A 2.5-V active low-pass filter using all-n-p-n Gilbert cells with a 1-Vp-p range

This paper describes the design and measured results of an all-n-p-n low-voltage (2.5 V), low-current (1 mA), large-swing (1 V_p-p), low-distortion (-53 dB, 1 V_p-p) active filter using a conventional bipolar process. The transconductors for the filter are composed of Gilbert cell transconductors. Distortion has been improved by feedback circuits without increasing the supply voltage and without using p-n-p transistors. The filter is a gyrator-capacitor type third-order Butterworth low-pass filter with a nominal cutoff frequency of 192 kHz. A voltage scaling technique has been applied directly to the gyrator-capacitor filter. This has improved the signal-to-noise ratio by 3 dB. Simulation results indicated that a fast operation up to tens of MHz is possible with a standard bipolar process, as the signal path is composed only of n-p-n transistors