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Title: Differential charge amplifier for marine seismic applications
Claims: What is claimed is: 1. A marine hydrophone assembly for an elongated streamer designed to be towed underwater for passive underwater sound detection of acoustic pressure variations and the like, comprising a marine hydrophone of the capacitive transducer type responsive to varying acoustic pressure to produce a varying electrical charge output that is proportional to the acoustic pressure to which the hydrophone is subjected, an electrical conductor pair connected to the hydrophone to conduct the output from the hydrophone to signal processing equipment spaced therefrom, and a differential charge amplifier connected to the pair of conductors to respond to the varying electrical charge output from the hydrophone and produce amplified output voltage signals representative of said varying electric charge output, the differential charge amplifier comprising a pair of amplifier circuits each having negative and positive polarity inputs and an output arranged to form a pair of charge amplifier stages, each including a paralleled resistance-capacitance combination connected from the outputs of each of the respective amplifier circuits to the respective negative input thereof forming negative feedback circuits having equal capacitance to keep the two inputs of each respective amplifier circuit at the same potential, means connecting the pair of conductors respectively to the negative polarity inputs of the two respective amplifiers to feed said varying electric charge output thereto, and a differential amplifier having negative and positive polarity inputs and means connecting the outputs from said two charge amplifiers to said last-mentioned inputs for cancelling common mode signals appearing at the negative polarity inputs of said two charge amplifiers. 2. A marine hydrophone assembly as defined in claim 1, wherein a pair of operational amplifiers form said amplifier circuits and the positive polarity inputs for each of said pair of operational amplifiers are connected to ground potential and the negative feedback circuits associated with each of said operational amplifiers are operative to keep the negative polarity inputs of said operational amplifiers at ground potential, whereby variations in shunt capacitance between the negative polarity inputs of said two amplifiers has no effect on signal response since no voltage is developed across the two negative polarity inputs. 3. A marine hydrophone assembly as defined in claim 1, including means connected to said positive polarity inputs to said pair of amplifier circuits forming said charge amplifier stages to maintain them at a selected common potential level, and said negative feedback circuits associated with the respective amplifier circuits being operative to maintain the negative polarity inputs to said amplifier circuits at the same potential level as said positive input terminals by the negative feedback voltages coupled therethrough, thereby preventing any voltage from developing across shunt capacitance existing between the negative polarity inputs of the two amplifier circuits whereby adding shunt capacitance across said negative polarity inputs has negligible effect on signal characteristics of the charge amplifier stages. 4. A marine hydrophone assembly as defined in claim 1, wherein a pair of operational amplifiers form said amplifier circuits and connection of said electrical conductor pair to said negative polarity inputs of said operational amplifiers causes the same current to be produced at both said negative polarity inputs and causes the output voltages of said two operational amplifiers to be of the same magnitude and opposite in polarity. 5. A marine hydrophone assembly as defined in claim 3, wherein a pair of operational amplifiers form said amplifier circuits and connection of said electrical conductor pair to said negative polarity inputs of said operational amplifiers forming said charge amplifier stages causes the same current to be produced at both said negative polarity inputs and causes the output voltages of said two operational amplifiers to be of the same magnitude and opposite in polarity. 6. A marine hydrophone assembly as defined in claim 1, including a differential amplifier input stage associated with each of said amplifier circuits having a pair of field-effect transistors each forming one-half of a dual transistor device in the respective differential amplifier input stages, means connecting said varying electrical charge on said electrical conductor pair from said hydrophone to the gates of the field-effect transistors forming a corresponding one-half of the dual transistor devices in the differential amplifier input stages selectively paired with said amplifier circuits in said charge amplifier stages and means intercoupling the field-effect transistors forming the two halves of each of the dual transistor devices and connecting such devices to two amplifier sub-circuits in said amplifier circuits, the negative feedback fed through said feedback circuits being applied upstream of said dual transistor devices. 7. A marine hydrophone assembly as defined in claim 1, including a differential amplifier input stage associated with each of said amplifier circuits having a pair of field-effect transistors each forming one-half of a dual transistor device in the respective differential amplifier input stages, means connecting said varying electrical charge on said electrical conductor pair from said hydrophone to the gates of the field-effect transistors forming a corresponding one-half of the dual transistor devices in the differential amplifier input stages selectively paired with said amplifier circuits in said charge amplifier stages, means connecting the gate of another field-effect transistor in each of said differential amplifier input stages to a selected constant potential level, and means intercoupling the field-effect transistors forming the two halves of each of the dual transistor devices in the two differential amplifier input stages and connecting the collectors of the transistors forming each of the two halves of each respective dual transistor device with the negative and positive inputs respectively of an amplifier sub-circuit, and means connecting the output of said amplifier sub-circuit by said resistance-capacitance negative feedback circuit to the gate of said first-mentioned field-effect transistor associated with each of said charge amplifier stages. 