Magnetic bubble transducer

A circuit arrangement is disclosed for sensing a presence or absence of a low-level output signal which may be used for detecting a change of resistance in a magnetic bubble detector. The circuit operates in a D.C. coupling mode whereby the low-level signals are directly coupled into a sense amplifier, which is preceded by the circuit above-mentioned. The circuit, which comprises a modified current mirror, is used as a resistance transducer which converts a change of resistance (.DELTA.R) into a direct change of voltage (.DELTA.E). The change of voltage provides an indication of the presence of, for example, a magnetic bubble.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the field of electrical signal detection 
of a low-level magnitude and, particularly, those low-level electrical 
signals that emanate from a bubble memory device. 
2. Description of the Prior Art 
The known prior art for detecting electrical low-level bubble signals is a 
complicated one and is not suitable for forming into an integrated circuit 
(IC) chip. As is well known, forming an electrical circuit which is 
composed of discrete circuit elements comprising resistors, inductors and 
capacitors, into an IC chip enables a design to become more economical, 
reliable, and results in a reduction of size. The known prior art 
detection circuitry, which will be discussed in greater detail 
hereinafter, utilizes AC coupling in combination with a complicated bridge 
circuit. In addition, this prior art scheme requires two DC sources of 
power. As is well understood by those skilled in the art, AC coupling 
requires more components such as high-valued capacitors in order to allow 
passage of a low-frequency component of non-repetitive digital 
information. Furthermore, in the prior art bridge circuitry, the losses of 
voltage signal value is considerable. 
In the present invention, DC coupling provides simplicity of operation and 
requires only one power source. Furthermore, the circuitry of this 
invention does not sustain the power losses mentioned above in the prior 
art bridge circuitry. 
SUMMARY OF THE INVENTION 
The instant invention is directed to a circuit arrangement for detecting a 
change of resistance which, in turn, will provide a corresponding voltage 
change. The voltage change may be used as an indication that a magnetic 
bubble has been detected in the circuit. The circuit arrangement comprises 
two transistors, which are connected to one another to carry two main 
currents which tend to mirror one another so that the voltage change 
becomes a function of a change of resistance only. In other words, since 
the circuit arrangement utilized in this invention maintains the two main 
currents (i.e., collector currents) which are relatively equal to one 
another, the change in voltage which is an indication of a bubble's 
presence in a memory is directly related to a resistance change only. The 
main collector currents through the two transistors are maintained at an 
equal value by fabricating them so that they have equal betas (current 
gain of a transistor) and equal respective Vbe (voltage across a base and 
emitter junction of a transistor).

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, which represents a known prior art technique for 
sensing the presence or absence of a magnetic bubble, a bridge circuit 10 
comprising resistors R1, R2, R3 and R4 are utilized. A bubble chip 11, 
which is surrounded by a dotted line, includes resistor sensors R2, R4. 
Resistors R2, R4 comprise magnetoresistive Permalloy sensor strips which 
are positioned over a well-known chevron-shaped stretcher (not shown) of a 
bubble propagation loop. It should be understood by those skilled in the 
art that bubble chip 11 includes major and minor bubble loops as well as 
other elements required for operation of a bubble memory, but which are 
not shown for purposes of simplicity. Chevron stretchers are used with a 
resistance sensor to stretch a bubble's dimensions in order to obtain a 
greater output signal therefrom. The output signal of a bubble element is 
on an order of 5 millivolts. Resistance sensors R2, R4 are elements whose 
resistance changes in a presence of a magnetic field such as a field 
emanating from a magnetic bubble when passing in its immediate vicinity or 
a proximately-positioned rotating magnetic field. A signal emanating from 
the bridge circuit 10 which indicates presence of a bubble is coupled via 
capacitors C1, C2 into input terminals C, D of an amplifier 12. Amplifier 
12 is a differential sense amplifier which converts analog input signals 
into a digital output. Details of this circuit may be found in "Linear 
Integrated Circuits Data Book," Third Edition, by Motorola Semiconductor 
Products, Inc., under type MC1544. 
