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Optical Receiver Amplifier Front End for Diffuse Infrared Communication

IP.com Disclosure Number: IPCOM000038527D
Original Publication Date: 1987-Jan-01
Included in the Prior Art Database: 2005-Jan-31
Document File: 3 page(s) / 53K

Publishing Venue

IBM

Related People

Bustamante, CM: AUTHOR [+2]

Abstract

This article describes an amplifier circuit arrangement for diffuse infrared (IR) communication which utilizes two symmetrical common base transistor amplifiers tied together at their respective bases. This design allows large photodiode areas required by IR media and provides a low noise solution. It is possible to connect personal computers (PCs) together in a local area network without the use of cables or fibers. This is achieved with technology borrowed from fiber optics without the use of a fiber, i.e., the transmitting light-emitting diodes (LEDs) and the (Image Omitted) receiving photodiodes are similar. Most IR receivers are based on fiber-optic receivers. In diffuse optical media the light enters the photodiodes from all different angles in a room.

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Optical Receiver Amplifier Front End for Diffuse Infrared Communication

This article describes an amplifier circuit arrangement for diffuse infrared (IR) communication which utilizes two symmetrical common base transistor amplifiers tied together at their respective bases. This design allows large photodiode areas required by IR media and provides a low noise solution. It is possible to connect personal computers (PCs) together in a local area network without the use of cables or fibers. This is achieved with technology borrowed from fiber optics without the use of a fiber, i.e., the transmitting light-emitting diodes (LEDs) and the

(Image Omitted)

receiving photodiodes are similar. Most IR receivers are based on fiber-optic receivers. In diffuse optical media the light enters the photodiodes from all different angles in a room. With fiber optics the light travels down the light guide to a small area on a die and there is no need for large photo areas. With diffuse IR receivers, the signal-to-noise ratio increases as the square root of area. To explain this simply, when the area is doubled the signal is doubled, but the noise only increases by the 2 because it is not correlated. Thus, it is desirable to have large photodiode areas. The circuit disclosed herein is shown in Fig. 1. Two symmetrical common-base transistor amplifiers T1 and T2 are tied together at their respective bases. A light sensitive P-Intrinsic-N (PIN) photodiode is connected between the emitters of T1 and T2. Dynamically, light currents that flow through the photodiode are forced to flow out of the emitter of T1 and into the emitter of T2, because of their low Z, and vice-versa. Since the bases are tied together and are both AC ground, the impedance at each emitter is low (228 ohms) which follows the relation re=KT/qIe. For both transistors the collector resistors are metal film 50K ohms. Emitter resistors are metal film 30K ohms. Base bias resistors are shared by the two transistors and are metal film 80K ohm and 40K ohm to ground. These parameter values are for a photodiode ar...