 



Figure 3.41 



resistance in the filter is also 0, and the cable capacitance has no effect. 

 When the slider of/* is at mid-travel the effect can be serious: thus, if P = 1 

 megohm, and the cable is 6 ft. long and is P.V.C. insulated, its capacitance 

 may well be 400 pF. At mid-setting of P, R^R^liRi + ^2) is maximal at 

 0-25 MQ, and the turn-over frequency is 



w = l/C/J 



1 



400 X 10-12 X 0-25 X 10« 



= 10^ radians/sec 



Dividing by 2tt, we find this to be a frequency of the order of 1-5 kc/s, much 

 too low for many applications, Thus if A were an electrophysiological pre- 

 amplifier, and B the main amplifier, manipulation of the ampHtude control 

 would be found to affect the shape, as well as the size, of action potentials, 

 due to attenuation of high-frequency components in the waveform at reduced 

 amplitude settings. Notice, however, that as the control is moved to below 

 \ maximum, the position improves again. 



R-C circuits which filter with limited phase shift 



Inspection of Graphs 9 and 10 reveals that the simple R-C filters, by the 

 time they are offering an attenuation of 20 dB's, are also introducing a phase 



in 



I 



C 

 Rl 



T 



Figure 3.42 



shift of nearly 90 degrees. In the chapter on feedback we shall see that there 

 are times when we require circuits which will filter, whilst holding the phase 

 shift within bounds. 

 The low-pass version of such a filter is shown in Figure 3.42; it is derived 



43 



