RESISTANCES AND CAPACITANCES 



Wien bridge transmission characteristic and phase shift are plotted for 

 a = 1 and a = 8 in Graphs 16 and 17. Evidently a = 1 gives the sharper 

 null, and in practical bridges this value is usually employed. In this case the 

 transmission characteristic reduces to 



out 



in 



1 



a/2 



oiCR 



1 +1 



mCRI 



and the phase shift to 



<j> = tan 



-1 



OiCR — 



1 



ojCR 



Parallel T filter 



It will become clear when we come to discuss valve circuits that a network 

 of the form of Figure 3.49 is not as convenient as one such as Figure 3.51, 

 where one terminal of the output is directly connected to one terminal of 

 the input, and both can, in practice, be earthed. Figure 3.51 is called the 

 parallel T filter, and has a performance similar to, but is rather more useful 

 than, the Wien bridge. 



R 



© 



2C 



'in 



^^ 



R/l 



K>ut 



Figure 3.51 



— vWA-p^WAA-* 



(a) 



Figure 3.52 



»■ t 



t o 



B 



B 



(b) 



A good way to analyse this circuit is to convert each T into an equivalent 

 pi ; taking first the T possessing shunt capacitance (Figure 3.52a) and con- 

 verting it to the pi in Figure 3.52b we get 



A = R+ R + 



R.R 2R 



X 

 -J2 



X 



iX + jR) 



50 



