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Bidirectional Magneto-Resistive Bias Generator with Symmetrical Output

IP.com Disclosure Number: IPCOM000111058D
Original Publication Date: 1994-Feb-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 2 page(s) / 56K

Publishing Venue

IBM

Related People

Burns, MJ: AUTHOR

Abstract

The illustrated circuit provides a voltage controlled, bidirectional current source for the DC biasing of the Magneto-Resistive (MR) element of a thin film head. Intended for use in head testing, the circuit allows rapid turn on/turn off cycling. The voltage at each terminal of the MR element will be equal in magnitude and opposite in polarity, thereby setting the 'center' of the element at ground potential.

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Bidirectional Magneto-Resistive Bias Generator with Symmetrical Output

      The illustrated circuit provides a voltage controlled,
bidirectional current source for the DC biasing of the
Magneto-Resistive (MR) element of a thin film head.  Intended for use
in head testing, the circuit allows rapid turn on/turn off cycling.
The voltage at each terminal of the MR element will be equal in
magnitude and opposite in polarity, thereby setting the 'center' of
the element at ground potential.

      A 'Howland' current pump is used to supply current to one leg
of the MR stripe, via a series resistor (402 ohms).  An inverting
operational amplifier provides a voltage exactly equal in amplitude
and opposite in polarity to the voltage at the other leg of the MR
stripe, again via a 402 ohm resistor.

      The 'Howland' current pump is unique in that it is
bidirectional, i.e., it is capable of both sourcing and sinking
current, in this case, up to 15 milliamps.  As connected, the
relationship between input voltage Vin and bias current Ibias is

   Ibias = Vin / 634
or approximately 1.5 ma per volt.  Note that the charge pump operates
entirely in it's 'active region'; i.e., it never 'saturates'.

      The impedance at the output nodes of both the charge pump and
inverting operational amplifier are very low (a few ohms) at the
frequencies at which the head is intended to operate (above 1
MegaHertz).  In order to provide a relatively high 'AC' impedance to
the head,...