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Common-Mode Feedback Circuit for Fully Differential CMOS Circuits

IP.com Disclosure Number: IPCOM000116248D
Original Publication Date: 1995-Aug-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 4 page(s) / 97K

Publishing Venue

IBM

Related People

Soyuer, M: AUTHOR

Abstract

One of the standard techniques to extend the signal swing of analog ICs is to use fully-differential circuits. However, in order to maintain the output common-mode voltage levels at preset values additional circuitry is always required. For example, a typical case encountered in frequency synthesizer applications is shown in Fig. 1. A fully-differential charge-pump filter generates voltages VP and VN for a voltage-controlled oscillator (VCO) with a fully-differential control input where the difference (VP - VN) controls the VCO frequency. However, the common-mode voltage of the VCO input should also remain at a well-defined level of 0.5*(VP + VN). This can be achieved with a common-mode feedback (CMFB) circuit as will be explained in detail in the next section.

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Common-Mode Feedback Circuit for Fully Differential CMOS Circuits

      One of the standard techniques to extend the signal swing of
analog ICs is to use fully-differential circuits.  However, in order
to maintain the output common-mode voltage levels at preset values
additional circuitry is always required.  For example, a typical case
encountered in frequency synthesizer applications is shown in Fig. 1.
A fully-differential charge-pump filter generates voltages VP and VN
for a voltage-controlled oscillator (VCO) with a fully-differential
control input where the difference (VP - VN) controls the VCO
frequency.  However, the common-mode voltage of the VCO input should
also remain at a well-defined level of 0.5*(VP + VN).  This can be
achieved with a common-mode feedback (CMFB) circuit as will be
explained in detail in the next section.

      This invention solves the problem mentioned above by using a
CMOS CMFB circuit and is applicable to any CMOS fully-differential
circuit architecture.  Fig. 2 shows the block diagram of such an
architecture for the example mentioned above.  Voltage-controlled
oscillator output provides the high-speed signal output of the
frequency synthesizer phase-locked loop.  Its output frequency is
controlled by the differential input level (VP - VN), but it is
crucial that the input common-mode level is also well-defined and
stable for proper operation.  The block labeled CMFB senses the
common-mode voltage and generates output currents IPUP, IPDN, INUP
and INDN to stabilize it.  This circuit is designed in such a way
that when the common-mode voltage goes up for any reason (such as
feedthrough, noise etc.)  IPDN and INDN increase whereas IPUP and
INUP decrease.  This forces the voltages VP and VN to decrease thus
compensating for the original increase.  Similarly, if the
common-mode voltage goes down then IPDN and INDN decrease whereas
IPUP and INUP increase thereby compensating for the reduction in the
common-mode voltage.

      The ...