18 



THE CATHODALLY SCREENED 

 CATHODE FOLLOWER 



In electrophysiological work with glass micro-pipettes it is usual to make use 

 of the high input impedance of the cathode follower and employ such a stage 

 as a buffer between micro-electrode and amplifier. The piece of wire joining 

 the electrode to the cathode-follower grid is extremely prone to pick up 

 interference and requires screening. 



In order that the capacitance between grid lead and screen shall have the 

 minimum effect, the screen is connected to the valve cathode instead of to 

 earth; if this capacitance is Cg and the cathode-follower gain is G, the 

 effective input capacitance is not Q + (1 — G)Cg„ + Cg^, as it would be 

 with the screen earthed, but only (1 — G)(C, + Gg,,) + Q„. The technique 

 is called cathodal screening and is due to M. Ryle. 



To prevent interference pick-up by the cathodal screening, it is itself 

 usually surrounded by an earthed screefi (Figure 18.1). The presence of the 



earthed screen has little effect on the performance, since it is in parallel with 

 the low output-impedance of the cathode follower. 



However, when use is made of the ability of the cathode follower to provide 

 cathodal screening and io drive a long piece of cable, as in the typical electro- 

 physiological circuit oi Figure 18.2, it is possible to run into difficulties; it has 

 been pointed out by W. J. Nastuk and A. L. Hodgkin (/. cell, coinp. Physiol. 

 35 (1950) 39) that the transient response of the cathode follower can be 

 oscillatory. 



Steady-state response 



The circuit may be drawn as in Figure 18.3. R^^ represents the micro- 

 electrode and Q is the capacitance of the cathodal screen. Q is the capaci- 

 tance between the inner conductor of the output cable and its screen. The 

 valve capacitances are likely to be relatively insignificant. 



259 



