A.C. COUPLING METHODS 



(2) the minimum current of many voltage reference tubes is as high as 

 2 mA. A Fj anode current of 10 mA is therefore required, an amount much 

 too great for low-level stages. 



A.C. COUPLING METHODS 



There are two a.c. coupling methods: transformer coupling and resistance- 

 capacitance coupling. Transformer coupling was de rigueur in the days when 

 the ampHfication obtainable for valves was not very great and the voltage 

 step-up obtainable with a transformer was welcome. Modern valves give 

 very high gains, and transformers are bulky, expensive and complicate 

 design; in consequence the intervalve coupling transformer is practically 

 extinct. 



Resistance capacitance coupling 



The resistance and capacitance referred to are Ry and C in Figure 9.5. C is 

 the couphng capacitance or 'blocking' capacitance, because it prevents the 

 positive anode potential of V^ from affecting the negative bias potential of 



HT+ 



HT+ 



Figure 9.5 



Figure 9.6 



Fg. The exact equivalent circuit of the device is shown in Figure 9.6. Since 

 r„ and Rj^ are usually much smaller th&n Rg this is with sufficient accuracy 

 reducible to Figure 9.7. If V^ is a pentode the equivalent circuit is Figure 9.8, 





l^2g''id 



-1^29^1^ 



^l- 



-^V'2grid 



QnP'^g 



* ^V. 



'99 



m 



/?,© 



Figure 9.7 



Figure 9.8 



Figure 9.9 



which, if R]^ <^ Rg, reduces to Figure 9.9. 



In both cases the coupling is seen to form a high-pass filter of time constant 

 CRy or turn-over frequency co^ = \l(CRg). co^ must therefore be below the 

 lowest frequency the amplifier is required to transmit without significant loss 



153 



