Browse Prior Art Database

Clamp Circuit for Controlled Voltage of a Variable Gain Amplifier

IP.com Disclosure Number: IPCOM000111937D
Original Publication Date: 1994-Apr-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 2 page(s) / 61K

Publishing Venue

IBM

Related People

Pham, B: AUTHOR [+2]

Abstract

Disclosed is a clamp circuit which permits the gain of a Variable Gain Amplifier (VGA) to be clamped precisely at an arbitrary gain value between minimum and maximum gain settings.

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This is the abbreviated version, containing approximately 59% of the total text.

Clamp Circuit for Controlled Voltage of a Variable Gain Amplifier

      Disclosed is a clamp circuit which permits the gain of a
Variable Gain Amplifier (VGA) to be clamped precisely at an arbitrary
gain value between minimum and maximum gain settings.

      A typical magnetic storage device read channel uses a variable
gain amplifier.  When the device is not reading, the Automatic Gain
Control (AGC) loop which controls the gain of the VGA clamps the VGA
control voltage at some fixed value.  If the clamped gain value is
too far beyond the operating range of the VGA control voltage,
recovery time for the VGA at the start of a read operation can be
very long.

      The present invention derives a clamp voltage which forces the
VGA gain to an arbitrary value within the VGA's operating range so
that the AGC loop's recovery time is minimized.  A reference circuit
(Qsub 21, Rsub e21 and Qsub 22) and a transconductance amplifier
(Qsub 31, Qsub 32 and Qsub 33) are added to the conventional VGA and
control voltage circuit of Fig.1.  See Fig.2.  Because of the
inherent matching between transistors and resistors of like type on
an integrated circuit, and because the current source transistor Qsub
22 is biased from the same reference as Qsub 1 and Qsub 2, the
voltage across Rsub E 21 is an exact proportion of the maximum useful
value of Vsub C, Isub C 1 X Rsub E 11.  When the AGC loop is clamped,
the control current is set to a fixed value charging capacitor C.
The...