8. A marine hydrophone assembly as defined in claim 7, including means connecting the emitters of each of said dual transistor devices in the emitter-collector circuits respectively of another pair of transistors having a zener diode connected to its base to maintain constant collector current in said lastmentioned pair of transistors. 9. A hydrophone hydrophoen assembly as defined in claim 8, including means connected to said positive polarity inputs to said pair of amplifier circuits forming said charge amplifier stages to maintain them at a selected common potential level, and said negative feedback circuits associated with the respective amplifier circuits being operative to maintain the negative polarity inputs to said amplifier circuits at the same potential level as said positive input terminals by the negative feedback voltages coupled therethrough, thereby preventing any voltage from developing across shunt capacitance existing between the negative polarity inputs of the two amplifier circuits whereby adding shunt capacitance across said negative polarity inputs has negligible effect on the charge amplifier stages. 10. A marine hydrophone assembly as defined in claim 3, including a differential amplifier input stage associated with each of said amplifier circuits having a pair of field-effect transistors each forming one-half of a dual transistor device in the respective differential amplifier input stages, means connecting said varying electrical charge on said electrical conductor pair from said hydrophone to the gates of the field-effect transistors forming a corresponding one-half of the dual transistor devices in the differential amplifier input stages selectively paired with said amplifier circuits in said charge amplifier stages, means connecting the gate of another field-effect transistor in each of said differential amplifier input stages to a selected constant potential level, and means intercoupling the field-effect transistors forming the two halves of each of the dual transistor devices in the two differential amplifier input stages and connecting the collectors of the transistors forming each of the two halves of each respective dual transistor device with the negative and positive inputs respectively of an amplifier sub-circuit, and means connecting the output of said amplifier sub-circuit by said resistance-capacitance negative feedback circuit to the gate of said first-mentioned field-effect transistor associated with each of said charge amplifier stages. 11. A marine hydrophone assembly as defined in claim 10, including means connecting the emitters of each of said dual transistor devices in the emitter-collector circuits respectively of another pair of transistors having a zener diode connected to its base to maintain constant collector current in said last-mentioned pair of transistors. 12. A marine seismic streamer assembly forming an elongated streamer designed to be towed underwater for passive underwater sound detection of acoustic pressure variations and production of electrical signals to be transmitted through a long pair of twisted conductors to signal processing equipment on a towing vessel, comprising an elongated seismic streamer formed of plural, serially interconnected streamer sections, a plurality of the streamer sections each containing an array of marine hydrophones of the capacitive transducer type each responsive to varying acoustic pressure to produce a varying electrical charge output that is proportional to the acoustic pressures to which the hydrophones in the array are subjected, means directly connecting the long pair of twisted electrical conductors to the hydrophone array at an input end of the conductors to conduct the varying electric charge output from the hydrophone array to the signal processing equipment, and a differential charge amplifier connected to an output end of said pair of conductors at a location remote from said hydrophone array to respond to the varying electrical charge output from the hydrophone array and produce amplified output voltage signals representative of said varying electrical charge output, the differential charge amplifier comprising a pair of charge amplifier stages forming a pair of parallel signal processing channels each having an operational amplifier therein having negative and positive polarity inputs and an output and parallel combinations of resistance and capacitance connected from the outputs of each respective charge amplifier to the respective negative inputs thereof forming negative feedback circuits to keep the two inputs of each respective charge amplifier at the same potential level, the capacitance in each of said negative feedback circuits being the same value, means connecting the pair of conductors to the negative polarity inputs of the two respective charge amplifiers, and a differential amplifier having negative and positive polarity inputs and means connecting the outputs from said two charge amplifiers to said last-mentioned inputs for cancelling common mode signals appearing at the negative polarity inputs of said two charge amplifiers. 13. A marine hydrophone assembly as defined in claim 12, including means connected to said positive polarity inputs to said pair of operational amplifiers forming said charge amplifier stages to maintain them at a selected common potential level, and said negative feedback circuits associated with the respective operational amplifiers being operative to maintain the negative polarity inputs to said operational amplifiers at the same potential level as said positive input terminals by the negative feedback voltages coupled therethrough, thereby preventing any voltage from developing across shunt capacitance existing between the negative polarity inputs of the two operational amplifiers whereby adding shunt capacitance across said negative polarity inputs has negligible effect on the charge amplifier stages. 