Neutralization Techniques for High-Frequency Amplifiers: An Overview

Neutralization techniques to mitigate the oscillatory currents/voltages in amplifiers due to instability goes as far back as the era of vacuum tube amplifiers. For instan ce, in U.S. patent 1,334,118, granted on 16 March 1920 to C.W. Rice, it was observed that "under certain conditions, a device employing an incandescent cathode, an anode, and a grid produces oscillatory currents in the circuits associated therewith and in some cases the oscillatory currents so produced interfere with the efficient reception, amplification and detection of the signals to be received." It then goes on by stating that "in order to compensate for the coupling due to the natural capacity between the grid 10 and anode 11 in Figure 1(a), which is represented by the dotted condenser 12, an electromotive force, equal and opposite to that impressed, is applied to the grid circuit through the condenser 13, upon the grid from the anode 11 across the capacity 12." [See Figure 1(a).]