DOUBLE-SIDED AMPLIFIERS 



Rj, becomes part of the load and introduces negative-feedback. We have 

 already seen that the gain of this circuit when defined as dVjdVg is only 



R^ 



Rl + R, l+AB 



2 



H 



where A is 



(t^-^"^) 



-2-+ ^^ + 2 



(triodes) or 2gA-^^RA (pentodes) 



and B is 



R. 



Rl 



2 



R. 



If ^5 ^ 1 , as it usually is, this reduces to Rj]2R^ . Thus if we make Rj, = Rj^, 

 the changes of potential produced at the anodes by an in-phase input dVg are 

 in the same direction and equal to | dVg, and the more Rj, is, the less the 



HT* 



f^9m.rh 



Input 



HT* 



\Rl/2 



h''^9m-rj2 



Input 

 1 and2o- 



^ 



— Output land 2 



JL 



\f^k 



HT- 

 Figure 12.20 



anode potential change produced. This is a most useful fact, for it means that 

 we have a differential amplifier, i.e. one which responds to inputs which affect 

 the difference of potential between the input grids, but is practically unaffected 

 by their absolute potential. Since the small output which does appear from 

 an in-phase signal is also in-phase, if another stage of amplification by a long- 

 tailed pair follows, this additional stage will further discriminate against the 

 original unwanted in-phase signal. 



Further, suppose an input 6Vg be applied to only one grid of a long-tailed 

 pair. From Figure 12.21 it is clear that this is the same as if we had applied 



182 



