TETRODE 



swing = (GH)I(2^2) and the R.M.S. current swing = (£F)/(2\/2)- The 

 R.M.S. output power is therefore 



Area of rectangle ABCD 



8 



Similarly for a square wave input 



R.M.S. output 



Area of ABCD 



Swing of anode voltage 

 Figure 8.17 



The mean input power to the stage = HT X Mean anode current 



= Area of rectangle OIKJ 

 Efficiency of power amplifier = 



Area ABCD 

 8 X area OIKJ 



Area ABCD 



X 100 per cent for sine waves 



or 



4 X area OIKJ 



X 100 per cent for square waves 



Notice that with transformer coupling the anode voltage swings above the 

 HT. The same construction may be used to find the power output and 

 efficiency of direct coupled power amplifiers. 



TETRODE 



The simple triode valve amplifier has two important disadvantages; one we 

 have already mentioned — its voltage amplification is not very great, being 

 (g„iR)l(\ + R/rJ and not g„,R as we might hope. Matters will be improved 

 if we can find some way of increasing r„; looked at another way, it will 

 help if we can reduce the dependence of the anode current on the anode 

 voltage. 



143 



