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Method for bias circuitry for bipolar differential amplifiers

IP.com Disclosure Number: IPCOM000011745D
Publication Date: 2003-Mar-12
Document File: 7 page(s) / 252K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for bias circuitry for bipolar differential amplifiers. Benefits include improved performance.

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Method for bias circuitry for bipolar differential amplifiers

Disclosed is a method for bias circuitry for bipolar differential amplifiers. Benefits include improved performance.

Background

        � � � � � Due to the unavailability of positive-negative-positive (pnp) transistors in heterojunction bipolar transistor (HBT) technology, innovative design techniques overcome the shortcomings of biasing circuitry. These techniques are imperative for the chip to conform to the specifications under various operating conditions.

General description

        � � � � � The disclosed method is bias circuitry for the differential amplifiers (trans-impedance amplifiers, TIAs) used in optical communication systems.

        � � � � � A biasing circuit is used to bias all of the circuitry without requiring separate external power supplies. The two biasing schemes are:

•        � � � � Constant trans-conductance bias: The overall gain of the differential amplifiers varies due to the variation of trans-conductance (Ic/VT) of the gain stages with temperature, especially for bipolar devices operating at a given constant current. The change in gain affects parameters such as bandwidth (in TIA), input sensitivity to the following stages, and possible device functions (such as peak detector circuitry). The constant trans-conductance scheme uses inherent properties of the bipolar transistors to change the bias current as the operating temperature changes, keeping the trans-conductance constant. As the temperature rises, the base-emitter turn-on voltage decreases. This reduction in turn-on voltage is tapped on the output to increase the biasing current. The benefit of this technique is that it keeps the overall system’s gain constant.

•        � � � � Supply insensitive bias: The biasing circuit is subject to system power supply variation (typically ±5%). A second biasing circuit can be designed to make the system less sensitive to supply variations. This implementation of bias circuitry can be used in an implementation of the 40 Gb/s TIA to keep the gain constant and make it less prone to supply-voltage fluctuation.

        � � � � � The prime advantage of the constant trans-conductance bias circuit is that it keeps the trans-conductance of the gain stages (differential pairs) constant with temperature. The advantages of using supply-insensitive bias is that it makes the chip insensitive to power supply variations.

        � � � � � The common advantages of the constant trans-conductance bias circuit and supply-independent bias circuit implementations are the following:

•        � � � � Symmetric implementation that tracks the process variations

•        � � � � Generic biasing scheme that can be implemented in any bipolar process

•        � � � � Performance improvement when applied to TIA (see Figure 1)

•        � � � � Avoidance of the shortcomings of biasing circuitry due to the unavailability of pnp transistors

        � � � � � Compared to conventional methods, the disclosed method keeps the trans-conductance of the differential pair...