Frequency offset Cartesian feedback system

1. An amplifier system, comprising:

a complex band pass error amplifier;

a quadrature up converter connected to the complex band pass error amplifier so as to receive as input, output from the complex band pass error amplifier;

an amplifier connected to the quadrature up converter so as to receive as input, output from the quadrature up converter;

a quadrature down converter connected at or beyond the amplifier output so as to receive as input a signal proportional to that delivered by the amplifier as output to a load, wherein the complex band pass error amplifier is connected to the quadrature down converter so as to receive as a first input, output from the quadrature down converter and as a second input, a quadrature reference signal.

2. The amplifier system, as recited in claim 1 , wherein the complex band pass error amplifier is a cross coupled polyphase amplifier.

3. The amplifier system, as recited in claim 2 , further comprising a coupler connected between the amplifier output and quadrature down converter to sample the amplifier output signal to the load and provide the sampled output to the quadrature down converter.

4. The amplifier system, as recited in claim 3 , further comprising a quadrature demodulator connected to the complex band pass error amplifier to generate the quadrature reference signal from a single ended radio frequency reference signal.

5. The amplifier system, as recited in claim 4 , wherein the cross coupled polyphase amplifier comprises at least two amplifiers that are cross connected together.

6. The amplifier system, as recited in claim 5 , wherein the complex band pass error amplifier can be fully differential or single ended in construction.

7. The amplifier system, as recited in claim 4 , wherein the cross coupled polyphase amplifier comprises at least two amplifiers that are cross connected with a network incorporating capacitors and digitally adjustable variable resistors.

8. The amplifier system, as recited in claim 4 , further comprising a magnetic resonance imaging coil connected to the amplifier, to receive as input, output from the amplifier.

9. The amplifier system, as recited in claim 3 , wherein the reference signal is grounded.

10. The amplifier system as recited in claim 4 , wherein the complex band pass error amplifier is implemented digitally using analog to digital converters to convert the reference analog input and the quadrature demodulated amplifier signal, and a digital to analog converter to provide the output to the quadrature up converter, with the complex band pass response computed digitally by DSP, FPGA, or CPLD.

11. An array of amplifiers incorporating at least one of the amplifier systems as recited in claim 1 .

12. The amplifier system, as recited in claim 10 , further comprising a magnetic resonance imaging coil connected to the amplifier, to receive as input, output from the amplifier.

13. The amplifier system as recited in claim 1 , further comprising a phase shifter providing input to the quadrature up converter and quadrature down converter.

14. The amplifier system, as recited in claim 13 , further comprising a local oscillator providing input to the phase shifter.

15. A method for providing linearized Cartesian feedback amplification, comprising:

providing a quadrature reference signal as input to a complex band pass error amplifier;

providing output from the complex band pass error amplifier to a quadrature up converter;

up converting the output from the complex band pass error amplifier by a factor;

providing output from the quadrature up converter to an amplifier, which amplifies the output from the quadrature up converter;

receiving as input a signal proportional to a signal delivered by the amplifier as output to a load to a quadrature down converter;

down converting the input signal by the factor;

providing the down converted input signal as input to the complex band pass error amplifier.

16. The method, as recited in claim 15 , further comprising providing output from a phase shifter as input to the up converter and the quadrature down converter.

17. The method, as recited in claim 16 , further comprising providing as input to the phase shifter, output from a local oscillator.

18. The method, as recited in claim 15 , wherein the complex band pass error amplifier is a cross coupled polyphase amplifier.