CA1229886A - Method of low noise current amplification - Google Patents

Abstract

Abstract of Disclosure This disclosure is concerned with novel techniques and circuit configurations for rendering high gain operational amplifiers and related differencing amplifiers adaptable for use with very low impedance signal sources to permit amplification of low-power signals with ultra-low noise, high D.C. stability, high gain, and wide frequency response, through development of the signal current as a very small increment on a much larger pre-existing current forced in opposite directions through an electronic switching device connected to such amplifier and producing minimal uncorrelated noise therein.

Description

The present invention relates to methods of low-noise signal current amplification, including the conversion of high gain operational amplifiers and the . like into low-noise inverting amplifiers of high D.C.
stability, high gain and wide frequency response.
Prior art techniques for low-noise amplification . of signal currents with the aid of amplifier electron tubes, solid-state amplifiers such as transistors and the like, and other types of amplifying relay devices, all hereinafter sometimes genericaIly referred to as electronic , switching devices, have largely relied upon the use of ; a series element to develop the input signal current as ~! a voltage, inherently thereby degenerating the signal capabilities of the device. The conventional approaches to l.ow noise circuikry is to use, for example, a single transistor with grounded emitter, and to introduce the i signal by means of a voltage at the base. As an ~;, illustration, to record from a moving coil signal source, ~, one uses . .
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a damping resistor across it and the voltage generated ' across the resistance is the signal. ~ut, in using such inputs as, e.g. those of moving coil cartridges and the ' like, it is clear that the use of a damping'resistor i corrupts the possible signal to be had by operating the ! coil as a pure current generator, ~or e~en as a purely ~, inductive generator when the coil resistance is cancelled I by a series negative resistance). The same is true of any jY low impedance, low'power signal device. Prior approaches -, to lo~t-noise amplification are described~ for example, in1 . .
~ Wlreless World, Aprii 1981, IlIntroduction to 10w-Noise ,j `-~i Ampli~ier Design", A. Foord, pages 71-73.

~! In accordance with the philosophy underlylng the '-i d~scoveries of the present 1nvention~ on the other hand, ~ electronic switching devices or amplifiers are operated to ;' measure current rather than voltage, and to do so ~ith a novel superpositlon of signal from a low input impedance source upon much larger opposing balancing currents passed from a common preferably constant current source in opposite dlrections through the electronic swltchlng device--this technlque having been found to result in the lowest possible noise in such ampli~ylng circuits.

8~i A ~eature of the present invention, accordingly, is to provide a new and improved method of low noise amplification and, more particularly, a novel method for low-noise signal current amplification.
A further feature is to provide a novel method for converting high gain operational amplifiers and the like into low noise current amplifiers of low input impedance, high D.C. stability and high yain, together with wide frequency response.
An additional feature is -to provide novel amplifying techniques for amplification that are adapted for use with a myriad of prior art types of components and circuits to provide highly improved low-noise operation and other benefits.
In summary, however, from one of its broader aspects, the invention relates to a method of low noise current amplification of a signal from low impedance source means of the signal sensor type having a moving conductor in a magnetic field, that comprises, applying signal forces to the signal sensor ~o move the conductor in the magnetic field in response to the signal forces, connecting a c~lrrent receiving circuit across the sensor e~fectively to short circuit the same, and pxoviding a measurement of the signal power applied to the sensor by the short ! circuiking in the circuit.

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The invention will now be described with reference to the accompanying drawings:
Figs. lA, B and C which are schematic circuit '~ diagrams explanatory of the underlying techniques and ' operation of the invention;
Fig. 2 is a similar diagram of a preferred elemental-form circuit illustrating the method and type ' of apparatus useful for the practice of the invention ." using transistors of complementary form;
Figs. 3 and 4 are circuit diagrams of further modified circuits run from a power supply, illustrating the application of the invention in more practical form and useful with three-terminal electronic switching devices or other transconductance relays or the like;

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i, ~, - Fig. 5 is the symbol for such a circuit, considered as an element with two input terminals and one output terminal, while Fig. 12 shows a pair of these circuits arranged so that the signal current generator (e.g. a "moving coil" transducer) can be connected directly therebetween;
~r,) '-~ Figs. 6 and 7 are cascade-type circuit modifications useful with operational amplifiers and the like and also embodying the basic circuit topology and methodology of the " invention;
Figs. 8 and 9 each illustrates a plurality of electronic s~itching devices or relays useful, respectively, , with the apparatus of the circuits of Fig. 3 and Figs. 6 and ; 7;

