A CMOS limiting amplifier and signal-strength indicator

Although all commercially available monolithic log amps today are bipolar ICs, CMOS is equally well-suited to implement the successive-detection architecture. We report here on the design and performance of such a logarithmic amplifier, which is part of a monolithic all-CMOS spread-spectrum 900 MHz wireless transceiver. In the intended use, a received 160 kb/s binary-FSK signal is amplified at RF, directly downconverted to DC, and applied to the logarithmic amplifier after channel-select filtering. The amplifier provides two useful outputs. First, the limited output from the cascade of clipping amplifiers contains the data encoded as signal phase in the zero-crossings. Second, the circuit produces a logarithmic signal-strength measurement to an accuracy of 1 dB over a 80 dB dynamic range.