US3274517A

US3274517A - Beam deflection tube modulation circuit with means for suppressing undesirable alternating output

Info

Publication number: US3274517A
Application number: US212251A
Authority: US
Inventors: Edward I Lynch
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1962-07-25
Filing date: 1962-07-25
Publication date: 1966-09-20
Anticipated expiration: 1983-09-20
Also published as: FR1364470A

Description

Sept. 20, 1966 BEAM DEFLECTION TUBE MODULATION CIRCUIT WITH E. I. LYNCH MEANS FOR SUPPRESSING UNDESIRABLE Filed July 25, 1962 FIG.I

ALTERNATING OUTPUT 2 Sheets-Sheet 1 SOURCE OF DIRECT CURRENT OPERATING POTENTIAL+E SOURCE OF DIRECT CURRENT OPERATING POTENTIAL-I- E SOURCE OF MODULATING VOLTAGE SOURCE OF DIRECT CURRENT OPERATING POTENTIAL-I-E I SOURCE OF LUMINANCE VOLTAGE -32 JWVW SOURCE OF DIRECT CURRENT OPERATING POTENTIAL+E2 'NVENTOR' EDWARD l. LYNCH, SOURCE OF T I COLOR DIFFERENCE BY A -68 L HIS ATTORNEY.

Sept. 20, 1966 E. 1. LYNCH 3,274,517

BEAM DEFLECTION TUBE MODULATION CIRCUIT WITH MEANS FOR SUPPRESSING UNDESIRABLE ALTERNATING OUTPUT Filed July 25, 1962 FIG.2

ANODE CURRENT MAI FIG.3

ANODE CURRENT -'(MA) 2 Sheets-Sheet 2 I I I I I I III 876 I I I I I I I 5432IOI DEFLECTOR VOLTAGE "(VOLTSI I I I I I I ll 5432IOI23456T8 DEFLECTOR VOLTAGE -(VOLTS) lNVENTOR EDWARD I. LYNCH,

BY W M HIS ATTORNEY.

United States Patent "Ice 3 274 517 BEAM DEFLECTION TU BE MODULATION CIR- CUIT WITH MEANS FOR SUPPRESSING UNDE- SIRABLE ALTERNATING OUTPUT Edward I. Lynch, Syracuse, N.Y., assignor to General Electric Company, a corporation of New York Filed July 25, 1962, Ser. No. 212,251 8 Claims. (Cl. 332-58) This invention relates to modulation circuits of the type utilizing a beam deflection electron discharge amplifying device and more particularly to improvements thereof.

A modulation circuit employing a beam-deflection electron discharge amplifying device as a [plural function modulating element is described in co-pending patent application Serial No. 204,983 filed June 25, 1962, now US. Pat. No. 3,219,754 and assigned to the assignee of this invention. Presently fabricated beam-deflection devices which are suitable for application in the modulation circuit of the referred-to patent application and in other similar modulation circuits exhibit a deflector-electrode voltage versus anode-electrode current transfer curve having an electron beam leakage segment. This beam leakage segment is of relatively low constant current over a range of relatively high deflector-electrode voltages and represents a flow of current to a first anode of the device when a deflection voltage is applied between first and second deflector electrodes of the device for directing the entire electron beam current to a second anode. In a modulation circuit the electron beam is intensity modulated by an alternating voltage which is coupled to a control electrode of the device and the referred-to leakage current causes a resultant alternating voltage to occur in an output circuit coupled between the first anode electrode and the cathode electrode of the device.

In addition to the generation of an alternating voltage by the leakage current, a capacitive coupling exists between the anode electrodes and input electrodes of the beam deflection device. This capacitive coupling which consists of stray and interelectrode capacitance results in a flow of alternating current from the input electrodes to the anode electrodes and causes a radio frequency voltage to exist in output circuits coupled to the anode electrodes.

In some electrical systems utilizing modulation circuits, it is desirable at times that an alternating output voltage from the modulation circuit be suppressed. For example, in a light valve color projection system of the type described in the above referred to patent application, and in other circuit applications having similar requirements as will occur to those versed in the art, it is desirable to reduce to zero volts the amplitude of the modulation circuit alternating output votlage when a modulating voltage which is coupled between the first and second deflector electrodes attains a selected level and polarity with respect to one of the deflector electrodes. Specifically, in the light valve projection system of the referred to patent application, it is desirable that the alternating output voltage from the modulation circuit be substantially suppressed when video information supplied to the system corresponds to the color black. The above described leakage current and capacitive coupling in presently manufactured beam-deflection electron discharge amplifying devices undesirably inhibits the reduction of the alternating output voltage to zero volts when such a reduction is desired.

