. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. 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 m
. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. 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 Crossover. 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
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