14. A marine hydrophone assembly as defined in claim 12, wherein connection of said electrical conductor pair to said negative polarity inputs of said operational amplifiers forming said charge amplifier stages causes the same current to be produced at both said negative polarity inputs and causes the output voltage of said two operational amplifiers to be of the same magnitude and opposite in polarity. 15. A marine hydrophone assembly as defined in claim 12, including a pair of differential amplifier input stages respectively coupled to each of said operational amplifiers each having a dual transistor set formed of first and second halves providing a pair of field-effect transistors in the respective differential amplifier input stages, means connecting said varying electrical charge on said electrical conductor pair from said hydrophones to the gates of the field-effect transistors forming a first half of the dual transistor sets in the differential amplifier input stages selectively paired with the operational amplifiers forming said charge amplifier stages, and means intercoupling the field-effect transistors forming the two halves of each of the dual transistor sets and connecting both halves of each set to the two respective operational amplifiers, the negative feedback fed through said feedback circuits being applied to the inputs of said first halves. 16. A marine hydrophone assembly as defined in claim 12, including a pair of differential amplifier input stages respectively coupled to each of said operational amplifiers having a pair of field-effect transistors each forming a first half of a dual transistor set in the respective differential amplifier input stages, each said set having a second half intercoupled with its first half also forming a field-effect transistor, means connecting said varying electrical charge on said electrical conductor pair from said hydrophones to the gates of the field-effect transistors forming a said first half of the dual transistor sets in the differential amplifier input stages selectively paired with the operational amplifiers forming said charge amplifier stages, means connecting the gate of the second half field-effect transistor in each of said differential amplifier input stages to a selected constant potential level, and means connecting the collectors of the transistors forming each of the two halves of each respective dual transistor sets with the negative and positive inputs respectively of the associated operational amplifier and means connecting the output of said associated operational amplifier by said resistance-capacitance negative feedback circuit to the gate of said first half field-effect transistor associated with each of said charge amplifier stages. 17. A marine hydrophone assembly as defined in claim 16, including means connecting the emitters of each of the halves of said respective dual transistor sets in the emitter-collector circuits respectively of another pair of transistors having a zener diode connected to the base of each to maintain constant collector current in said last-mentioned pair of transistors. 18. A marine hydrophone assembly for passive under-water sound detection of acoustic pressure variations and production of electrical signals to be transmitted through a pair of twisted conductors to signal processing equipment at a processing station spaced from an underwater listening station comprising a marine hydrophone of the capacitive transducer type at the underwater listening station responsive to varying acoustic pressure to produce a varying electrical charge output that is proportional to the acoustic pressures to which the hydrophone is subjected, means directly connecting the pair of twisted electrical conductors to the hydrophone at an input end of the conductors to conduct the varying electric charge output from the hydrophone to the signal processing equipment, and a differential charge amplifier connected to an output end of said pair of conductors at the processing station remote from said hydrophone to respond to the varying electrical charge output from the hydrophone and produce amplified output voltage signals representative of said varying electrical charge output, the differential charge amplifier comprising a pair of charge amplifier stages forming a pair of parallel signal processing channels each having an operational amplifier therein having negative and positive polarity inputs and an output and parallel combinations of resistance and capacitance connected from the outputs of each respective charge amplifier to the respective negative inputs thereof forming negative feedback circuits to keep the two inputs of each respective charge amplifier at the same potential level, the capacitance in each of said negative feedback circuits being the same value, means connecting the pair of conductors to the negative polarity inputs of the two respective charge amplifiers, and a differential amplifier having negative and positive polarity inputs and means connecting the outputs from said two charge amplifiers to said last-mentioned inputs for cancelling common mode signals appearing at the negative polarity inputs of said two charge amplifiers. 19. A marine hydrophone assembly as defined in claim 18, including means connected to said positive polarity inputs to said pair of operational amplifiers forming said charge amplifier stages to maintain them at a selected common potential level, and said negative feedback circuits associated with the respective operational amplifiers being operative to maintain the negative polarity inputs to said operational amplifiers at the same potential level as said positive input terminals by the negative feedback voltages coupled therethrough, thereby preventing any voltage from developing across shunt capacitance existing between the negative polarity inputs of the two operational amplifiers whereby adding shunt capacitance across said negative polarity inputs has negligible effect on the charge amplifier stages. 