Accordingly, it is an object of this invention to provide an improved modulation circuit of the type employing a beam deflection electron discharge amplifying device.

Another object of this invention is to provide a modulation circuit of the type employing a beam deflection 3,274 ,5 l7 Patented Sept. 20, 1966 device and having means for suppressing an alternating output voltage when an electron beam current flowing between a cathode electrode and a first anode electrode of the device has a pre-selected amplitude.

Another object of this invention is to provide means for elminating an alternating output voltage resulting from a leak-age current occurring in a modulation circuit of the type referred to.

A further object of this invention is to provide means for suppressing an alternating voltage occurring in an output circuit as a result of capacitive coupling between electrodes of a beam deflection device in a modulation circuit.

In accordance with the present invention, a modulation circuit having a beam deflection electron discharge amplifying device is provided for generating an amplitude modulated alternating output voltage. The output voltage is generated in an output load circuit which is coupled between a cathode electrode and a first anode electrode of the device. Means are provided for automatically suppressing the alternating output voltage when a preselected value of current i comprising all of the electron beam conduction current which flows between the cathode and first anode electrodes. The voltage suppression means includes a second load circuit coupled between the cathode electrode and a second anode electrode, and includes means for intercoupling the second and output load circuits. Neutralization circuit means are also provided and are coupled between a source of alternating voltage of frequency f and the output load circuit for suppressing in the output load circuit a voltage generated therein by an alternating current flowing between an input electrode and an anode electrode by stray capacity and interelectrode capacitance in the device.

Further objects, features and the attending advantages of this invention will be apparent with reference to the following specification and drawings in which:

FIGURE 1 is a diagram of a modulation circuit embodying the present invention,

FIGURE 2 is a diagram of an ideal deflector-voltage versus anode-current transfer curve for a beam deflection amplifying device used in a modulation circuit,

FIGURE 3 is a diagram of a deflector voltage versus anode current transfer curve for a presently manufactured beam deflection amplifying device, and

FIGURE 4 is a diagram of another form of modulation circuit embodying the present invention.

In FIGURE 1 an amplitude modulation circuit embodying the present invention is shown having a modulating element 11 comprising a beam deflection electron discharge amplifying device. The device 11 includes a cathode electrode 12, a control electrode 13, an accelerating electrode 14, a pair of

deflector electrodes

15 and 16, and a first and a

second anode electrode

17 and 18, respectively. For purposes of clarity, an electrode for focusing an electron beam into an electron sheet beam is not shown. An output load circuit is coupled between the first anode 17 and the cathode 12. Although other suitable load circuits will occur to those versed in the art, the output load circuit shown comprises a top coupled, 1r filter

network including inductances

19 and 20, associated tuning capacitances 21 and 22 which may comprise winding and stray capacitance, a coupling capacitance 23, and damping

resistances

24 and 25. A first source of operating potential 26 for supplying a direct current voltage +E to the device 11 is provided and is coupled to the anode 17 via the inductance 19 and a power supply decoupling circuit including a resistor 27 and a capacitor 28. Operating potential for the electron beam accelerating electrode 14 is provided via the above described decou pling circuit and a voltage dropping resistor 29 which may be by-passed, by means not shown. The

deflector electrodes

15 and 16 obtain an operating voltage from a second source 30 of direct current operating potential +E via limiting

resistors

31 and 32. In order to intensity modulate an electron beam flowing from the cathode 12 to the

anodes

17 and 18, a source of radio frequency alternating voltage 33 of frequency f is provided and is coupled between input electrodes for the device comprising the control and

cathode electrodes

12 and 13. A source of modulating voltage 34 is also supplied and coupled between

deflector electrodes

15 and 16 for amplitude modulating a radio frequency output voltage which radio frequency voltage exists in the output load circuit.

