DIFFERENTIAL VOLTAGE AMPLIFIER 



an anti-phase signal dVJl to both grids, quickly followed by an in-phase 

 signal dVJl. The former causes the anodes to move in opposite directions 

 and the latter has almost no effect. If the anodes move equally in opposite 



Grid 1 



Grid 2 



Figure 12.21 



directions we have all the benefits of push-pull voltage amplification (no 

 decouphng required etc.) without the need for a special phase-splitter to drive 

 the stage. It makes no difference in what ratio the input is divided between 

 the grids, the effect at the output is substantially the same. In other words, the 

 stage responds similarly to an anti-phase input whether the input is balanced 

 or not. 



The discrimination ratio or rejection ratio of a complete differential amplifier 

 is the ratio of the deflections produced at the indicating device by a unit anti- 

 phase and a unit in-phase signal. Its magnitude depends on the way the 

 indicating device is fed ; since the anodes of the last differential stage in the 

 amplifier move equally in opposite directions for any anti-phase signal it is 

 possible, convenient but theoretically undesirable to take the output from 

 between one anode only and earth. To take an example, suppose the indicat- 

 ing device is a cathode ray tube of the classical type having a pair of Y 

 deflector plates, and suppose we have two similar differential voltage- 

 amplifying stages employing pentodes having g,„ = 1-5 mA/volt, anode loads 

 of 100 kQ and a cathode resistance in each stage of 50 kQ (as indicated by the 

 incomplete circuit in Figure 12.22). Then the two deflector plates are con- 

 nected to the final stage anodes and the deflection produced is proportional to 

 the difference of final anode potential. 



The stage gain for an anti-phase signal isg„^R = 150, so the anti-phase gain 

 of the whole amplifier is 22,500. The in-phase stage gain is RjKlR^) = 1. 

 Since for the in-phase signal the final anodes move in the same direction, and 

 since the C.R.T. spot is deflected by difference of deflector plate potential, 

 no deflection is produced by the in-phase signal and the discrimination ratio 

 is theoretically infinite. 



Now suppose we change the cathode ray tube for one of the Cossor 

 double-beam type. In the latter tube each spot is deflected by varying the 

 potential of one deflector plate with respect to a beam spHtter plate whose 



183 



