THERMIONIC VACUUM TUBES 



53 



type of circuit is in general the easiest to handle. Due to the insertion 

 of condensers it will, of course, not serve as a direct current amplifier, 



Fig. 24 



but with sufficiently large condensers it can handle low frequencies 

 with little or no distortion. 



20. Retard-Condenser Coupled Circuit. The substitution of retard 

 coils for resistances (see Fig. 25) in the circuit last described affects 



Output 



Fig. 25 



the behavior of the circuit in several ways. An advantage in the 

 change lies in the fact that a given plate potential can be secured by 

 a smaller plate battery (Sec. 14). Since the tubes in all but the last 

 stage of an amplifier generally act as voltage amplifiers, it is desirable 

 that the inductance of the retard coils be made large. By employing 

 retard coils of the proper inductance and resistance and shunting 

 them with condensers, such an amplifier may be tuned for any par- 

 ticular frequency. 



It follows that the width of the frequency band which can be am- 

 plified without distortion is likely to be less than for the resistance 

 coupled amplifier. Since the impedance of the retard coils increases 

 with increase of frequency the higher frequencies will, in general, be 

 amplified more than the low. However, it is impossible to make 

 retard coils without a certain amount of distributed capacity, the 

 shunting effect of which tends to limit the amplification of the higher 

 frequencies. By the proper design of coils it is possible to construct 

 a retard-coupled amplifier which will give practically uniform ampli- 

 fication, e.g. throughout the speech range of 200 to 3,000 cycles. It 



