Browse Prior Art Database

DIGITAL PROGRAMMABLE CURRENT SOURCE

IP.com Disclosure Number: IPCOM000008355D
Original Publication Date: 1997-Sep-01
Included in the Prior Art Database: 2002-Jun-10
Document File: 3 page(s) / 126K

Publishing Venue

Motorola

Related People

Jeffrey C. Durec: AUTHOR

Abstract

In the application it is desired to switch between bias conditions and it is very important that the bias conditions that you switch between are accurately switched. The application for the circuit is in 900 MHz cordless phones. In the application, a signal is received from an antenna, down converted to a lower frequency and demodulated and amplified, so that the voice on the RF signal can be sent to the speaker.

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MOTOROLA Technical Developments

DIGITAL PROGRAMMABLE CURRENT SOURCE

by Jeffrey C. Durec

BACKGROUND

  In the application it is desired to switch between bias conditions and it is very important that the bias conditions that you switch between are accurately switched. The application for the circuit is in 900 MHz cordless phones. In the application, a signal is received from an antenna, down converted to a lower frequency and demodulated and amplified, so that the voice on the RF signal can be sent to the speaker.

THE INVENTION

  In the invention, switchable currents are used in each circuit block so that the biases can be tweaked by the use of digital signals. With the invention, you can switch between multiple bias conditions just by changing some logic. Ideally you want to operate at the lowest power dissipation, but you also want to achieve high performance. If the performance at the low power condition is not sufficient, the bias is switched to a higher power condition to obtain the performance.

The device is presently a custom part for VTECH and will become a standard product a year

later. The function is easily implemented with CMOS, but we are limited to the MOSAIC 5 bipolar process.

  Referring to Engineering notebook page I, Figure 1, a method is shown for selecting between two bias currents that are internally regulated. The goal is a stable current source that is independent of supply, so that if you have a supply variation the regulated current source does not vary.

PRIOR ART

  The way this has been accomplished in the past is to have two current regulators with two outputs. To obtain the desired bias, the outputs are either summed, or one of the outputs is used individually. To sum the currents, you choose between current sources by turning one "on" or the other "on" by selecting Iin, or Iin,. The problem with the prior art is that a lot of transistors are required to sum the currents. Also, in many cases there are multiple out- puts which have higher capacitances associated with the output summing node, and the capacitances are detrimental at higher frequencies. For each stage in the prior art, five transistors are added. In the inven- tion, only two additional transistors are required.

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208 September 1997

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Technical Developments

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