Additional electrical components to those above-mentioned, namely, C1, C2, 
R1 and R3, are required for operation of the prior art circuitry described 
in FIG. 1 such as, for example, resistors R5, R6 and R7. Resistors R5 and 
R7 are necessary in order to reference the bridge output signals emanating 
from output terminals A and B to ground after passing through coupling 
capacitors C1, C2. In other words, if these signals were not referenced to 
ground by means of resistors R5, R7, the signals would tend to float after 
passing through capacitors C1, C2. In addition to the above-mentioned 
circuit components, elements R6 and C3 are required because the prior art 
circuitry requires an additional power supply, -V. Capacitor C3 is of 
relatively large value so as to provide a low impedance so that noise may 
be shorted to ground. 
In summary, therefore, it will be shown that prior art circuit arrangement 
of FIG. 1 for detecting bubble memories requires five resistors (R1, R3, 
R5, R6, R7) and three capacitors (C1, C2, C3) which are not required in 
the circuitry provided by this invention. This aspect will become apparent 
as the instant invention is described below. 
Referring now to FIG. 2, there is depicted a preferred circuit arrangement 
which is utilized in the instant invention. The shown circuit is referred 
to as a current mirror which consists essentially of two NPN transistors, 
Q1, Q2, connected in a front-to-back configuration such that their 
respective collector currents travel over two separate lines. It will be 
shown below that the magnitude of the two collector currents remain 
substantially equal, either quiescently or when in a dynamic or active 
state. 
The two transistors, Q1, Q2 of the circuit arrangement of FIG. 2 are 
designed with matched high betas (.beta.), which may be defined as the 
current gain of a transistor, or a ratio of a small change in base current 
to a resulting larger change in collector current, collector voltage being 
constant. The betas of Q1, Q2 may be on an order of 200-300. Quiescently, 
the circuit operates in the following manner. 
In the quiescent operation, the base-emitter junctions 16, 18 and 22, 24 of 
transistors Q1, Q2 are forward-biased since respective emitters 18, 24 are 
at ground potential, whereas base junctions 16, 22 are connected to +V 
through resistor Rs2. The forward biasing of the base-emitter junctions of 
transistors Q1, Q2 causes currents Ib1 and Ib2 to flow therethrough to 
ground. The forward biasing of the base-emitter junctions of transistors 
Q1, Q2 causes collector currents Ic1 and Ic2 to flow through the 
collector-emitter junctions 26, 24 and 20, 18 to ground. Therefore, the 
output voltage Eo is approximately at ground potential. It should be noted 
that collector currents Ic1 and Ic2 originate from main currents I1, I2 
that emanate from +V and thence through sensor resistances Rs1, Rs2, and 
sensor resistances Rs1 and Rs2 are equivalent to R2 and R4, respectively 
(FIG. 1). It should be further noted in FIG. 2 that main current I2 is 
equal to Ic2, whereas main current I1 is equal to Ib1+Ic1+Ib2. 
As previously mentioned, the betas (.beta.1, .beta.2) of transistors Q1, Q2 
are matched to one another. Furthermore, the voltage (Vbe) across the 
respective base-emitter junctions 16, 18 and 22, 24 are equal. The 
voltages Vbe1 and Vbe2 of transistors Q1, Q2, respectively, are equal 
because they are connected across the same two nodes and, moreover, 
because the base-emitter junctions 16, 18 and 22, 24 are matched. If Vbe 
across both base-emitter junctions 16, 18 and 22, 24 of transistors Q1, Q2 
are equal, then base currents Ib1 and Ib2 are equal. Accordingly, the 
collector currents Ic1 and Ic2 are equal, since: 
Ic1=.beta.Ib1 and 
Ic2=.beta.Ib2 
where .beta.=.beta.1=.beta.2 
Main currents I1, I2 are nearly equal to each other as can be demonstrated 
below. Since transistors Q1 and Q2 are matched, it is understood that 
their respective Betas are equal, that is, .beta.1=.beta.2, their 
base-emitter voltages Vbe are equal (i.e., a voltage across the 
base-emitter junction 16, 18 and 22, 24), and respective intrinsic base 
resistances rb of each transistor Q1, Q2 are equal. 
