Variable-transconductance four-quadrant Gilbert-type modulator with feedback circuitry in improved NMR transmission

In the transmission circuitry of a nuclear magnetic resonance imaging system, a variable-transconductance four-quadrant Gilbert-type modulator having reduced non-linearity for generating pulses of nuclear magnetic resonance imaging radio-frequency current, said modulator comprising:

• a differential-transconductance amplifier, having an input port for receiving an input voltage signal, an output port for a differential output current, and a common port having two leads, for providing a change in its differential output current on its output port when the voltage signal on its input port changes;
• a fixed current source for providing drive currents to the two leads of the common port of the differential-transconductance amplifier, the sum of the drive currents being approximately equal to the sum of the output currents of the differential transconductance amplifier;
• a current sensing resistor coupled across the two leads of the common port of the differential-transconductance amplifier;
• variable-transconductance circuitry, operatively connected to the output port of the differential-transconductance amplifier to receive the differential output current, having a nuclear magnetic resonance imaging carrier input port for receiving a nuclear magnetic resonance imaging carrier input signal, and having a modulated output port, responsive to the differential output current of the differential-transconductance amplifier to vary the gain of the carrier input signal to provide a modulated output in the form of the pulses of nuclear magnetic resonance imaging radio-frequency current for the transmission circuitry of the nuclear magnetic resonance imaging system;
• and
• feedback circuitry having a nuclear magnetic resonance imaging modulation input port for receiving a nuclear magnetic resonance imaging system modulation voltage signal, a modulation output port operatively connected to the input port of the differential-transconductance amplifier for providing the modulation voltage signal to the differential-transconductance amplifier, and a feedback port having two leads operatively connected across the current sensing resistor, for causing the voltage across the current sensing resistor to follow the voltage of the modulation voltage signal and cause the differential output current of the differential-transconductance amplifier to be directly proportional to the voltage signal on its input port.