. Figs. 10 and 11 are similar circuits embodying pro-~- tective circuits for limiting the ~ and - input differential ~i~
excursions applied to the operational amplifiers or similar cc~ponents;

~i Fig. 12 is a further circuit modification embodying the system of Fig. 5 and employing the switching operational , .
amplifier circuits or Fig. 3;
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Figs. 13A, B and C are circuits using the general configuration of Fig. S particularly adapted fo~ use with low-resistance signal coils such as those of moving-coil cartridges and the like, J

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Flgo 14 is a ~urther modificat~on illustrating _ how a ~negative" resistance can be generated to cancel the l equivalent serles resistance of a coil so as to treat the coil as a pure inductance; and Fig. 15 is a modlfied circult conflguratlon . involving lmpedances that allow actlve fllters to be made .. in the range of operation o~ the device.
. E`ig. 16 is similar diagram of a modi~ied circuit ~; permitting grounding o~ the sensor input~
Re*erring to the drawings, as above lndicated, Figs.. 3 and 4 are complementary verslons o~ circuits ; embodylng electronlc switchlng transistors Tl ~ed equal ~ and opposite currents ~rom a constant current source I and ;~........... connected to a dif~erencing amplifier, such as an opera-f," tional ~mplifier A, wlth ~eedback from its output ("OUT") . . through resistance Ro to the emitter ~ Tl For an under-. standing o~ the operation, re~erence is made to the . explanatory diagrams of Figs. lA, B and C.
In Fi.g. lA, the current generator I is shown . implemented by the FET T2, being connected in circuit with ; a D.C. source v and the transistor switching device Tl~

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~A shown with base grounded at ~. No possible current can ~i flow except leakages which can be assumed to be less than 10-1 amps. Thus the drain-source voltage of T2 will be shown with base grounded at G. No possible current can ~low except leakages which can be assumed to be less than 10-1 amps. Thus the drain-source voltage of T2 will be fS zero and the gate-source voltage ~ill also be zero so that the drain-source resistance is low.
i:, !, This system is connected to a di~ferenclng ~¦ ampliXier as illustrated in Fig. lB. The differenclng ¦ ampllfier preferably has a high input impedance (l~e. has ~ an FET flrst stage) and has its * input connected to the ,,',,r;,"` grounded base of Tl, its-input connected to the collector thereof, and its phase-in~erting output ~ed bac~ through ¦ Ro to the emitter of Tl. The input slgnal Veb~ as from a ~, very low impedance source ~such as the coil sensors before discussed and the like)~ is applied to the emitter but is independent of the 1'floating" current~ and the signal cur-rent passed through Tl ls of magnitude small compared to ,, the equal and opposite currents forced from the constant . . , ., .