For I1 to equal I2 the following relationship must exist: 
EQU Ib1+Ib2+.beta.Ib1=.beta.Ib2 (1) 
where .beta.Ib1=Ic1 and .beta.Ib2=Ic2 
The following equation results from the fact that respective voltages are 
taken across a same node (i.e., across terminal of el and ground): 
EQU Ib1(Rs+rb)=Ib2(rb) (2) 
wherein rb is a basic intrinsic resistance of transistors Q1, Q2 and Rs is 
an external resistor in circuit of transistor Q1. 
Solving for Ib1 in equation (2) and substituting therefore in equation (1), 
the following is established; 
EQU Rs=(2rb)/(.beta.-1) (3) 
The relationship for resistor Rs expressed in equation (3) assures that 
main current I1 will equal current I2 for all practical purposes. It 
should be noted however that if .beta.&gt;&gt;rb, Rs can be ignored in the above 
equation since rb is small (i.e., less than 10 ohms whereas .beta. has a 
value magnitude which is 100 or greater). 
Let us turn now to an operation of the circuit of FIG. 2 when it is in an 
active state. The active state results when one of the two sensor 
resistors Rs1, Rs2 changes its value. The reason for this change of value 
will be discussed in a later paragraph. In any event, let us assume that 
resistor Rs1 decreases in value. A voltage drop across this resistor, 
therefore, will decrease and a voltage output at terminal Eo will increase 
as depicted by signal 30. It should be appreciated that the voltage +V 
exists across resistor sensor Rs1 and across transistor Q2. It should be 
noted hereat that the current I2 (substantially equal to I1) remains the 
same during this resistor change, because its value is determined by 
voltages +V, Vbe and sensor resistor Rs2 by reason of 
EQU I1=(+V-Vbe2)/(Rs2) 
as long as Rs2.gtoreq.Rs1. In other words, I2 mirrors the current I1. Since 
the current I2 remains unchanged even though resistor Rs1 is reduced in 
value, the only parameter that may reflect this resistance change is the 
voltage output Eo. Accordingly, it will be recognized that the voltage 
output Eo directly reflects a change of resistance in resistor Rs1. When 
resistor Rs1 returns to its normal state by increasing its value, the 
signal 31 is generated, as explained hereinafter. Thus, when resistor Rs1 
increases in value, the voltage drop across resistor Rs1 increases. 
Therefore, the output Eo decreases in view of the fact the voltage drop 
across the collector-emitter junction of transistor Q2 decreases. In other 
words, the output voltage Eo again directly changes as there is a change 
in resistor Rs1 since current I2 does not change. 
Let us now assume that the sensor resistive element Rs2 decreases in value 
immediately after resistor Rs1 returns to its normal state. When sensor 
Rs2 decreases in value, the main current I1, which flows from +V to 
ground, increases in magnitude. If I1 increases in magnitude, the base 
currents Ib1, Ib2 must also increase as must the collector currents Ic1, 
Ic2. This results from the previously-discussed formula wherein 
Ic=.beta.Ib and .beta. is constant. It can, therefore, be appreciated that 
an increase of the main current I1 will cause an equal increase of current 
of Ic2, which is equal to I2. As a result of I2 gaining in magnitude, the 
voltage drop across sensor Rs1 will increase and the corresponding output 
Eo will decrease as shown by signal 32. In other words, the total voltage 
drops across resistor Rs1 and transistor Q2 must be +V. As soon as 
resistor sensor Rs2 returns to its normal state, by an increase in its 
resistance value, the current I1 will decrease which causes a 
corresponding decrease in circuit I2. Therefore, the voltage drop across 
Rs1 will decrease and the voltage drop across the collector-emitter 
junctions 26, 24 of transistor Q2 will increase as depicted by signal 33. 
On the other hand, when both resistors Rs1, Rs2 change simultaneously, 
there will be no change of the output voltage Eo as will be discussed 
below. 
