Grid-current measurements with a D-C analog computer

INCREASED USE of amplifiers at frequencies of 0 to 20 cycles per second has focused attention on the problem of designing direct-coupled amplifiers. Grid current, in particular, is a problem, inasmuch as it causes a change in tube bias that cannot be distinguished from a signal. For this reason, direct-coupled amplifiers are designed to operate in the region where grid current is normally considered negligible. Operation in this region, however, is not in itself a solution because even these small currents are significant in direct-coupled amplifiers. Since grid-current characteristics for this region are not included in handbooks, the designer must solve the new problem of measuring these currents.