20. A marine hydrophone assembly as defined in claim 18, wherein connection of said electrical conductor pair to said negative polarity inputs of said operational amplifiers forming said charge amplifier stages causes the same current to be produced at both said negative polarity inputs and causes the output voltages of said two operational amplifiers to be of the same magnitude and opposite in polarity. 21. A marine hydrophone assembly as defined in claim 18, including a pair of differential amplifier input stages respectively coupled to each of said operational amplifiers each having a dual transistor set formed of first and second halves providing a pair of field-effect transistors in the respective differential amplifier input stages, means connecting said varying electrical charge on said electrical conductor pair from said hydrophone to the gates of the field-effect transistors forming a first half of the dual transistor sets in the differential amplifier input stages selectively paired with the operational amplifiers forming said charge amplifier stages, and means intercoupling the field-effect transistors forming the two halves of each of the dual transistor sets and connecting both halves of each set to the two respective operational amplifiers, the negative feedback fed through said feedback circuits being applied to the inputs of said first halves. 22. A marine hydrophone assembly as defined in claim 18, including a pair of differential amplifier input stages respectively coupled to each of said operational amplifiers having a pair of field-effect transistors each forming a first half of a dual transistor set in the respective differential amplifier input stages, each said set having a second half intercoupled with its first half also forming a field-effect transistor, means connecting said varying electrical charge on said electrical conductor pair from said hydrophone to the gates of the field-effect transistors forming a said first half of the dual transistor sets in the differential amplifier input stages selectively paired with the operational amplifiers forming said charge amplifier stages, means connecting the gate of the second half field-effect transistor in each of said differential amplifier input stages to a selected constant potential level, and means connecting the collectors of the transistors forming each of the two halves of each respective dual transistor sets with the negative and positive inputs respectively of the associated operational amplifier and means connecting the output of said associated operational amplifier by said resistance-capacitance negative feedback circuit to the gate of said first half field-effect transistor associated with each of said charge amplifier stages. 23. A marine hydrophone assembly as defined in claim 22, including means connecting the emitters of each of the halves of said respective dual transistor sets in the emitter-collector circuits respectively of another pair of transistors having a zener diode connected to the base of each to maintain constant collector current in said last-mentioned pair of transistors. 24. A marine hydrophone assembly for passive underwater sound detection of acoustic pressure variations and production of electrical signals to be transmitted through a pair of ungrounded twisted conductors to signal processing equipment at a processing station spaced from an underwater listening station comprising a marine hydrophone of the capacitive transducer type at the underwater listening station responsive to varying acoustic pressure to produce a varying electrical charge output that is proportional to the acoustic pressures to which the hydrophone is subjected, means directly connecting the pair of twisted electrical conductors to the hydrophone at an input end of the conductors to conduct the varying electric charge output from the hydrophone to the signal processing equipment, and a differential charge amplifier coupled to an output end of said pair of conductors at the processing station remote from said hydrophone to respond to the varying electrical charge output from the hydrophone and produce amplified output voltage signals representative of said varying electrical charge output, the differential charge amplifier comprising an operational amplifier having negative and positive polarity ungrounded inputs and an output, circuit means providing a negative feedback circuit comprising a parallel combination of resistance and capacitance coupling feedback voltage to the negative input of the operational amplifier to form a charge amplifier circuit having the property of operating substantially independent of shunt capacitance variations across its input, means coupling signals from said pair of conductors to the two inputs of the operational amplifier, and a branch circuit having a parallel combination of resistance and capacitance connected between said positive polarity input and electrical ground and having the same resistance and capacitance values as said negative feedback circuit, said last-mentioned branch circuit and said negative feedback circuit being connected directly to the positive and negative polarity inputs respectively of said operational amplifier for causing cancellation of common mode signals appearing at the inputs of said operational amplifier. 