Reference is now made to FIG. 3, wherein the circuit arrangement of FIG.2 
is utilized for sensing a presence or absence of a magnetic bubble in a 
bubble memory system. A plurality of bubble chips 13, 15, 17, 21, 23, 25, 
are arranged such that eight different bubble circuits are symmetrically 
connected to a plus (+) as well as to a negative (-) terminal of sense 
amplifier 27 (i.e., N=4 and four chips are connected to the +, and four 
chips are connected to the - terminal of sense amplifier 27). The bubble 
chip was previously described with respect to FIG. 1. It will be recalled 
that each bubble chip or circuit utilizes two sensor resistor strips, 
R.sub.A (active resistor), R.sub.D (dummy resistor). Resistors R.sub.A, 
R.sub.D correspond to sensor resistor strips R2, R4 (FIG. 1) as previously 
described, and Rs1, Rs2 (FIG. 2). Sensor resistor R.sub.D is required to 
provide a common-mode rejection scheme of noise induced into sensor 
resistor R.sub.A. Noise is induced into the resistor sensors R.sub.A and 
R.sub.D because each bubble chip is associated with a rotating magnetic 
field (not shown) which is generated in the plane of these resistors. As 
is understood in the art, an associated rotating field is switched into an 
active state one bubble chip at a time with respect to any one of the 
eight bubble circuits when a read/write cycle is to be performed. The 
rotating magnetic field which is generated is sinusoidal in nature, and 
causes the values of resistors R.sub.A, R.sub.D to rise and fall in unison 
since it will be recalled that the value of each resistor changes in the 
presence of a magnetic field. This corresponding change of value in 
resistors R.sub.A, R.sub.D is considered a noise signal because it is 
deleterious to circuit performance, and is eliminated by the common-mode 
rejection scheme in the system shown in FIG. 3, as will be discussed in 
detail below. Transistors Q3, Q4 and Q5, Q6 are matched pairs so as to 
produce identical outputs. 
Let us assume that values of sensor resistors R.sub.A, R.sub.D increase in 
unison in view of the presence of the magnetic rotating field. For a 
moment, it will be considered that no bubble propagation occurs so that 
the effect of noise in the circuit can be considered. Therefore, a 
quiescent current I3 flowing in resistor R.sub.D from +V to ground through 
respective base-emitter junctions of transistors Q3, Q4 is changed by 
decreasing in value and a voltage drop across R.sub.D decreases so that 
output voltage E1 increases. In other words, the fixed voltage +V provides 
a voltage drop across resistor R.sub.D and transistor Q4 and if the 
voltage drop across resistor R.sub.D decreases, the voltage drop across 
the transistor Q4 (i.e., the collector-emitter junction) must increase. 
When quiescent current I3 decreases through R.sub.D, current I4 will 
decrease through sensor resistor R.sub.A for the reason previously 
discussed and a voltage drop across R.sub.A will increase and the output 
voltage E1 will decrease. The simultaneous increase and decrease of output 
voltage at output E1 will effectively cancel each other at the + input 
terminal. Accordingly, it is apparent that use of the active sensor 
resistor R.sub.A in combination with a dummy resistor R.sub.D in the 
circuit of this invention achieves noise cancellation at the + input 
terminal of sense amplifier 27. It should be understood that like 
cancellation will occur when the resistance values of resistors R.sub.A, 
R.sub.D decrease in unison, due to the rotating magnetic field. 
It should be noted that transistors Q5, Q6 are also in a conducting 
quiescently at this time, and are conducting through their respective 
base-emitter and collector-emitter junction from +V to ground. Therefore, 
the voltage output E2 of transistor Q6 which is directed to the - input of 
sense amplifier 27 is near ground potential. In this respect, the noise 
signal discussed above with respect to the+ terminal of sense amplifier 27 
will be superimposed upon the near ground signal at which the collector of 
transistor Q4 sits. Accordingly, the output signal E1 which is a noise 
signal will be self cancelling because of the circuit comprising 
transistors Q3, Q4, whereas the near ground signals E1, E2 resulting from 
the conduction of transistors Q3, Q4, Q5, Q6, will be cancelled by 
differential sense amplifier 27. 