j- current ~enerator or source I lnto the emitter and collec-~or of Tl. It should be noted that these equal and oppo-site currents fed at the emitter and collector through Tl ~re noise-correlated by virtue of the common source. This results-in minimal uncorrelated and uncancelled noise ~herein; and thus with the signal current applied between ,i .
emitter and base (Ieb~ being superimposed as a tiny incre-~'" ment much smaller than such currents lying above the noise current of Tl, there is generated an output ~mpli~ied si~-nal Vc~ o~ the operational ampli~ier A that is propor--~ tional thereto wlth very low nQise.
,; Specifically, with the clrcuit of Fig. lB~ the ~- output voltage ~OUT rises until OUT ~ eb = I~b = ~ `~
. where ~ ~s the amplification factor of Tl. Once Tl is "
turned on, the emitter-collector current IeC i~ that of ,, , ~
~ the current source I; that is, IeC = I~ and is unaf~ected '; in princlple by the signal current Ieb. In fact, however~
there is some lea~age between collector and base, but none of` it ~lows, since by the feed-back clampinæ~ the ~oltage between collector and base ~eb ~ O. Under th~se condi-, ~g~6 tions, the voltage gain is ( ec) - lO4with most i d (Veb3 low-level transistors~ insofar as that galn can be measured dlrectly. The gain of the operational amplifier A is cnlO5 so that the open-loop gain is v~ lO9. For this reason,C is used between the output and input terminal -of A to prevent parasitic oscillations.
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' With no input, as it stands~ if the collector ; were grounded to the base~ the resistance o~ emitter-to-j ground is VT, where VT - KT ~2~ mv ak room temperaturè., 1 Then the systém would be an inverting amplifier with a V
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gain of (R) tI). But the great gain of Tl changes all 26 mv that, even though Vec is clamped to Veb. The e~fectlve input resistance at the emitter of Tl is ~ell below .OOl ohm by calculation and by practice, so that the emltter is quite as good a ground as one end of a short stout copper wire w~ose other end is grounded~ effectively short-cir-cuiting the before-mentioned coil sensor or otner very low impedance signal source represented by Veb applied between emitter and grounded base of T11.
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Equally gratifying is the noise of the device.
As second sta~e~ the operational ampllfier A contributes ne~ligibly because of the gain of the first stage Tl. At the same time, the current between emitter and collector of Tl contributes only by the correlated noise of T2 divided by hfe, with such noise being relatively low ~in the first place--so it contributes little if h~e~ 100.

~ihat is left ls the noise of the equlvalent resistance Re through the emitter plus the base spreading resistance ~bb' In good bipolar transistors, for example, Rbb'c 5 ; ohms and Re-26IV, so that the equivalent resista~ce from 1 which to calculate the noise is C 130hms if ICe ~ 3ma.
i This technique thus enables substantially pure ;-1 1 current amplif~cation with substantially no voltage-induc-ed noise, particularly, though by no means exclusively, useful-wIth the before mentioned very lo~ impedance slgnal sources o~ 20 ohms or less. EfLect~ively all of the base, emitter and collector electrodes are clampled ~the base, to ~round), and the switching devlce operates in a pure .~. current mode w-lth the noise of only the emitter junction ; and the base spreading resistance; and with output gain of , any desired value (such as the be~ore-mentioned loop gain , . . .
at about 109). The current through the switching device effectively nev~r changes significantly because of the feedback.
This 1s thus a general way of convertlng any ; hlgh gain operatlonal amplirier into a low-noise signal r, ~ .

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current amplifier of high D.C. stability and high gain together wi~h wide frequency response. It replaces signal transformers, betterin~ them in noise and contrlbuting negligible phase shift over a very wide band.
Displayed in Fig~ lC and Fig. 2 are t~o comple-mentary versions o~ the circuit illustratlng the same top-ology of the invention. Again, the same two independent currents are created, that between collector and emitter of T1 ~or T11 in Fig. 2) and that between base and emitter. ~y using a floating current ~enerator I forthe collector-e~itter current, it not only is lnsured that such remains constant, but ~hat it cannot affect ~he base current. An~ signal current applied between emitter and asa can onl~ flow through that ~unc'ion and so permits making a common-base amplifler instead of the conventional common-emitter amplifier.
The open-loop gain, belng about 108-109, the closed loop behavior at a gain o~ 1000 or less ls not only linear over the permitted dynamic range Or the operational ampli~ler A~ but has the frequency response of the opera-tional amplifier. Under suitable choice of C, ~he . . .
'Z~- high rrequency response can be tailored. It sXould be noted that the emitter and collector are protected by diode action so that no reverse currents can ever flow . through the ~unctions.
i Any three-terminal electronic switching config-;' uration, characterlzed by a transconductance, can be used for the purpose of the invention if not directly for Tl in the circuits of Figs. 3 and 4, by the equivalent arrange-ments shown by Figs. 6 and 7. Thus, FET 7 S, MOS-FET's, : thermion~c devices, and all manner of cascode combinations i~ of them can be accommodated, either by a voltage offset of ,ii *lN of th~ operational amplifier A from REF (such as ground~ in Figs. 3 and 4~ or by the arrangements of Flgs.
.~ 6 and 7. ~igs. 3 and 4 are complementary versions of mod--t'', ified clrcuits wherein the circuits T1-A and T1l-A of res-pective Figs. l~ and 2 are fed from the source I through a compleme~tary pair o~ transistors T2 and T3 (or T2', T3' in Fig. 4) with appropriate reverse diode-protected po~er supply voltage resistor networks Rl, R2, R3, R4 (or Rl R4'). Preferably R1=R2 50 that with R3=R4, there is a substantial part of the current passing through Tl that .

