| 摘要 |
An illustrative embodiment of the invention relates to a circuit for a nuclear magnetic logging tool in which two sets of two parallel connected field effect transistors (FET), are coupled to respective gate voltages that are of equal magnitude and of opposite polarity. During polarization, the FETs are in a low impedance condition to avoid being subjected to large voltages. The method of biasing, together with the parallel connection of the FETs ensures symmetrical voltages with respect to ground across the coil. At the end of polarizing, the coil voltage reverses causing conduction to occur in a Zener diode which is connected across the coil. When the reverse voltage of the coil drops below a predetermined level, the Zener diode stops conducting and the circuit rings at its resonant frequency for a few milliseconds. To further reduce the charge in the circuit, a low value critical damping resistance is applied across the coil through another field effect transistor. After most of the stored charge has been dissipated, both FET sets are switched to a high impedance state in order to apply the formation fluid nuclear magnetic resonance signal to the downhole amplification system input.
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