SIMPLE AMPLIFICATION WITH THE TRIODE 



such as a penwriter or a loudspeaker, or it may be included so that fluctua- 

 tions in the anode current caused by a signal at the grid are converted to 

 fluctuations of potential diff"erence across the load so that these may be passed 

 on to operate another valve. The former case is called power amplification 

 and the latter voltage amplification. We consider voltage ampUfication first. 



Simple voltage amplifier 



In Figure 8.10 we see a triode valve whose anode potential is derived from 

 a HT supply — in this case a battery — via a load resistance R. The grid is 

 given a mean negative bias by another battery as shown, and a signal Fgjg 

 is superposed on this mean bias by placing the generator, which we regard 



Figure 8.10 



as the source of the signal, in series with it. V^ is thus — F^ + f^sig- A 

 mean current flows down the load resistance and through the valve, pro- 

 ducing a voltage drop across the load so that the anode potential is less than 

 the HT voltage. The bias and anode voltages are chosen so that the valve 

 operates on a linear part of the anode and grid characteristics, so that the 

 parameters g^, r„ and ^ are as nearly as possible constant. 



At first sight it might appear that as dVg^ Fgig, 61 ^ = gmVsig and there- 

 fore dV^ — RgmKis- Then the amplification would be SVjVsig = g^l^. In 

 fact this is not so, for if V^ changes, SlJdVg ^ g,,,. In words, suppose the 

 grid be made a certain amount less negative. The anode current increases 

 but there is an increased voltage drop across the load and the anode becomes 

 less positive. As a result the anode current does not increase as much as it 

 would have done had the anode potential been fixed. The actual amphfica- 

 tion can be found either analytically or graphically. The answer can be found 

 in a few seconds from the analytic solution for an existing design. In creating 

 a new design the graphical approach has to be adopted anyway because 

 it is the quickest way of finding the required load resistance and bias. 



Analytic expression of amplification 



If 

 then 





SL 



dV.. 



uJVa 



and /•„ = 



1 



'dV„ 



SI. 



aJl 





137 



