Patent ID: 8198932

Claim:
A circuit comprising: a digital-to-analog converter (DAC), coupled to a power supply, that provides a first current at a first output terminal of the DAC and a second current at a second output terminal of the DAC, the first current being differential to the second current; a first circuit, coupled to the first output terminal of the DAC and to the second output terminal of the DAC, that generates a first voltage and a second voltage, the first voltage being non-linear with respect to the first current and the second voltage being non-linear with respect to the second current; an attenuator coupled to the first circuit, and responsive to the first voltage and the second voltage to attenuate an input signal of the attenuator and to generate linear attenuation characteristics in decibels with respect to the first current and the second current; wherein the first circuit comprises: a first transistor coupled to the first output terminal of the DAC; a second transistor in series connection with the first transistor to define a first path for the first current and to generate the first voltage at a gate of the second transistor in conjunction with the first transistor; a third transistor coupled to the second output terminal of the DAC; and a fourth transistor in series connection with the third transistor to define a second path for the second current and to generate the second voltage at a gate of the fourth transistor in conjunction with the third transistor; a first isolation circuit coupled to the gate of the second transistor to isolate voltage at the gate of the second transistor from interfering with the input signal; and a second isolation circuit coupled to the gate of the fourth transistor to isolate voltage at the gate of the fourth transistor from interfering with the input signal.