Capacitively coupled magnetic amplifiers

Amplification of a-c signals with magnetic amplifiers is generally more difficult than d-camplification. The conventional full-wave circuits, such as the doubler circuit and the bridge circuit, can yield high performance only by demodulating a-c signals and amplifying the resultant direct current. These circuits remain, in effect, d-c amplifiers and suffer from several shortcomings. For example, the sensitivity is limited by the forward voltage drops of the demodulating elements, the output is affected by d-c drifts of the control characteristics, and the component duplication and ineffeciencies of push-pull circuitry must be used to obtain phase sensitivity. The work of Ramey has led to the development in recent years of a number of high-speed magnetic amplifiers of the half-wave type. These circuits can amplify a-c signals without demodulation, but they also have certain shortcomings. The power gain per stage is generally less than the gain of full-wave circuits, the amplifiers remain subject to the effect of zero drift caused by unbalance of component characteristics, and low-level sensitivity is particularly difficult to obtain. Geyger has shown that the effect of amplifier zero drift on a-c servomotors (or other frequency discriminating output elements) can be eliminated by operating half-wave circuits at a carrier frequency several times the signal frequency.1 None of these circuits, though, entirely achieves the advantages relative to d-c amplification techniques which are associated with true a-c amplifiers.