TRIODE 



charge limited conditions and cannot be made to saturate unless: (1) the 

 cathode temperature is reduced below that intended, which we have seen to 

 be bad ; or (2) the anode voltage is excessive, which is also bad. 



TRIODE 



When Lee de Forest put a grid between the anode and cathode of a hard 

 diode he produced the key device from which the whole technology of elec- 

 tronics has grown up, the triode valve (Figure 8.6). Not only does this 



Figure 8.6 



device amplify, which facilitates the detection of weak effects, but it does so 

 whilst imposing a load on the effect which is for many applications quite 

 negligible — that is, its input resistance can be made very high. This distin- 

 guishes it from the other amplifiers used in electronics, the transductor and 

 the transistor. At present its field of application is much wider than that of 

 the transductor and transistor, though it is neither as robust as the former 

 nor as small and economical in power consumption as the latter. 



If a triode be set up as shown in Figure 8.7, and the anode current plotted 

 as a function of anode voltage for various values of negative grid bias, then 

 a series of diode-like characteristic curves are produced, each one displaced 

 further to the right as the grid bias is increased {Figure 8.8). The mechanism 



\ 



v_ 



I^T 



+ 



\' 



reasing 



Figure 8.7 



Figure 8.8 



of this is that the negativity of the grid reinforces the effect of the space 

 charge in reducing the anode current, and a higher anode voltage is necessary 

 to achieve the same anode current. Figure 8.8 is called the 'anode charac- 

 teristic' of the valve. 



There are three fundamental 'valve parameters' which describe the per- 

 formance of a triode. They are: (1) ,a, the ampUfication factor; (2) g„„ the 

 mutual conductance; and (3) r^, the anode incremental resistance. 



jLi is defined as {SVjdVg)j^ and is the most nearly constant of the valve 

 parameters, both with regard to the age of the valve and to the particular 

 voltages and currents at which it is worked, /n is a pure negative number. 



135 



