Harmonic Distortion in Amplifiers

To correlate nonlinear distortion with the various parameters in vacuum-tube circuits and to determine methods for reducing this distortion, a theoretical analysis, supported by experimental measurement, was made of amplifier and cathode-follower circuits. The analysis shows that under certain operating conditions minimums may exist for the various harmonics in a vacuum-tube circuit. These minimums do not ordinarily occur for different harmonics under identical operating conditions. Although measurements were made using 6AK5 and 6C4 tubes at audio frequencies, the analysis is also valid for broadband amplifiers at higher frequencies. Curves obtained experimentally illustrate the relation between harmonic distortion and such parameters as plate voltage, grid bias, and load resistance. For a limited range of operating conditions, second- or third-harmonic distortion will drop from an average value of several percent to less than 0.01 percent.