Abstract
The noise and drift set a limit of lowest input voltage of a DC amplifier. To decrease the effects of noise and drift, a suitable modulating amplifier is used instead of a direct coupled amplifier.
The advantages of the 2nd harmonic type of Magnetic Modulator for this porpose have been discussed by Williams and Noble in 1950. Afterwards, the authors proposed the ultra low noise Magnetic Modulator used Ferrite core which has the smallest value of 2.6×10-6A/m referring to the input noise of Magnetic Modulator.
In this paper, we attempt to apply the ultra low noise Magnetic Modulator to a DC amplifier which has an input impedance as low as several hundred ohms or less and discuss theoretically the limitation on the low input voltage due to noise and drift on the modulator core, and find that the input drift voltage of the DC amplifier using the Magnetic Modulator can be held at 5×10-9V/hr, when the source impedance of this DC amplifier is 5 ohm. And more, to consider the limitation for a small signal on DC amplifier, we compare ours with other types of DC amplifier which are incorporating mechanical choppers, semiconductor choppers and so on.
