. Electronic apparatus for biological research . Figure Figure 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 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
. Electronic apparatus for biological research . Figure Figure 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 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 ). We define an 'in-phase input' as one which moves the grids an equal amount in the same direction ( 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 181
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