Browse Prior Art Database

Crystal oscillator with lower current consumption

IP.com Disclosure Number: IPCOM000240309D
Publication Date: 2015-Jan-21
Document File: 6 page(s) / 213K

Publishing Venue

The IP.com Prior Art Database

Abstract

A crystal oscillator design with lower current consumption is proposed. The proposed design is able to use gain of both PMOS and NMOS transistors and also able to work with automatic gain/level controlled circuit without disturbing automatic gain controlled circuit operation/output bias voltage. Since it uses the gain of both PMOS and NMOS transistors, the current consumption of the design is reduced across all PVT.

This text was extracted from a Microsoft Word document.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 42% of the total text.

Crystal oscillator with lower current consumption

Abstract

A crystal oscillator design with lower current consumption is proposed. The proposed design is able to use gain of both PMOS and NMOS transistors and also able to work with automatic gain/level controlled circuit without disturbing automatic gain controlled circuit operation/output bias voltage. Since it uses the gain of both PMOS and NMOS transistors, the current consumption of the design is reduced across all PVT.

Introduction

A crystal oscillator circuit includes an amplifier with a crystal resonator connected between the input and output terminals of the amplifier.  The amplifier could be of controlled or uncontrolled gain.  An inverter amplifier is an example of an uncontrolled gain amplifier, while a gain transistor with variable biasing current is an example of a controlled gain amplifier.  The advantage of the uncontrolled gain amplifier is that it uses the gain of both PMOS and NMOS transistors.  However, it is not feasible with automatic level/gain controlled (ALC) circuit.  Further, the advantage of the controlled gain amplifier is that it automatically controls the gain for optimum amplitude of oscillations, reduces current consumption, and has lower signal distortion.  However, it uses the gain of only one transistor without using a coupling capacitor.

Figure 1 shows a controlled gain amplifier coupled to an Automatic Level Control (ALC) circuit.  Transistors P1 and N1 are part of an amplifier.  Here the amplifier uses the gain of only NMOS transistor, N1.  In steady-state the ALC output (pbias_alc) is a DC signal voltage with minimum ripple.  Extal and xtal are outputs of the oscillator.  When extal is at near zero voltage then there is very little gain of amplifier.

                              

Figure 1: Controlled gain amplifier working with Automatic Loop Control (ALC)

So there is need for a design that can use the gain of both PMOS and NMOS transistors and also work with automatic gain/loop controlled circuit without disturbing automatic gain/loop controlled circuit operation/output bias voltage.

Proposed Circuit

The proposed crystal oscillator circuit is shown in Figure 2.

In the proposed design, extal node is AC coupled to gain amplifier bias node. Due to AC coupling, the PMOS transistor is also able to provide gain. So for fixed amplifier current, gain of the amplifier increases compared to conventional architecture. It means for same gain, current requirement of proposed architecture reduces compared to conventional architecture.

There should be minimum ripple voltage at ALC output bias node (pbias_alc) for proper operation of ALC (otherwise ALC gain transistor will go in linear region). But due to AC coupling of PMOS transistor (of amplifier), ripple will come at PMOS transistor bias node of amplifier (pbias_amp).  To filter this ripple, one RC filter is added between amplifier PMOS bias node (pbias_amp) and ALC output ...