The operation of the circuit as described may briefly be explained as follows. An electron stream is generated by the cathode 12, focused by the referred to focusing electrode (not shown), and is accelerated by the accelerating electrode 14 toward the

anodes

17 and 18. An alternating voltage which is coupled from the source of radio frequency voltage 33 between the cathode electrode 12 and control electrode 13, intensity modulates the electron stream at a frequency h. A modulating voltage provided by the source 34 deflects the intensity modulated electron stream between the

anodes

17 and 18 and causes an amplitude modulated radio frequency output voltage of frequency f to occur in the 1r filter load circuit. The amplitude modulated voltage is then coupled to a utility circuit, not shown.

FIGURE 2 illustrates desirable deflector electrode voltage versus anode electrode current

transfer characteristic curve

35 and 36 for

anodes

17 and 18, respectively, for use in the modulation circuit of FIGURE 1. It will be understood that the curves of FIGURE 2 are exemplary, as are the curves of FIGURE 3 described hereinafter, and are indicative of the general characteristics of a sheet beam deflection amplifying device without reference to a specific beam deflection device. From the curves of FIGURE 2, it can be seen that when a zero difference of potential exists between the

deflector electrodes

15 and 16, the electron beam current is divided and flows equally to the

anodes

17 and 18. When the modulating voltage drives the deflector electrode 16 positive with respect to the deflector electrode 15, the electron beam current flowing between cathode 12 and anode 17 represented by curve 35 decreases uniformly until it attains a value of zero amperes, indicated in FIGURE 2 as occurring at 3 volts. Similarly, when the deflector electrode 15 is driven to a positive potential with respect to the electrode 16, the electron beam current flowing between the cathode 12 and the anode electrode 18 will decrease uniformly to a value of zero amperes at 3 volts. The

curves

35 and 36 illustrate a desired linearity in the area of operation and desirable zero leakage current above specific deflector voltage drive levels.

However, presently manufactured beam deflection electron discharge amplifying devices do not exhibit the desirable characteristics of the idealized curves as shown in FIGURE 2, but rather exhibit characteristic curves as indicated in FIGURE 3. In FIGURE 3 it can be seen that the anode current deflector voltage curve 37 for anode 17, and the curve 38 for anode 18 include

linear segments

39 and 40, non-linear segments 41 and 42, and segments of relatively low

constant current

43 and 44. For the purpose of this application, the term non-linear leakage segment will be understood to mean a composite segment of curve 37 consisting of

segments

42 and 44 or a composite segment of curve 38 consisting of segments 41 and 43.

Segments

43 and 44 indicate the existence of leakage currents which cannot be eliminated by increased deflector voltage alone. A leakage current thus flows to one anode electrode when it is desire-d that the entire beam current flow to another anode electrode. In the circuit of FIGURE 1, leakage currents existing along the segment 44 of curve 38 in FIGURE 3, will cause a radio frequency voltage to occur in the output load circuit. As previously indicated, it is desired in some applications that this radio frequency output voltage be reduced to zero volts.

In accordance with one embodiment of the present invention, inductive circuit means are supplied for providing suppression of an alternating output voltage of frequency h which is generated in the output load circuit by a current i The current i of frequency f comprises the entire electron beam conduction current flowing between the cathode electrode 12 and the anode electrode 17. The inductive circuit suppression means includes an inductance comprising a load circuit coupled between the second anode 18 and cathode electrode 12 and coupled inductively with an inductance in the output load circuit for attenuating to Zero amplitude, an undesired alternating output voltage.

Referring once again to FIGURE 1 and an embodiment of the present invention illustrated therein, an inductance 45 is shown coupled between the second anode 18 and the cathode 12. The inductance 45 is shown coupled between the second anode 18 and the cathode 12. The inductance 45 is tuned to the frequency h by a capacitance 46 which may be a winding, stray, and associated plate capacitance of anode 18 of the device 11. When the modulating voltage at the deflector electrodes attains an amplitude at which value it is desired that the alternating output voltage be suppressed, a current i will be flowing between the cathode 12 and the anode 17. This current i may represent the current flowing at a preselected point along the non-linear leakage segment of the curve 37 of FIGURE 3, as for example :at point 47 or even in the linear region 39. A corresponding current i having a value indicated at point 48 on curve 38 will flow between the cathode 12 and anode 18 and in the inductance 45. The

inductances

19 and 45 .are arranged to be closely coupled and to have a coefficient of coupling k substantially equal to 1 and polarized to be phase bucking. Each inductance includes a number of turns N and N respectively for providing a turns ratio N defined as 1 N N2 for providing the following polar relation between the currents i and i kN1i1=N2i2/i18() which when k=1 reduces to the expression:

Ni =i /i'180 Thus, by inductive coupling, a current i flowing in the inductance 45 causes suppression in the output load circuit of an alternating output voltage. Since this suppression causes a zero amplitude alternating output voltage of frequency h to be established at point 47 on the curve in the example given, it can be seen that the amplitude of the alternating output voltage will increase from its zero value at this point to a maximum value at a saturation segment 49 of the curve with decreasing positive potentials at electrode 16 with respect to electrode 15.

Under some operating conditions, it may be deemed necessary or desirable to provide operation of the amplifying device so that a zero alternating output voltage of frequency f occurs when the device is operating in the linear region 39. For example, it may be desirable under certain conditions to reduce the output voltage to zero amplitude when the modulating voltage drives the deflector electrode 16 positive by 2.5 volts with respect to the electrode 15. From the curve of FIGURE 3, it can be seen that the point 50 corresponding to 2.5 volts of deflector drive represents an increased anode current i over the point 47 of the previous example and causes an increased alternating output voltage amplitude to exist over the voltage amplitude occurring at point 47. In addition, the current i at anode 18 has decreased to a value to the input modulating phase voltage.

12 and 13 to the anodes 17v and 18.

at a corresponding point 51 from point 48. .However, by arranging the referred to coupling between the

inductances

19 and 45 in accordance with the above defined relation such as by adjusting theturns ratio N, a zero alternating output voltage amplitude may be established at point 50.

It can be seen from the curves of FIGURE 3 that a radio frequency output voltage will occur when the input modulating voltage to the deflector electrodes increases so that i decreases in amplitude from its value at an established zero output voltage point of operation. As the deflector voltage increases beyond this point, the alternating radio frequency output voltage will increase in amplitude. The polarity of this radio frequency output voltage will be 180 reversed when operation of the device along curve 37 occurs at points to the left on the curve of the established point pf zero output voltage. Similarly, operation in this same region will provide 180 phase reversal of the modulation envelope with respect If desired, various means may be utilized to suppress an output voltage generated by this overdrive operation of the device.

'For example, the amplitude of the modulating voltage applied to the deflector electrodes maybe limited so as to limit deflector electrode voltage drive to the zero output point 50 or to other desiredpoints on the curve. Alternatively, a voltage which is generated by operation along .these segments may be blanked out either by means external to the modulation circuit or within the device itself. In systems wherein a blanking voltage is available such as in the light valve projection system described in the above referred .to patent, the point 50 may be selected as the black level for a television picture and a blanking voltage may be utilized ,to blank out the electron beam during excursion in the blacker than black region occurring along

segments

42 and 44.

As indicated above, interelectrode capacitance provides an undesirable coupling of an alternating current i of frequency f from input electrodes comprising electrodes In accordance with another feature of this invention, a neutralizing circuit is provided for suppressing a radio frequency voltage generated in the load circuit by the flow of, the alternating current i through interelectrode and stray capacitive coupling. In FIGURE 1, a neutralizing circuit comprising a series connected capacitor60 and a circuit including an inductance 61 and stray and winding capacitance 62 is provided and coupled to a source of alternating voltage of frequency f The inductance 61 is tightly coupled to the inductance 19. An alternating current i flows in the neutralizing series connected circuit and the inductances 19 and 61 are arranged to be closely coupled and to have a coeflicient of coupling k substantially equal to 1, and polarized to be phase bucking. Each of the inductances includes a number of turns N and N respectively for providing a turns ratio N defined as o N3 for providing the following polar relation between the current 1' and i kN1i =N i 3/i18() which when k=1 reduces to'the expression:

N i =i 180 Thus, an undesirable alternating voltage which is generated in the output load circuit as the result of interelectrode and stray capacitive coupling is suppressed.

