SIMPLE AMPLIFICATION WITH THE TRIODE 



the case. That is, the valve is overloaded more easily*. This may easily be 

 seen from the working region of the power amplifier valve anode characteris- 

 tic (Figure 8.14). Clearly, when the load is small (load hne steep) the maximum 



Figure 8.14 



possible input 'swing' dVg is much less than for the flatter load line representing 

 the higher load. 



Simple transformer-coupled power amplifier 



If the response of the system does not have to extend down to zero fre- 

 quency, as for example in sound reproduction, design is much easier, for a 

 transformer may be used to match up any convenient valve and load. The 



HT+ 



Figure 8.15 



resistance of a loudspeaker is commonly as low as 3 Q. An appropriate valve 

 might have r„ = 5,000 O.. The two can be matched for approximate maxi- 

 mum power transfer by using a transformer of turns ratio (5,000/3)^/^ 

 {Figure 8.15). 



* An unfortunate but accepted piece of jargon. Is nothing to do with excessive values 

 of load. 'Overloading' simply means an excessive input that takes the working point out- 

 side the working region. 



141 



