An autozeroing floating-gate amplifier

We have developed a bandpass floating-gate amplifier that uses tunneling and pFET hot-electron injection to set its dc operating point adaptively. Because the hot-electron injection is an inherent part of the pFET's behavior, we obtain this adaptation with no additional circuitry. Because the gate currents are small, the circuit exhibits a high-pass characteristic with a cutoff frequency less than 1 Hz. The high-frequency cutoff is controlled electronically, as is done in continuous-time filters. We have derived analytical models that completely characterize the amplifier and that are in good agreement with experimental data for a wide range of operating conditions and input waveforms. This autozeroing floating-gate amplifier demonstrates how to use continuous-time floating-gate adaptation in amplifier design.

I. Introduction

We PRESENT a bandpass floating-gate amplifier that uses tunneling and pFET hot-electron injection so that it can return to its sensitive region despite large changes in the dc input voltage. Offsets often present a difficult problem for designers of MOS analog circuits. A time-honored tradition for addressing this problem is to use a blocking capacitor to eliminate the input dc component. However, for integrated filters, this approach requires enormous input capacitors and resistors to get time constants of less than 1 Hz. Existing on-chip autozeroing techniques rely on clocking schemes that compute the input offset periodically, then subtract the correction from the input [1]. These autozeroing techniques add significant complexity to the circuit, as well as to clock noise, aliasing, etc. Fig. 1. An AFGA that uses pFET hot-electron injection. The ratio of to sets the gain of this inverting amplifier. The nFET is a current source and sets the current through the pFET. Steady state occurs when the injection current is equal to the tunneling current. The capacitance from the floating gate to ground represents both the parasitic and the explicitly drawn capacitances. Increasing will increase the linear input range of the circuit. The capacitance connected to the output terminal is the load capacitance. Between and is our symbol for a tunneling junction, which is a capacitor between the floating-gate and an n-well.