INTERSTAGE COUPLING 



of amplification. A reasonable design procedure is as follows: Take the 

 lowest frequency the amplifier is required to handle and divide it by the 

 number of RC couplings there are going to be. This gives w^. Take the 

 reciprocal, getting the minimum product CRg for each coupling. There is 

 usually a maximum value of Rg which the makers of V^ will allow. It is 

 called the maximum grid resistance and is given in the manufacturers' litera- 

 ture. Having found this, the economic minimum value of C follows. There 

 is no point in making C larger, as 'microfarads cost money'. 



Automatic grid bias — ^With RC coupled amplifiers it is possible to dispense 

 with the bias batteries by using 'automatic grid bias', which is illustrated in 

 Figure 9.10. The bottom of the grid resistance is taken to earth and instead 



HT + 



Figure 9.10 



of making the grid slightly negative to earth we make the cathode slightly 

 positive, by causing the cathode current to flow down a resistance. The value 

 of resistance Rj^ required is simply 



Requisite bias 



Rt.^ = 



^ Cathode current 



Notice the use of the term cathode current rather than anode current. In 

 Figure 9.10 a triode is shown and the two are the same thing. If the valve 

 had been a pentode or tetrode, the cathode current would have been the 

 sum of the anode and screen currents. The screen current is usually small 

 but it ought not to be forgotten. The cathode current fluctuates with the 

 input signal and produces a fluctuating bias voltage across it. This is fre- 

 quently undesirable ; the fluctuation in bias may be 'ironed out' by connecting 

 across the bias resistance a large capacitance C^, the 'cathode bypass capaci- 

 tance'. This must be chosen so that its reactance is always much lower than 

 the resistance of i?^, even at the lowest frequency the amplifier is to transmit. 

 Values of the order of 100 microfarads are often necessary. The biasing 

 scheme is automatic in the sense that as the valve wears out and the anode 

 current fafls, the bias is automatically reduced and a measure of compensa- 

 tion obtained. 



FREQUENCY RESPONSE OF INTERVALVE COUPLINGS 



We have already seen that whilst the frequency response of a direct-coupled 

 pair of valves extends down to zero, the response of an a.c. coupled pair is 

 that of a simple high-pass filter, that is, the response curve is 3 dB's down at 



154 