FIGURE 4 illustrates another form of modulation circuit which includes an embodiment of the present invention. Cir-cuit components performing similar functions as similar elements in FIGURE 1 are represented by the same numerals. The modulating circuit of FIGURE 4 includes an arrangement as described in the above referred to co-pending application for providing self-sustained intensity modulation of the electron beam by an oscillator circuit intercoupled between the cathode electrode 12, control elect-rode 13 and accelerating electrode 14. The oscillator arrangement which replaces the source of radio frequency voltage 33 of FIGURE 1 includes a tapped inductance 63 having one terminal of the inductance coupled to the control electrode 13 via an RC grid bias arrangement including a resistor 64 and a capacitor 65 and an opposite terminal of the inductance to accelerating electrode 14 via a RF by-pass capacitor 66. The cathode 12 of the device 11 is connected to a tap 67 on the inductance 63. Frequency of oscillation is determined tralizing circuit comprising the capacitor 60, and the inductance 61 coupled between the cathode electrode 12 and accelerating electrode 14 for alternating currents and the winding of inductance 61 is phased with relation to the winding of inductance 19 in order to provide the de sired relation Although I have illustrated in one embodiment of my invention an inductive coupling means for coupling a voltage having an amplitude and phase suitable for buck- 'ing out a voltage generated in the load circuit by a leakage current, it will be apparent to those versed in the art that other circuit arrangements may be provided for generating and coupling into the output load circuit the desired voltage of required amplitude and phase.

While I have illustrated and described and have pointed out in the annexed claims certain novel features of my invention, it will be understood that various omissions,

substitutions and changes in the forms and details of the system illustrated may be made by those skilled in the art without departing from the spirit of the invention and the scope of the claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A modulation circuit comprising: (A) a beam deflection electron discharge amplifying d evice having a cathode electrode, first and second anode electrodes, an electron beam intensity control electrode, and first and second deflector electrodes;

(B) an output load circuit coupled between said first anode and cathode electrodes;

(C) means coupled to said control electrode for providing alternating intensity modulation at a frequency 7; of an electron beam generated by said device;

(D) means for providing and coupling a modulating voltage between said first and second deflector electrodes;

(E) means providing direct current operating potentials for said device;

(F) said amplifying device having a current i and a corresponding current i flowing therein; said current i comprising the total electron beam current flowing between the cathode and first anode elect-rode and said current i comprising the total electron beam currentflowing between the cathode and the second anode electrode;

. (G) said current. i generating an alternating output voltage in said output load circuit; and

(H) means connected between said second anode and cathode electrodes responsive to said current i for automatically suppressing the alternating output voltage when said current i has a preselected amplitude smaller than the amplitude of said corresponding current i 2. A modulation circuit comprising:

(A) a beam deflection electron discharge amplifying device having a cathode electrode, first and second anode electrodes, an electron beam intensity control electrode, and first and second deflector electrodes;

(B) an output load circuit coupled-between said first anode and cathode electrodes;

(C) a second load circuit coupled between said second anode and said cathode electrodes;

(D) means for providing alternating intensity modulation at a frequency f of an electron beam generated by said device;

(E) means for providing and coupling a modulating voltage between said first and second deflector elec trodes;

(F) means providing direct current operating potentials for said device;

(G) said amplifying device having an electron beam conduction current i flowing between said cathode and first anode electrodes and a corresponding electron beam conduction current i flowing between said cathode and second anode electrode;

(H) said current i generating an alternating output voltage in said output load circuit;

(I) said second load circuit including means responsive to said current i intercoupling said second and said output load circuits for suppressing said output voltage at a preselected amplitude of current i smaller than the amplitude of said corresponding current i 3. The apparatus of claim 2 wherein said means intercoupling said second and said output load circuits comprises inductive coupling means.

4. The modulation circuit of claim 2 wherein said means for intensity modulating said electron beam in said device comprises a positive feedback network coupled between said accelerator, cathode, and control electrodes for providing self-sustained intensity modulation of the electron beam.

5. A modulation circuit comprising:

(A) a beam deflection electron discharge amplifying device having a cathode elect-rode, first and second anode electrodes, an electron beam intensity control electrode, and first and second deflector electrodes;

(B) an output load circuit including a first inductance coupled between said first anode and cathode electrodes;

(C) a second load circuit including a second inductance coupled between said second anode and said cathode electrodes;

(D) means for providing alternating intensity modulation at a frequency of an electron beam generated by said device;

(E) means for providing and coupling a modulating voltage between said first and second deflector electrodes;

(F) means providing direct current operating potentials for said device; V k

(G) said amplifying device having an electron beam conduction current i and an'electron beam conduction current i flowing between said cathode and first and second anodes respectively;