is completely correlated in noise since there is only a unitary source of current I. The uncorrelated part due to R1 and Rz and R3 and R4 being separate elements contri-butes noise well belo~ the level of the input noise of the circuit; The effective symmetric current reflector action provided by the net-work resistors insures that the noise of the power supply is pro~oundly attenuated. The larger the resistances in the reflector circuits, the less un-correlated noise is generated, with the correlated noise being cancelled as before explained.
In the embodiment of Fig. 6, as distinguished ~rom Fig. 3, a further stage Tl~ ls employed with its coll-ector connected with that of T3 and to the - input of the operational ampli~ier A, with the emltter o~ T3 connected to the collector of T1. This provides for the collector current of Tl to be applied through the emitter of T3 and the resulting collector current T3 to be pitted against a reflected or inverted current from the same constant cur-rent source. Flg. 7 ~llustrates the complementary version thereof. These clrcuits are used when the voltage of the drain of an FET or the plate Or a triode must be offset from the REF yolta~e by a definite amount.

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Other types of electronic switching devices Tl (or Tll) useful with this technique are shown in Fig. 8A-F
(or Fig. 9A-F), embodying various FET, transistor, elec-tron tube and cascode arrangments thereof; ~ith the de-vlces of Fig. 8 being useful in clrcuit configurations such as those o~ Figs 3 and 6, and the de~ices of Fig. 9 being useful with circuits as represented in Figs. 4 and 7. The reference to electrodes herein such as base, coll-ector and emitter types for solid state transistor devices is thus intended to describe in a generic way equivalent or similar electrodes of other solid state, electron tube r~, or related electronic switchin~ devices.
.~ For some operational amplifiers it is desirable to limit the differential excursions of the + and - in-puts~ which can be done by such devices 3S protective diodes or the circuit variants of Figs. 10 and ll~ wherein . ~ , supplemental compllmentary transistor stages To and To~

are inserted in the respective circuits of Figs. 4 and 3.

, The collectors of the complementary stages are connected : together as are the bases, with equal and opposite current ,"; !~
fed to the symmetrical but complementary emltters~
Fig. 5 is a schematlc representation of a slngle current source using a pair Or electronic transistor ~;

s~-itching-o~er2tional amplilier circuits o~ the ~n-en~ion, with the ~row at -lN (~s in Fig. 13) diagr2mctically indic~tin~ whether the emitter is ~Ipll or "~", The explicit c~cui~ so schematic.~lly represented in Fig. ~ is illustrated in Fi~. 12 in connection with switching-op-ra-tional amplifier circ~its of the iype shown in Fig. 3, with its p~.~er supply resistor volta~e networks P~1-R
Two such a~-e sh~m in Fig, 12 as T2-Tl- A-T3 and T2"-Tl"-A"-T3~ ith the ~ormer having signal input -L~ A
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and output OUT A, and the latter, signal input -IN B ~3d output OUT B. I~nen power supply resistor P.3 is so ad~st-ed sligh'~ly less than R4, whatever current flo~Ys into ~he em~tter is I 1 Ieb, while the current that flows 'rom '~e collector is I. ~rOu~ can be readily brougr.t to O vol~ '-10 ~ V. S~nce the currents through Tl or Tl" never a~r3-ciably cha~ge auring c~peration, nor the volta~e acrocs them, the systzm is ~uite thermally stable in palred ~or-ation as in Fig. 12. ~uch a s~stem is particul~rly ~a-~?
ted for use with 'ow im~e~2nce sensGrs such as the berore-mentioned low resistance coils as Or the movin~-coil cartri~ge type or similar aevices. ~oth operati~
a~pli~iers cre sho~n ~iith co~on rererence (I~EF) inpu~
but ~ith th~ two ~F inpu~s capable o~ hav~ng a volt~
dir~erence ~et~een them.

