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Bias Circuit for Cascaded Sallen-Key Filters

IP.com Disclosure Number: IPCOM000116849D
Original Publication Date: 1995-Nov-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 2 page(s) / 62K

Publishing Venue

IBM

Related People

Bailey, JA: AUTHOR

Abstract

The circuit described herein compensates for level shift introduced when cascading multiple Sallen_Key filter stages implemented with emitter-follower or source follower amplifiers. A bias current source added to the network is designed to produce a voltage drop in one of the Sallen-Key filter resistors which compensates for the base-emitter or gate-source voltage drop of the preceding stage.

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Bias Circuit for Cascaded Sallen-Key Filters

      The circuit described herein compensates for level shift
introduced when cascading multiple Sallen_Key filter stages
implemented with emitter-follower or source follower amplifiers.  A
bias current source added to the network is designed to produce a
voltage drop in one of the Sallen-Key filter resistors which
compensates for the base-emitter or gate-source voltage drop of the
preceding stage.

      When using emitter-follower or source-follower amplifier stages
for cascaded Sallen-Key low-pass filters, signal swing can be
severely limited by the level shift of each follower stage.  The
level shift is due to the V sub be or  V sub gs of each follower
circuit.  As each stage is cascaded to the output of the previous
stage, the level shift adds to the previous stage.  For n stages
cascaded together, the resulting level shift is n times V sub be.

      The circuit shown makes use of a special bias circuit to
compensate for the level shift of each stage.  An added current
source is connected to each Sallen-Key low-pass filter stage (except
for the first stage) that produces a compensating level shift upward
for each V sub be shift downward.  Refer to the second stage of the
3-stage design shown.  Current source produced by T13, I sub D13, is
connected to R5.  The bias circuit causes the drain current for T13,
I sub D13 to be one-half of the output sink current of Q3, I sub C3,
from the first stage.  By Kirchoff's current law:
  I sub R5 = I sub C3 - I sub D13
  Since:
  I sub D13 = I sub C...