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Modified Sallen and Key Low Pass Filter Using FETs as Source Followers

IP.com Disclosure Number: IPCOM000109999D
Original Publication Date: 1992-Oct-01
Included in the Prior Art Database: 2005-Mar-25
Document File: 4 page(s) / 91K

Publishing Venue

IBM

Related People

Buchholtz, TC: AUTHOR [+3]

Abstract

Many filtering schemes suffer from the inability of circuits to drive them and the inherent difficulties of building high bandwidth amplifiers required by numerous filter topologies. Disclosed is a modification of the classic Sallen and Key low-pass filter. This modified low-pass has extremely high input impedance while maintaining the same transfer function as the Sallen and Key. In addition, filter Q's can be built using lower gain amplifiers than the unmodified version.

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Modified Sallen and

Key

Low

Pass

Filter Using FETs as Source Followers

       Many filtering schemes suffer from the inability of
circuits to drive them and the inherent difficulties of building high
bandwidth amplifiers required by numerous filter topologies.
Disclosed is a modification of the classic Sallen and Key low-pass
filter.  This modified low-pass has extremely high input impedance
while maintaining the same transfer function as the Sallen and Key.
In addition, filter Q's can be built using lower gain amplifiers than
the unmodified version.

      A second-order Sallen and Key positive feedback low-pass filter
is shown in Fig. 1, also referred to in the literature as a KRC
low-pass.  This KRC filter configuration is capable of achieving any
Q with an amplifier gain of less than 3.  In addition, it is observed
that the KRC filter topology decouples filter corner frequency and
filter Q, allowing them to be controlled independently.  Since
high-frequency filtering requires smaller RC time constants, the KRC
topology lends itself to the resistances and capacitances found in an
integrated environment.  The most common method of constructing the
resistances in the filter is to utilize an FET as a pass gate.
Variable resistance is achieved by varying the voltage at the gate.

      A modification of the KRC low-pass can be constructed by using
the transconductance of an FET in a source-follower mode where
resistance is varied by adjusting the bias current of the FET.  Using
the FET in this way alters the expression for Q due to the addition
of a unity gain a...