Let us now turn to an operation of the system of FIG. 3 wherein the 
rotation of magnetic field with respect to bubble chip 13 will cause a 
bubble to be detected. As understood from the above discussion, the 
induced noise from the rotating field will be eliminated. As soon as a 
bubble is propagated under the Permalloy sensor strip or resistor R.sub.A, 
its resistance (in the manner of Rs1 in FIG. 2) will decrease. As also 
previously described with respect to FIG. 2, the output voltage E1 will 
increase to directly reflect the change in resistance of sensor R.sub.A. 
At this time, the output voltage E2 remains near ground. The positive 
output 50 is shown as produced by the sense amplifier 27. When the 
resistor strip returns to its normal state, a negative signal 51 is 
generated. The reason for this negative signal is that when the resistor 
strip R.sub.A increases in returning to its normal state, the current I4 
decreases so that the voltage drop across R.sub.A increases and, 
therefore, the voltage drop across the collector-emitter junction of 
transistor Q4 must decrease. It will be recalled that +V is the voltage 
drop across these two elements. 
Upon passing the resistor strip R.sub.A, the bubble is next propagated past 
strip R.sub.D, whereby the value of strip R.sub.D will decrease and the 
output voltage E1 will also decrease. Output voltage E1 decreases because 
when current I3 decreases, current I4 decreases, thereby causing a voltage 
across R.sub.A to increase and correspondingly, the voltage across 
transistor Q4 to decrease. This negative signal is shown as signal 52. The 
analysis for this mirror circuit behavior was described with respect to 
FIG. 2 and, in particular, when sensor resistor Rs2 decreased. A positive 
signal 53 is generated when sensor resistor strip R.sub.D returns to its 
normal value in a manner previously described with respect to the signal 
33 of FIG. 2. 
Referring now to the bubble chips 21, 23 and 25 in the lower half of the 
system, it is noted that the resistor strips R.sub.A, R.sub.D are 
connected in a slightly different manner to transistors Q5, Q6 than the 
bubble chips 13, 15, and 17 are connected to transistors Q3, Q4 except 
that a collector electrode of transistor Q4 is directed to a positive 
input terminal of amplifier 27, whereas its negative terminal is connected 
to a collector electrode of transistor Q6. The operation will be precisely 
the same as above described with respect to bubble chips 13, 15 and 17, 
except that the output signals 50', 51', 52' and 53' directed into input 
terminal E2 are opposite in polarity from signals 50, 51, 52 and 53. The 
reason for this is that the resistors R.sub.A, R.sub.D connected to chips 
21, 23 and 25 are interchanged from a connection of resistors R.sub.A, 
R.sub.D connected to chips 13, 15, 17, because operation of sense 
amplifier 27 requires that the output E1 must go positive with respect to 
E2, or E2 must go negative with respect to E1. Therefore, output signal 
50, 50', 53, 53', can be detected by sense amplifier 27. 
Referring now to FIG. 4, there is depicted another circuit which may be 
substituted in the system described above in FIG. 3. The lower half of the 
circuit, comprising transistors Q9, Q10 is similar to the circuit 
described in FIG. 2, but used in a current source configuration. FIG. 2 of 
this invention does not use the circuit as a current source. In the 
circuit comprising transistors Q7, Q8, the currents I5, I6, are equal when 
these transistors are matched. A bubble chip 60 including resistor sensors 
R.sub.A, R.sub.D are utilized in the manner previously described. Any 
change in value of resistors R.sub.A, R.sub.D will result in a voltage 
change at the +, - input terminals of a differential amplifier 65. In this 
circuit, it is important that transistors Q7, Q8 stay out of saturation. 
The current source comprising transistors Q9, Q10 is used to supply 
currents I5, I6, in such a manner that the voltage drops across R.sub.A, 
R.sub.D do not exceed the +V power supply voltage. 
In summary, it can be readily appreciated that the present invention 
provides simplicity in organization over that shown in the prior art. In 
particular, DC instead of AC coupling, a single power supply, as well as 
providing a circuit which acts as a transducer to convert a change of 
resistance (R) into a change of voltage output (E) are deemed salient 
advantages which are not realized by the prior art.