Bandwidth Limits for Neutralized Input Capacity Amplifiers

Criteria for bandwidth limits for neutralized input capacity amplifiers are developed. The behavior of different neutralization schemes is analyzed in terms of the root locus of the system gain function. This technique permits quantitative design and evaluation of the different circuits that have been used. It makes it possible to judge the effectiveness of new approaches to the problem. It is shown that second-order amplifiers may achieve inherently faster response than amplifiers with a single time constant. Using a critically damped criterion, the maximum bandwidth of the second-order amplifier is equal to the cube root of the product of the input circuit bandwidth times the square of the amplifier bandwidth with the input circuit removed. A single time constant amplifier has a maximum bandwidth equal to the square root of the product of the input circuit bandwidth times that of the amplifier alone. It is shown also that one may have to choose between speed of response and excess noise. The noise figure of these systems may increase markedly with bandwidth and increases to a lesser degree with the system complexity.