DIFFERENTIAL VOLTAGE AMPLIFIER 



Class B amplifiers unfortunately suffer from having their mean working 

 point outside what would generally be regarded as the working region, where 

 there is curvature at the foot of the characteristic curves. In consequence there 

 is distortion of the output waveform near the base line. This distortion is 

 called 'crossover distortion' and takes the form of flattening of the bases of 

 the half-sine v/ave loops due to the cramping of the characteristic curves. A 

 more reahstic total output waveform is that of Figure 12.17. Crossover 



Figure 12.17 



Figure 12.18 



distortion is overcome by reducing the bias shghtly so that the mean working 

 point moves into the working region : the half-loops then have the form of 

 Figure 12.18 and the distortion cancels. Some standing anode current now 

 flows and so the stage is slightly less efficient. This mode of working is called 

 class AB. Alternatively, if efficiency is vital (as in miniature battery operated 

 gear) it may be better to reduce crossover distortion with negative feedback. 



DIFFERENTIAL VOLTAGE AMPLIFIER 



If we take a direct coupled push-pull voltage amplifier and enormously 

 increase the common cathode resistance beyond the sort of value that would 

 be used for automatic biasing, and if we maintain the anode currents by 

 returning this large resistance to a negative supply, a very useful device 

 known as a 'long-tailed pair' results {Figure 12.19). 



We define an 'in-phase input' as one which moves the grids an equal amount 

 in the same direction (i.e. both positive or both negative) and a 'balanced 

 anti-phase input' as one which moves them an equal amount in opposite 

 directions. 



When a balanced anti-phase input is applied to a long-tailed pair the com- 

 bined cathode current remains the same and no change in voltage drop occurs 

 along the cathode resistance. So far as the signal is concerned the circuit 

 behaves as if Ry. were not there and amphfication of the input occurs. If the 

 signals apphed to the grids are + dVg and — 6Vg, then the potential changes 

 at the anodes are plus and minus (dVg/LiRjJI(Rj^ + rj (triodes) and 

 dVgg^Rj^ (pentodes) in the usual way. 



When an in-phase signal is applied the situation is quite different. Both 

 valves change their anode current similarly and the two halves of the circuit 

 are eff"ectively in parallel: an equivalent circuit is given in Figure 12.20. 



181 