25. A seismic detection system for geophysical prospecting and the like by detection of acoustic pressure variations forming seismic signals and production of electrical signals to be transmitted through a pair of ungrounded twisted conductors to signal processing equipment at a processing station spaced from a seismic signal detection station, comprising a seismic sensor of the capacitive transducer type at the detection station responsive to varying acoustic pressure to produce a varying electrical charge output that is proportional to the acoustic pressures to which the sensor is subjected, means directly connecting the pair of twisted electrical conductors to the sensor at an input end of the conductors to conduct the varying electric charge output from the sensor to the signal processing equipment, and a differential charge amplifier coupled to an output end of said pair of conductors at the processing station remote from said sensor to respond to the varying electrical charge output from the sensor and produce amplified output voltage signals representative of said varying electrical charge output, the differential charge amplifier comprising an operational amplifier having negative and positive polarity ungrounded inputs and an output, circuit means providing a negative feedback circuit comprising a parallel combination of resistance and capacitance coupling feedback voltage to the negative input of the operational amplifier to form a charge amplifier circuit having the property of operating substantially independent of shunt capacitance variations across its input, means coupling signals from said pair of conductors to the two inputs of the operational amplifier, and a branch circuit having a parallel combination of resistance and capacitance connected between said positive polarity input and electrical ground and having the same resistance and capacitance values as said negative feedback circuit, said lastmentioned branch circuit and said negative feedback circuit being connected directly to the positive and negative polarity inputs respectively of said operational amplifier for causing cancellation of common mode signals appearing at the inputs of said operational amplifier. 26. A seismic detection system for geophysical prospecting by sound detection of acoustic pressure variations forming seismic signals and production of electrical signals to be transmitted through a pair of twisted conductors to signal processing equipment at a processing station spaced from a seismic signal detection station comprising a seismic sensor of the capacitive transducer type at the detection station responsive to varying acoustic pressure to produce a varying electrical charge output that is proportional to the acoustic pressures to which the sensor is subjected, means directly connecting the pair of twisted electrical conductors to the sensor at an input end of the conductors to conduct the varying electric charge output from the sensor to the signal processing equipment, and a differential charge amplifier connected to an output end of said pair of conductors at the processing station remote from said sensor to respond to the varying electrical charge output from the sensor and produce amplified output voltage signals representative of said varying electrical charge output, the differential charge amplifier comprising a pair of charge amplifier stages forming a pair of parallel signal processing channels each having an operational amplifier therein having negative and positive polarity inputs and an output and parallel combinations of resistance and capacitance connected from the outputs of each respective charge amplifier to the respective negative inputs thereof forming negative feedback circuits to keep the two inputs of each respective charge amplifier at the same potential level, the capacitance in each of said negative feedback circuits being the same value, means connecting the pair of conductors to the negative polarity inputs of the two respective charge amplifiers, and a differential amplifier having negative and positive polarity inputs and means connecting the outputs from said two charge amplifiers to said last-mentioned inputs for cancelling common mode signals appearing at the negative polarity inputs of said two charge amplifiers. 27. A seismic detection system as defined in claim 26, including means connected to said positive polarity inputs to said pair of operational amplifiers forming said charge amplifier stages to maintain them at a selected common potential level, and said negative feedback circuits associated with the respective operational amplifiers being operative to maintain the negative polarity inputs to said operational amplifiers at the same potential level as said positive input terminals by the negative feedback voltages coupled therethrough, thereby preventing any voltage from developing across shunt capacitance existing between the negative polarity inputs of the two operational amplifiers whereby adding shunt capacitance across said negative polarity inputs has negligible effect on the charge amplifier stages. 28. A seismic detection system as defined in claim 26, wherein connection of said electrical conductor pair to said negative polarity inputs of said operational amplifiers forming said charge amplifier stages causes the same current to be produced at both said negative polarity inputs and causes the output voltages of said two operational amplifiers to be of the same magnitude and opposite in polarity. 29. A seismic detection system as defined in claim 26, including a pair of differential amplifier input stages respectively coupled to each of said operational amplifiers each having a dual transistor set formed of first and second halves providing a pair of field-effect transistors in the respective differential amplifier input stages, means connecting said varying electrical charge on said electrical conductor pair from said sensor to the gates of the field-effect transistors forming a first half of the dual transistor sets in the differential amplifier input stages selectively paired with the operational amplifiers forming said charge amplifier stages, and means intercoupling the fieldeffect transistors forming the two halves of each of the dual transistor sets and connecting both halves of each set to the two respective operational amplifiers, the negative feedback fed through said feedback circuits being applied to the inputs of said first halves. Other info: Inventors: Mastin, Robert H. (Dallas, TX, US) Application Number: 431723 Filing Date: 1974-01-08 Publication_date: 1976-02-17 Assignee: Whitehall Corporation (Richardson, TX) Primary Class(es): 367/22 330/7, 367/65 Other Classes: US Patent Ref:
Other Refs: Primary Examiner: Farley, Richard A. Assistant Examiner: Buczinski, S. C. Attorney: Mason, Fenwick & Lawrence |
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