HARD VALVES 



conditions seem to be optimum the signal handling capacity is still not good 

 enough, begin the design again with a different valve of lower [x. 



Simple direct-coupled power amplifier 



To achieve power amplification the circuit diagram is similar to the 

 voltage amplifier {Figure 8.12) but the component values are rather different. 

 A typical value of load resistance for a voltage amplifier might be 50,000 ohms, 



Vsig 



Figure 8.12 



while the r„ of the valve could be 10,000 ohms. For a power amplifier we 

 want the maximum power transferred to the load and this happens when 

 R. = r^, for the valve behaves like a constant voltage generator of e.m.f. 

 fidVg in series with a resistance r„ {Figure 8.13). R^ = r„ may not be very 

 easy to arrange. If the frequency response of the system has to extend down 

 to zero frequency the load must be placed directly in series with the valve 

 as in Figure 8.12. The problem is usually to get the load resistance high 

 enough and the r^ low enough. The load might typically be the coils of a 



lihVg 



Figure 8.13 



penwriter or the magnetic deflexion system for a cathode ray tube, and high 

 resistance is only usefully obtained by winding the coils with enormous 

 numbers of turns of very fine wire, a procedure which leads to expense and 

 fragility. The necessary low r^ can only be obtained by using a valve which 

 tends to be large, since its electrode structure may be regarded as a number 

 of valves of more ordinary r„ connected in parallel. 



In practice we may have to accept a considerable mismatch — Rj^ several 

 times less than r„ — and this is unfortunate on two counts; (1) because of 

 the mismatch the power transfer conditions are less than optimum ; (2) it is 

 not possible to apply as large an input to the valve as would otherwise be 



140 



