SIMPLE AMPLIFICATION WITH THE TRIODE 



the reciprocal of the slope of this line gives the value of anode load required 

 in order that the valve anode voltage and anode current shall be correct 

 when the bias has the proposed value. This line is called the load line (Figure 

 8.11). A signal applied to the grid adds to and subtracts from the standing 

 bias, altering the grid voltage, and shding the working point along the load 

 line. The amplification of the proposed circuit may be found by seeing how 



Pmax 



Ia„ 



Figure 8.11 



much the change of anode voltage is for a unit change of grid voltage, and 

 the largest signal the valve can accept is read off as the largest change of grid 

 voltage in either direction which just does not carry the working point out- 

 side the working region. 



We now modify the design in the light of particular requirements. If the 

 valve is a 'low level' amplifier — that is, the input signals are small (of the 

 order of microvolts) — it will be shown later that it is helpful to use a low 

 anode potential and convenient to have a low high tension voltage because 

 it is best derived from a battery. Because the working point is not moved 

 very far by the signal it is quite safe to arrange it near the edge of the working 

 region, so choose one at the bottom left-hand corner. Mark the new HT 

 point— HT batteries are commonly 60, 90 or 120 V— and read off the load 

 and bias required, and the gain. Do not worry if the latter is not very large. 



In a 'medium level' amplifier — input signal of the order of millivolts — 

 concentrate on securing the maximum possible gain. Choose a high tension 

 as large as convenient and the flattest load line which will allow a working 

 point somewhere near the middle of the bottom of the working region. 

 Read off the load, and the gain which would be obtained and the bias required. 



With a 'high level' amplifier— input signal of the order of volts— the prime 

 requirement is for the valve to be able to accept the large input signal without 

 the working point passing outside the working region; a low gain is to be 

 expected. The provisional design is probably about right for a high level 

 amplifier, but see whether by juggling with the load line, the HT and the 

 mean working point the valve can be made to accept a larger input. If when 



139 



