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Low-Noise High-Bandwidth Voltage-Biasing MR-Element Preamplifier with Short Circuit Protection

IP.com Disclosure Number: IPCOM000106413D
Original Publication Date: 1993-Nov-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 2 page(s) / 108K

Publishing Venue

IBM

Related People

Contreras, JT: AUTHOR [+2]

Abstract

A high-input-impedance low-noise amplifier for MR sensors eliminates the high frequency response limitations caused by parasitic input lead inductance from transducer to amplifier. This allows operation for higher data rates with the advantage of Delta R divslash R detection and voltage biasing of sensor for optimal signal-to-electronics-noise ratio.

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Low-Noise High-Bandwidth Voltage-Biasing MR-Element Preamplifier with Short Circuit Protection

      A high-input-impedance low-noise amplifier for MR sensors
eliminates the high frequency response limitations caused by
parasitic input lead inductance from transducer to amplifier.  This
allows operation for higher data rates with the advantage of Delta R
divslash R detection and voltage biasing of sensor for optimal
signal-to-electronics-noise ratio.

      The prior art low-noise amplifiers have been of low input
impedance type because of the topology needed for the lowest possible
electronics noise.  The low-input-impedance amplifier's high
frequency response depends on the differential input loop inductance
and MR sensor's resistance.  The high frequency response decreases
with an inductance increase or a resistance decrease.  The disclosed
circuit offers the best statistical signal-to-electronics-noise
ratio, sensor short-circuit protection, high bandwith not dependent
on parasitic input inductance, and the advantages of Delta R divslash
R signal sensing for minimal signal amplitude variation at the
amplifier output.

      An amplifier which has both low noise, electrical protection
for the MR head, a high input impedance,  Delta R divslash R sensing,
and voltage biasing is shown in the Figure.  The supply voltages are
indicated as +V and -V, where it may be possible that -V is equal to
ground as long as V sub <r e f> is between these voltages.  The MR
element R sub <m r> is connected between the bases of input devices Q
sub 1 and Q sub 2.  Constant and substantially equal currents J sub
<r e f sub 1> and J sub <r e f sub 2> provide a voltage drop across
the series combination of resistors R sub 7 and R sub 8 bias the
sensor with a predetermined bias voltage.  Current source J sub 1 is
the bias source for the input devices and sets emitter resistance ( r
sub e) of these devices.  Collector resistors R sub 4 and R sub 5 and
r sub e of the input devices set the midband forward gain of the
amplifier.

      There are two feedback loops, and the feedback path of the
first feedback loop includes the differential transconductance stage
g sub 2.  The RC combination of R sub 9 and C sub 3 provide stability
of the first feedback loop by providing a low frequency zero and pole
in the forward path of the loop, where the zero is below the pole in
frequency.  The first feedback loop minimizes the low frequency and
dc part of the signal at the output of the amplifier stage at the
collectors of the input devices.  This feedback substantially
eliminates the dc component (bias voltage) across the sensor and
provides a low frequency rolloff in the forward response...