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Low-Power CMOS Offset Control Circuit for Fixed Gain Differential Amplifiers

IP.com Disclosure Number: IPCOM000116341D
Original Publication Date: 1995-Sep-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 2 page(s) / 57K

Publishing Venue

IBM

Related People

Girard, P: AUTHOR [+2]

Abstract

Disclosed is a control circuitry for accurate control of the output offset in fixed gain differential amplifiers. The conventional solution (*) to this problem uses a feedback amplifier placed around the main amplifier. The feedback signal is substracted to the input signal to cancel the output offset. A more efficient implementation of this scheme can be achieved in BiCMOS technologies that does not have the drawbacks of the conventional structure.

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This is the abbreviated version, containing approximately 63% of the total text.

Low-Power CMOS Offset Control Circuit for Fixed Gain Differential
Amplifiers

      Disclosed is a control circuitry for accurate control of the
output offset in fixed gain differential amplifiers.  The
conventional solution (*) to this problem uses a feedback amplifier
placed around the main amplifier.  The feedback signal is substracted
to the input signal to cancel the output offset.  A more efficient
implementation of this scheme can be achieved in BiCMOS technologies
that does not have the drawbacks of the conventional structure.

      The principle of the disclosed solution is shown in the Figure.
The signal amplifier is built around a bipolar differential pair that
uses a emitter degeneration resistor scheme to precisely set the gain
of this circuit to 2xRc/Re.  The offset control circuit uses a
feed-back filter, a feedback amplifier and two voltage controlled
current sources.

      The feedback filter has a very low time constant to keep only
the DC component of the output signal that represents the offset.
The large resistors are realized with MOS transistors.  Therefore,
the capacitor value is kept small to be integrated at almost no
silicon area cost.

      The feedback amplifier amplifies the filter output and drives
the current sources.  The gain of this circuit has to be higher than
the main bipolar amplifier gain to achieve a good offset
cancellation.  A single CMOS differential stage can do the work.  The
feedback signal is added in...