(H) said current i generating an alternating output voltage in said output load circuit;-

(I) said first and second inductances having windings positioned relative to each other for providing close inductive coupling between said inductances, said windings polarized with respect to each other and having a turns ratio N for providing the relation (I) the inductive coupling between said inductance and said turns ratio N being chosen to suppress said alternating output voltage in said output load circuit when said current i has a preselected amplitude smaller than the amplitude of said current i 6. A modulation circuit comprising:

(A) a beam deflection electron discharge amplifying device having input electrodes including a cathode and a beamintensity control electrode; first and second anode interelectrodes and first and second deflector electrodes;

(F) means providing direct current operating potentials for said device;

(G) said amplifying device having electron beam conduction current i and an electron beam conduction current i flowing between said cathode and first and second anodes respectively;

(I) said first and second inductances having windings positioned relative to each other for providing close inductive coupling between said inductances, said windings polarized with respect to each other and having a turns ratio N for providing the relation (I) said device having an output voltage generated in said output load circuit by a current i flowing through undesired capacitance coupling from an input electrode to said first anode electrode; and

(K) neutralizing circuit means for suppressing said output voltage generated by said current flowing through interelectrode capacitance.

7. The modulation circuit of claim 6 wherein:

(A) said means for providing alternating intensity modulation at a frequency f of an electron beam generated by the device includes a source of alternating voltage of frequency h;

(B) said neutralizing circuit means comprising a series connected capacitance and a third inductance having a winding and means coupling said source of alternating voltage of frequency f to said series connected circuit;

(C) said third inductance having a current i flowing therein;

(D) said first and third inductances positioned relative to each other for providing close inductive coupling therebetween; the windings of said first and third inductances having a turns ratio N and polarized with respect to each other for providing the relation N i =i /i180 8. The circuit as defined in'claim 5 wherein said current i has-an amplitude defined by theleakage current to said first anode.

References Cited by the Examiner UNITED .STATES PATENTS ROY LAKE, Primary Examiner.

Claims

1. A MODULATION CIRCUIT COMPRISING; (A) A BEAM DEFLECTION ELECTRON DISCHARGE AMPLIFYING DEVICE HAVING A CATHODE ELECTRODE, FIRST AND SECOND ANODE ELECTRODES, AN ELECTRON BEAM INTENSITY CONTROL ELECTRODE, AND FIRST AND SECOND DEFLECTOR ELECTRODES; (B) AN OUTPUT LOAD CIRCUIT COUPLED BETWEEN SAID FIRST ANODE AND CATHODE ELECTRODES; (C) MEANS COUPLED TO SAID CONTROL ELECTRODE FOR PROVIDING ALTERNATING INTENSITY MODDULATIN AT A FREQUENCY F1 OF AN ELECTRON BEAM GENERATED BY SAID DEVICE; (D) MEANS FOR PROVIDING AND COUPLING A MODULATING VOLTAGE BETWEEN SAID FIRST AND SECOND DEFLECTOR ELECTRODES; (E) MEANS PROVIDING DIRECT CURRENT OPERATING POTENTIALS FOR SAID DEVICE; (F) SAID AMPLIFYING DEVICE HAVING A CURRENT I1 AND A CORRESPONDING CURRENT I2 FLOWING THEREIN; SAID CURRENT I1 COMPRISING THE TOTAL ELECTRON BEAM CURRENT FLOWING BETWEEN THE CATHODE AND FIRST ANODE ELECTRODE AND SAID CURRENT I2 COMPRISING THE TOTAL ELECTRON BEAM CURRENT FLOWING BETWEEN THE CATHODE AND THE SECOND ANODE ELECTRODE; (G) SAID CURRENT I1 GENERATING AN ALTERNATING OUTPUT VOLTAGE IN SAID OUTPUT LOAD CIRCUIT; AND (H) MEANS CONNECTED BETWEEN SAID SECOND ANODE AND CATHODE ELECTRODES RESPONSIVE TO SAID CURRENT I2 FOR AUTOMATICALLY SUPPRESSING THE ALTERNATING OUTPUT VOLTAGE WHEN SAID CURRENT I1 HAS A PRESELECTED AMPLITUDE SMALLER THAN THE AMPLITUDE OF SAID CORRESPONDING CURRENT I2.