23.2 Dynamically Biased 1MHz Low-pass Filter with 61dB Peak SNR and 112dB Input Range

In conventional analog filters operated in class-A mode, bias currents are large enough to accommodate the largest expected peak input signal. When only a small input signal (including in-band and out-of-band components) is present, these large bias currents unnecessarily waste power and create noise. A more optimal situation results if bias currents can be dynamically set by the system, depending on the peak value of the total signal being applied or anticipated at the input. However, dynamically varying the bias currents in a filter normally causes unacceptable disturbances at its output. This filter has bias currents that can be varied over several orders of magnitude—for optimum power consumption and noise at each total input signal level—without disturbing the output. Although the filter is internally nonlinear and time-varying, it is externally linear and time-invariant [1].