A method and apparatus for dynamically controlling a programmable gain amplifier
(PGA) in a radio receiver to provide a plurality of gain steps thereby providing
automatic gain control (AGC) in a receiver intermediate frequency (IF) stage comprises
an analogy peak detector formed to including a constant current source and a plurality
of MOSFETs all configured to produce an output voltage (DC) whose value reflects
a peak amplitude of a received differential quadrature phase shift keyed (QPSK)
signal. A first circuit portion generates currents that are proportional to the
square of the magnitude of the gate to source voltage for each of a plurality of
MOSFETs coupled to receive the differential QPSK signal and a second circuit portion
produces a voltage that is equal to the square root of the sum of the squares of
the currents produced (drawn) by the MOSFETs of the first circuit portion.