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Variable Gain Amplifier for Use in Communication Application Specific Integrated Circuits

IP.com Disclosure Number: IPCOM000115089D
Original Publication Date: 1995-Mar-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 4 page(s) / 93K

Publishing Venue

IBM

Related People

Gowda, SMAM: AUTHOR [+2]

Abstract

Disclosed is a scheme that is used to obtain balanced differential signals. The scheme is used in applications where a large input dynamic range is required. The scheme is also useful in applications where the input signal cannot be complemented because of inherent properties, such as in the case of a light pulse.

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This is the abbreviated version, containing approximately 52% of the total text.

Variable Gain Amplifier for Use in Communication Application Specific
Integrated Circuits

      Disclosed is a scheme that is used to obtain balanced
differential signals.  The scheme is used in applications where a
large input dynamic range is required.  The scheme is also useful in
applications where the input signal cannot be complemented because of
inherent properties, such as in the case of a light pulse.

      A typical receiver in data communication Application Specific
Integrated Circuits (ASICs) consists of a low-noise pre-amplifier, a
high-gain post-amplifier, and an off-chip driver.  To provide further
noise immunity, the individual blocks mentioned above are designed to
be fully differential i.e., they operate on normal and inverted
signals simultaneously.

      If the normal and inverted signals are not balanced in such a
circuit, the output of the receiver suffers distortion.  There are
several reasons why the normal and inverted signals may not be
balanced.  In applications such as fiber optics, the input-signals
are inherently single-ended since there cannot be a "negative" light
pulse.  During ASIC manufacture, mismatch of components can also
cause mismatch between normal and inverted signals.

      A solution that is used to balance the signals involves a
level-restore circuit which monitors the output of the post-amplifier
and feeds back this information to the input of the post-amplifier,
thereby forcing a balance between the inverted and normal signals.
The scheme is shown in Fig. 1.  The effectiveness of such a circuit
depends upon the availability, at the output of the post-amplifier,
of signal representing the imbalance of the amplifier.

      Several communication applications require the ability to
receive signals with a very large dynamic range.  In such
applications, using conventional saturating post-amplifier circuits
would result in a saturated signal at the output of the
post-amplifier.  Since the amplifier output has saturated...