A 55 μW programmable gain amplifier with constant bandwidth for a direct conversion receiver

A fully differential programmable gain amplifier (PGA) with constant transfer characteristic and very low power consumption is proposed and implemented in a 130 nm CMOS technology. The PGA features a gain range of 4 dB to 55 dB with a step size of 6 dB and a constant bandwidth of 10–550 kHz. It employs two stages of variable amplification with an intermediate 2nd order low-pass channel filter. The first stage is a capacitive feedback OTA using current-reuse achieving a low input noise density of 16.7 nV/ √ Hz. This stage sets the overall high-pass cutoff frequency to approximately 10 kHz. For all gain settings the high-pass cutoff frequency variation is within ±5%. The low-pass channel filter is merged with a second amplifying stage forming a Sallen-Key structure. In order to maintain a constant transfer characteristic versus gain, the Sallen-Key feedback is taken from different taps of the load resistance. Using this new approach, the low-pass cutoff frequency stays between 440 kHz and 590 kHz for all gain settings (±14%). Finally, an offset cancelation loop reduces the output offset of the PGA to less than 5 mV (3σ). The PGA occupies an area of approximately 0.06 mm and achieves a post-layout power consumption of 55 μW from a 1V-supply. For the maximum gain setting the integrated input referred noise is 14.4 μVRMS while the total harmonic distortion is 0.7 % for a differential output amplitude of 0.5 V.