. Electronic apparatus for biological research . read off. Draw the dynamic load through the working point, having the appropriate slope {Figure ). Choice of transformer ratioâFor a given input signal the maximum power is transferred to the load when the optimum transfer conditions are met. In practice these conditions are deliberately not met; let us see why this is. If the load resistance reflected into the valve circuit is other than r^ the system is mismatched, and more voltage swing will be necessary at the valve grid to develop the same power in the load. This is usually a minor prob
. Electronic apparatus for biological research . read off. Draw the dynamic load through the working point, having the appropriate slope {Figure ). Choice of transformer ratioâFor a given input signal the maximum power is transferred to the load when the optimum transfer conditions are met. In practice these conditions are deliberately not met; let us see why this is. If the load resistance reflected into the valve circuit is other than r^ the system is mismatched, and more voltage swing will be necessary at the valve grid to develop the same power in the load. This is usually a minor problem and can be dealt with by a bit more amplification at an earlier stage. A much more serious difficulty is to get a load condition such that the greatest voltage swing may be appUed to the valve grid without taking the working point outside the working region. The 'optimum load' is the load into which the valve will deliver the maximum signal power without distortion. We saw that with the direct coupled power amplifier low values of load were unfavourable from the point of view of possible input swing. For a triode it can be shown that the optimum load is twice r^, for the loss of output power due to the mismatch is overshadowed by the possibility of increasing the input. We shall not prove it rigorously, but the reader may care to experiment with some load fines on an actual published anode characteristic. The output power and efficiency of a power amplifier may be read off" as follows. In Figure , if the grid swings between Vg-^ and Vg^, the anode current swings between £â and Fand the anode voltage between G and H. Now these are peak-to-peak values. For a sine wave input the voltage 142
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