Some Considerations fo the Construction of Low-noise Amplifiers in Very Low Frequency Region

conference paper

English

The fundamental requrement on the amplifier for noise measurements is that the noise level referred to the input should be more than 20db below the noise to be measured. This requirement is not difficult to fulfill in audio frequency region, and it is possible to make white noise level of the system referred to input less than -190 dbV/Hz. In the very low frequency range extending to sum-milli-Hz there are several difficulties to overcome. Those are the problems concerning to the (de-)coupling capacitors, the excess noise in amplifying devices, the effects of ambient temperature variation, and so on. The capaticance necessary for the lower cutoff frequency of 0.1 mHz is 17 mF for the input resistance of 100 k\Ohm. The electrolytic capacitors are the exclusive selection for such value of capacitance, but they accompany considerable amount of leakage current causing additional noise. In addition, the coupling capacitor presents high source impedance for the successive amplifyin device. For example the impedance of 17 mF is 10 k\Ohm at 1 mHz, and this enhances the contribution of the equivalent input current noise of active devices. The coupling capacitor also causes the long transient when the power supply of the amplifier is switched on. The active devices for the first stage amplifier are selected by the measurements of equivalent input noise and the trans-conductance. The offset voltage and its drift are serious in a direct-coupled amplifier. The offset is cancelled by inserting a combination of small batteries. The drift is suppressed by attaching heat sinks to the active device and applying thermal feedback. By these two means the effects of temperature fluctuation of the first stage device is reduced down below the intrinsic 1/f noise of the device.

noise, nonlinearity, nanoscale devices

ŠIKULA, J., HASHIGUCHI, S., OHKI, M., TACANO, M.

2003

1. 1. 2003

CNRL

Brno

23

23

1