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- Fies. 13A, B and C em?loy the schematic reoresentation of Fig. 5 to show such use to , record from coil sensors or sienal sources desig-, nate~ by L. Fie. 13A is preferable since it , ~ ~rovides less noise than the ~rounded coil terminal '¦ embodiment of Fi~. 13B. Fi~. 13C represents a ¦ ~,ood compromise using a single circuit of the Fig. 5 ~yp~ with a grounded-ter~inal coil L.
.Fig. 14 sho~.7s a circuit similar to Fig~ 13A
~, , which enables th~ of th~ device to b~ connected ,' s'o as to put an equivalent resistance, varyin~
from ~r to -r in series with the coil through the ,, ne.~ork P.
i. , The ~opology of ,~he invention has another "l unique feature. By using emitter in'~ecti.on of .I current, the collector volta~e s~ing is directly '1, conlrolled instead of re~uiring an amplifyin~ -~ stage. This pres~rves phase even a~ very high .; frequencies. For e~ample, at 3ma for I, and 30~.d for C~ th~ system has been found to be co.,-pletely ~ s~able wit,h a slew rate of th~ operational a~plifier ,, ; al~ost indisti.n~uishable Lrom its nor.~al slew rate :

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at unity gain. Practically speakin~, when run at a cur-rent I of the order of 3ma, the circuits of the invention have a voltage noise referred fromm output 'GO input~ of about .6~V/V~ between lOHz and 100 KH~, corresponding to that of about 10-12~L . This suggests that the Rbb' of the PNP transistors is about 3-5lL, which are as they are specified. The noise measure can be reduced much further by using several transistors in parallel at Tl. Then the noise drops by ~ where n is the number used.
e circuits of the invention, moreover, are not limited to working only wlth feed-back resistance (Ro).
Two impedances Zl and Z2 can be used, as in Fig. 15~ to extend the high frequency range or to provide other spe-cial operations, such as that of active filters.
Operation of circuits as shown in Fig. 3 and other embodiments has demonstrated successful use wlth low impedance signal devices of input resistance less than iO
ohms producing signal amplification wlth noise (at 3ma current) of ~006 nanovolts/~ from 10 Hz to 1 megahertz and with a drift over several weeks o~ -~ lO~uV. Linear dynamlc range greater than 130 db has been obtained with rrequency response from D.C. to the megahertz range.

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~ he modification o Fig. 16 is some~hat like ~hat of Fig. 11 ~ut aliows the input to be at ground level by use of a pair of symmetrical sub-circuits that enable reduction of the noise of a single dual sub-circuit by j 3db. The complementary series-connected diodes ~1 and D11 I (collectors and bases strapped together) allow such s~me-j trical ground level input from the sensor or trznsducers or other low impedance signal source to the emitters of ¦ t~is comple~entary pair of series-connected switching , transistors T1 and T11 across which D1 and D1 are il shunted. A most unconventional ~eedback is employed of Ro o from the output OUT to the non-inverting input + of the .,l operational amplifier A and wh.ich is direc~ly connected by I 10 to the sensor or other input IN. This diminishes dri~t ¦ ~minimal as it may be) more than if the non-inverting ~ input were tied to ground (or other reerence) as in Fig.
,jl 11. ~dditional paralleled stages T3 and T~ may also be Il employed, i~ desired.
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Y¦ The invention thus, in effect, replaces signal trans~ormers~ bettering them in noise and contributing negligible phase.shift over a ~-ery wide band. Not only is it useful in converting differencing amplifier circuits "
such as operational amplifiers or low input impedance, wlth high D.C. stability and gain over wide frequency responses, but lt is particularly use~ul more generally .J
for amplification of currents generated by coils across ¦ which the magnetic field is changing. Examples of such .'! .
are siGnal sources or sensors of the moving coil cartridge type (as of the designs used in phonograph apparatu~), ribbon microphones, and coils used in sensors or detectors of various scientific, biological and medical instruments and the l~ke. The invention also provldes for ampllfica-tion of low currents or low-power signals generated by low-impedance devlces (lo~Y-power signals) such as thermo-couples, photo-current generators, straln c~nd pressure transducers, bioelectrlc signal sensors such as used in EKG's, muscle activity detectors as in electromyograms~
electroretinograms, etc. I~le prlnciples of the lnvention are further useful t~ provide very linear hi~h-power ampl~fiers, for use as FM recelver and other front end and low-noise antenna pic~-ups or term~nals, enabling improvement in weak s~gnal reception as a result of low antenna impedance matching in a current mode. Improve-ments in audio and stereo amplificat~on, in low-noise power supply performance, in current driving from constant j current sources and the li~e are other useful applications of the invention. The invention~ moreover, provides a , rather universal design for inverting mode amplifiers generally, well-suited to fabrication in solld-state chip form, also enabling wide use for active filter purposes.
While the invention is not dependent upon any particular theory of operation, it being sufficient to describe, as above, the circuits and adiustments required ! to produce the novel results thereo, it is ~elie~ed that the reason for the unusual performance of the invention is the effective operation of the circuits as a substan-tially perfect electronic ammeter connected across and substantially short-circuiting the moving coil sensor or transducer or other low impedance signal source, as before described. Such moving coil or conductor low impedance ;

