Automatic self-balancing operational amplifier impedance bridge

Claim 2

In an AC impedance bridge for measuring the two elemental parameters of an unknown impedance, a first bridge arm for receiving the unknown impedance, a second bridge arm connected to said first bridge arm and having a fixed resistance, a third bridge arm connected to said second bridge arm and having a first standard for balancing out one of the elemental parameters of the unknown impedance and a second variable standard for balancing out the second elemental parameter of the unknown impedance, a fourth bridge arm connected between said first bridge arm and said third bridge arm and having an operational-type currentto-voltage converter having a first input in series with the unknown and a second input at a fixed reference potential for producing a reference voltage output indicative of the current through the unknown, means for connecting a source of AC between the junction of said first and second arms and a point at said fixed reference potential, an operational amplifier connected to the junction between said second and third arms for producing an output voltage indicative of the current summed at the junction of said second and third arms, and balancing means responsive to the outputs of said operational amplifier and said current-to-voltage converter for varying the values of the first and second variable balancing standards to balance out the elemental parameters of the unknown impedance when said output voltage is zero.

Claim 3

In an AC impedance bridge as recited in claim 1, wherein: AC the unknown impedance is an inductor having elemental parameters of inductance and resistance; the first variable balancing standard is a variable capacitor connected between said second and fourth arms; and the second variable balancing standard is a variable capacitor and including phase-shifting means for connecting said second variable caPacitor across said first variable capacitor.

Claim 4

In an AC impedance bridge as recited in claim 2, wherein: said phase-shifting means includes an integrator and a current inverter connected on opposite sides of said second variable capacitor.

Claim 5

In an AC impedance bridge as recited in claim 1 wherein said first and second variable standard value-varying means comprises: a first phase detector having a signal input and a reference input and having a DC output indicative of the phase relationship between the signal and reference inputs; a second phase detector having a signal input and a reference input and having a DC output indicative of the phase relationship between the signal and reference inputs; means for connecting the output voltage of said operational amplifier to the signal inputs of both said first phase detector and said second phase detector; means for connecting the reference voltage output of said current-to-voltage converter to the reference input of said first phase detector; means for imparting no more than a 90* phase shift to the reference voltage; means for connecting the phase-shifted reference voltage to the reference input of said second phase detector; and means responsive to the outputs of said first and second phase detectors for adjusting the values of said first and second variable standard elements until the output voltage of said operational amplifier is zero.

Claim 6

In an AC impedance bridge as recited in claim 1, wherein: the unknown impedance is a capacitor having elemental parameters of capacitance and resistance; the first variable balancing standard is a variable resistor connected between said second and fourth arms; the second variable balancing standard is a variable capacitor and including means for connecting said variable capacitor as the feedback element of an integrator connected in series with a current inverter across said variable resistor.

Claim 7

In a system for ascertaining the series inductance and effective resistance values of an unknown inductor, a closed four-arm bridge circuit having the inductor connected in a first arm and a pair of parallel selectable banks of capacitors connected in a second opposing arm, means for applying an AC current through the inductor, a third arm connected between a first end of the first arm and a first end of the second arm, means connected in said third arm for presenting a virtual ground to one side of said inductor and responsive to the current flow through the inductor for generating a first reference voltage proportional to the current through said inductor, means responsive to said first reference voltage for imparting a 90* phase shift to generate a second reference voltage, a fourth arm connected between said first and second arms and having a fixed resistance, means responsive to the currents summed at the juncture of the fourth arm and the second arm for generating a signal voltage indicative of the unbalance condition of said bridge, means responsive to the phase difference between said first reference voltage and the signal voltage for selectively connecting predetermined ones of a first of said banks of capacitors to balance the inductive effect of the inductor, and means responsive to the phase difference between the second reference voltage and the signal voltage for selectively connecting predetermined ones of the second of said banks of capacitors to balance the resistive effect of the inductor.

Claim 8

In a system for ascertaining the series inductance and effective resistance values of an unknown inductor, a closed four-arm bridge circuit having the inductor connected in a first arm and a pair of parallel selectable banks of capacitors connected in a second opposing arm, means for applying an AC current through the inductor, a third arm connected between a first end of the first arm and a first end of the second arm, means connected in said third arm for presenting a virtual ground to one side of said inductor and responsive to the current flow through the inductor for generating a first reference voltage proportional to the current through said inductor, means responsive to said first reference voltage for imparting a 90* phase shift to generate a second reference voltage, a fourth arm connected between said first and second arms and having a fixed resistance, means responsive to the currents summed at the juncture of the fourth arm and the second arm for generating a signal voltage indicative of the unbalance condition of said bridge, means responsive to the phase difference between said first reference voltage and the signal voltage for selectively connecting predetermined ones of a first of said banks of capacitors to balance the inductive effect of the inductor, and means responsive to the phase difference between the second reference voltage and the signal voltage for selectively connecting predetermined ones of the second of said banks of capacitors to balance the resistive effect of the inductor.