Book Description

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1920 edition. Excerpt: ... the third term and having double the frequency of the fundamental is present, as was to be expected from the parabolic shape of the characteristic. It is shown by the curve ff in Fig. 83. This is the undesirable term which causes distortion. The last term, which is proportional to the square of the input voltage is the change in the d-c. component due to the alternating input voltage (shown by the broken line in Fig. 83), and is the only effective component of the output current when using the device as a radio detector. If the output transformer To (Fig. 82) has a sharp frequency characteristic, and a pure note be impressed on the input of the tube, a current meter A2 inserted in the load circuit would indicate a current which is proportional only to the second term of equation (19), that is, to the fundamental, and the distortion produced by the curvature of the characteristic would not be a serious matter. In telephony we have to deal, however, with frequencies ranging up to about 3000 cycles per second, and it is desirable to use a transformer with a flat frequency characteristic, so that all frequencies are transmitted with more or less equal facility. In such case the harmonic term would cause serious distortion of the speech wave. Distortion can, however, be reduced to a negligible quantity by properly choosing the impedance of the output transformer. We have so far assumed that the transformer impedance in the direction Z0Z1 is small compared with the plate resistance of the tube. In practice this is not so. In fact the best operation as power amplifier is obtained when this impedance is approximately equal to the plate resistance. In such case the characteristic of the circuit is different from that of the tube alone, as we...