HARD VALVES 



r„ is defined as (dVjdlJy^ and is given by the reciprocal of the slopes of 

 the curves in Figure 8.8. Clearly it is moderately constant if 4 is not too 

 small. It rises as the valve wears out. r„ is measured in ohms. 



The third parameter g^ can be found by re-plotting the information of 

 Figure 8.8 in terms of the anode current against grid voltage for various 

 anode voltages. It then appears as in Figure 8.9, which is called the grid 



increasing 



Figure 8.9 



characteristic of the valve. The mutual conductance is (dlJdVg)^^ and is 

 given by the slopes of the curves in the grid characteristic. It is moderately 

 constant if /„ is not too large or too small; it falls as the valve wears out. 

 Its units are amperes per volt, but it is usually measured in milliamperes per 

 volt. 



It is not necessary to plot out the grid characteristic to find g^. It can all 

 be done from the anode characteristic — usually supplied by the valve manu- 

 facturers — for if 



11 = 





and 



fa^ 





then 



A« 



SL 



Vd 



g-i 



= gr> 



is 



As we have seen, /j, and r^ can be read off the anode characteristic; g 

 given by their ratio. 



In general it is sufficiently true to say that, because the triode valve is 

 operated with the grid biased negatively with respect to the cathode the 

 electrons will be obliged to pass between the interstices of the grid on their 

 way to the anode — being repelled away from the grid wires — and that 

 therefore no current can flow into or out of the grid. This is the same as 

 saying that the resistance 'looking in' at the grid of the valve is infinite. 

 We say that the valve is 'voltage operated' rather than 'power operated'. 



SIMPLE AMPLIFICATION WITH THE TRIODE 



To get amphfication or 'gain' with a triode valve we cause the anode current 

 to flow through a load resistance. This resistance may be an actual load 



136 



