NPN Sziklai pair small-signal amplifier for high gain low noise submicron voltage recorder

Abstract

Small signal-to-noise ratio (SNR) and multiple noise sources, coupled with very weak signal amplitudes of bio signals make brain-computer interface (BCI) application studies a challenging task. The front-end recorder amplifiers receive very-weak signal (few μV) from high impedance electrodes and for efficient processing of such weak and low frequency (<1 kHz) signals a high gain amplifier with very low operating voltage and low total harmonic distortion (THD) is required. Existing amplifiers suffer from problem of high non-linearity and low common mode rejection. A good sense amplifier at predeceasing stage can solve this problem. Utilizing very high amplification factor of Sziklai Pair, this paper proposes two circuit topologies of common-emitter and common-collector negative-positive-negative (NPN) Sziklai Pair small signal amplifiers suitable for use in preamplifier stages of such signal acquisition circuit. Present study provides broad-spectrum of analysis of these amplifiers covering effect of additional biasing resistance RA, variation of ‘ideal forward maximum beta’ β, temperature dependency, noise sensitivity and phase variation. The tunable capability of first topology makes it a suitable candidate in wide variety of other applications. The first amplifier operates on very low input voltage range (0.1μV-6mV) whereas the second amplifier works on 100 μV-11 mV range of input voltage.