Detector cell for logarithmic amplifiers

A detector cell for detecting the strength of an input signal applied to a logarithmic amplifier comprising:

• a first transistor having a first terminal, a second terminal, and a control terminal;
• a second transistor having a first terminal, a second terminal, and a control terminal, wherein the first terminals of the first and second transistors form a pair of differential outputs of the detector cell;
• a third transistor having a first terminal, a second terminal, and a control terminal, wherein the first terminal of the third transistor is coupled to the first terminal of the first transistor, and wherein the control terminals of the first and third transistors form a pair of differential inputs of the detector cell;
• a current source coupled to the second terminals of the first, second, and third transistors;
• a first resistor coupled between the control terminals of the first and second transistors;
• and
• a second resistor coupled between the control terminals of the second and third transistors,
• wherein the current through each of one of the pair of differential outputs is substantially equal when no input signal is applied across the pair of differential inputs.

A method of determining a signal strength of a signal generated by a logarithmic amplifier having a plurality of serially connected gain stages each having a pair of differential inputs and a pair of differential outputs, and a plurality of detector cells each having a pair of differential inputs coupled to the differential output pair of a corresponding gain stage and a pair of differential outputs including first and second differential outputs, each detector cell producing first and second output signals on their first and second differential outputs, respectively, the method comprising:

• summing the first output signals generated at the first differential outputs of the detector cells;
• summing the second output signals generated at the second differential outputs of the detector cells;
• and
• determining the difference between the summed first output signals and the summed second output signals, wherein the difference is proportional to the signal strength of the signal generated by the logarithmic amplifier and wherein the difference is approximately equal to zero when there is no signal applied to the logarithmic amplifier.