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An Improved Schmitt Inverter

IP.com Disclosure Number: IPCOM000213524D
Publication Date: 2011-Dec-20
Document File: 5 page(s) / 115K

Publishing Venue

The IP.com Prior Art Database

Abstract

A Schmitt inverter circuit with dynamic body bias and low current consumption in transition window is proposed. The circuit’s operational principle is described, analyzed and simulated. The simulation results show this new Schmitt inverter circuit has good transient performance.

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An Improved Schmitt Inverter

Abstract

A Schmitt inverter circuit with dynamic body bias and low current consumption in transition window is proposed. The circuit’s operational principle is described, analyzed and simulated. The simulation results show this new Schmitt inverter circuit has good transient performance.

Key Words

Schmitt inverter, dynamic body bias, low current consumption, transition window

I. Introduction

The Schmitt inverter is a basic but widely used circuit as an interface cell in both analog and digital circutis. With built-in hysteresis logic, the input threshold for output changing to high state is higher than the threshold for output changing to low state. Thus when the input is near the threshold point, the Schmitt inverter could output a clean and stable state, while a regular inverter will switch several times.  Conventional Schmitt inverter designs are more concerned with hysteresis window than the current consumption when input is near threshold, which is always unexpectedly very high. In this paper, a Schmitt inverter circuit with dynamic body bias and low consumption current is described. Since Schmitt inverter circuits are widely used, the described circuit is of highly practical importance.

II. Principles of Schmitt Inverter in Conventional Designs

A conventional Schmitt inverter is shown in Fig.1.  When IN rises from ground, the voltage of ‘A’ is high. M0, M1 and M5 are on, while M2, M3 and M4 are off. Thus, the voltage of ‘B’ is pulled up to. When IN is higher than , M3 will be turned on.  However, ‘A’ will not be pulled down until M2 goes on so IN continues rising until and M2 is turned on. Then M2 and M3 pull ‘A’ down to ground.  Here the positive threshold is described as:

……………………………………………. (1)

Similarly, when IN falls from VDD, the voltage of ’A’ is pulled down to ground and ‘C’ is.  Thus, IN will fall until and M1 is turned on so the negative threshold is:

……………………………………………………. (2)

The characteristics of and are shown to the right of Fig.1.

Fig.1 – Conventional Schmitt Inverter (left) and transfer characteristic (right)

This Schmitt inverter will consume high current when IN is around the threshold point.  For example, when IN rises near, both M5 and M3 are on and there will be a current consumption path between VDD and ground.  When IN falls, there will be another path between VDD and ground since both M0 and M4 are on. When IN is applied with low frequency signal, the average current consumption will be a non-ignorable value.  One possible solution for improving this situation is to define smaller size of M4 and M5 to reduce current consumption, but this will result in worse hysteresis threshold shape.

III. Proposed Schmitt inverter structure

Based on the above description of the convention Schmitt inverter circuit, this paper proposes a Schmitt inverter with dynamic body bias and low current c...