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transducers or sensors, as they move in a uniform magnetic field with a particular velocity in response to a signal, permit of recording from this coil or conductor either by open circuit mode, in ~hich case the voltage is recorded, or by short-circuited mode, in which case the current is recorded~ Xn using t~e latter mode, as in the present invention~ the sensor moving coil or conductor, as it moves in the magnetic field, brings to bear the condition of Lense's Law, setting up its own counter maynetic field . .
as a result of the short-circuited operation, diminishing the field immediately behind it and thereby setting up conditions that impede its own progress, achieving a limi-ing or terminal velocity that is proportionate ~o the applied signal force, and with such limiting or terminal .; I _ . . - - . .
i. velocity being created immediately upon the application o ,, /,j, 1 the force. In other word~, the moving wire cannot coast 1 along under momentum but is damped and stops the moment .the force ceases to be applied. Such eedbac~ ~orces the limiting or terminal velocity such that the operation is i analogous or eq~ivalent to the nega~ive feedbacX in an . . , .

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J op-amp itself which reduces the excursion o the input in J
;. order both to linearize the device and extend its band with~ The dynamic range is enormously increased because I of dealing with fractional increments of what ormerly s were large magnitudes, and no one part of the coil or con-¦ ductor can move faster that any other part because of the inherent damping, somewhat comparable to vibrating a ~,1 string or dropping a ball in a container of viscous ! honey. Harmonic distortion is ruled out because forces are always in balance and no electrical storage tcapaci-i~ tive or other~ise) can exist; but the signai has nonethe-fl less been read as the total power of the signal is a~sorbed in this damping (of course, with heat genera-7~ tion). Thus, unlike open-circuited measuring techniques I wherein some index of the actual signal power is employed ,~i for.measurement, such as a representative voltage or f: current Lndicative of the same, the operation of the ~, invention involves direct measurement oE the total signal power itsel. The circuits of the invention are connected across such moving conductor type sensors in a magnetic ,~r~JI field effectively to short circuit the sensor conductor ..

, and thereby cause counter-magnetic field generation dissi-pation of all of the signal power that is applied to the - sensor and in so doing drain all of the signal driving power, such directly measuring the power applied by the signal. It should also be remarked that the first stage ~such as T1, etc.) of the circuit of the invention which is short-circuiting the sensor should not be construed as analogous to conventional power amplifiers and the like, in that it merely allows the sensor-developed current that flows to generate, in effect, its own phenomenon, with no power ampli~ieation, and with the circuit contributing no current at all~ -Further modifications will also occur to those skilled in this art and such are considered to fall wi'hin the spirit and scope of tne invention as deflned in the appended claims.

Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of low noise current measurement of a signal from low impedance source means of the signal sensor type having a moving conductor in a magnetic field, that comprises, applying signal forces to said signal sensor to move the conductor in said magnetic field in response to the signal forces, connecting a current receiving circuit across the sensor effectively to short circuit the same, and providing a measurement of the signal power applied to the sensor by the short circuiting in said circuit.

2. A method as claimed in claim 1 and in which the measurement is provided by the dissipation of the signal power applied to move the said conductor of the sensor as counter magnetic field forces are developed by the short-circuited sensor conductor analogous to negative feedback, and damping its movement so that the conductor stops the moment the signal force ceases to be applied, thereby linearizing the operation and extending bandwidth of the sensing operation.