The parallel-T d.c. amplifier: a low-drift amplifier with wide frequency response

Abstract

The difficulties associated with zero-drift in direct-coupled amplifiers have been overcome to a greater or lesser extent by various artifices, which have themselves often introduced disadvantages in other directions. The paper describes a design of amplifier which possesses a very low zero-drift, whilst at the same time avoiding most of the drawbacks—such as limitation of bandwidth—incurred in similar schemes. The performance is shown to be substantially that of a direct-coupled amplifier as regards frequency response, with the distinction that it may easily be designed to have zero-errors—including drift—below 1 mV (referred to the input) without the necessity for special adjustment. Phase-shifts within the range of frequencies from zero to the high-frequency cut-off can be made low enough to be neglected in closed-loop applications. The amplifier, which is of the “parallel hybrid” type, is considered to be especially applicable to high-speed servo systems, to electro-biological measurements and to precision measurements in conjunction with the cathode-ray oscillograph, as well as to precision analogue computing. The amplifier for which circuit details are given was designed for use with a high-speed pen recorder, for recording voltages over the range of frequencies from 0–90 c/s.