JPH0822024B2 - Method and apparatus for controlling electron beam amount of image pickup tube - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a scanning electron beam amount control method and apparatus for a low afterimage type image pickup tube equipped with a bipolar electron gun.

[Background of the Invention]

In the photoconductive type image pickup tube, a charge pattern corresponding to the illumination of the object is generated on the photoconductive film, and the photoconductive film is scanned by an electron beam generated by an electron gun to sequentially discharge the charge pattern, The charging current corresponding to the discharge is taken out as a signal. In the photoconductive type image pickup tube, it has been conventionally required to reduce the afterimage and to expand the light amount range.

The afterimage is a phenomenon that occurs because the electric charge generated by the subject cannot be completely discharged by one beam scan.
This occurs because the charges left unread in the previous scan are also read out in the subsequent scans. Particularly in an image pickup tube using a blocking type photoconductive film, a capacitive afterimage having a time constant determined by a product of a capacitance of the photoconductive film and a beam resistance of a scanning electron beam is mainly used.

The beam resistance is required to have a narrow velocity distribution of the electron group forming the electron beam. The electron group emitted from the cathode has a Maxwellian velocity distribution, but the current density of the beam rises in the process of forming a narrow beam, and the velocity distribution is expanded by the energy relaxation phenomenon due to the Coulomba of the electrons. It is known that This phenomenon is called the Beige effect, and it is known that the expansion rate of the velocity distribution is approximately proportional to J (Z) ^1/3 with respect to the axial current density J (Z) of the beam. Therefore, in an image pickup tube intended for low miscellaneous images, it is necessary to suppress the increase in beam current density as much as possible. Therefore, the first grid electrode facing the cathode is operated at a positive voltage with respect to the cathode, and electrons are emitted from the cathode in parallel with the tube axis to generate a laminar flow beam having a high current density and not forming a crossover 2. A polar electron gun has been proposed (for example, Japanese Patent Laid-Open Nos. 50-39869 and 54-129).
871).

FIG. 6 shows an example of a bipolar electron gun satisfying the above-mentioned object, 1 is a cathode, 2 is a first grid electrode, and a positive voltage E ₁ is applied to the cathode 1. Reference numeral 3 denotes a second grid electrode, which has a minute opening 33.
And a positive voltage E ₂ is applied. Reference numeral 4 denotes an electron beam generated, but when the applied voltage E ₁ is made variable,
The electron beam 4 changes from a laminar flow beam shown by a broken line to a concentrated beam (a beam forming a crossover) shown by a solid line. FIG. 7 is a diagram showing the relationship between the first lattice voltage E ₁ and the beam current reaching the photoelectric conversion surface. Point A is a laminar flow beam state where afterimages can be reduced at a normal operating point, and point B is a state where a crossover is formed. As described above, the image pickup tube equipped with the electron gun shown in FIG. 6 can obtain a large beam current by changing the first grid voltage E _1. Therefore, the ABO that controls the beam amount according to the illuminance of the subject is used.
The operation (Automatic Beam Optimizer) can be performed.

A conventional ABO circuit is shown in FIG. In the figure, 10 is an image pickup tube equipped with a bipolar electron gun as shown in FIG. In the above conventional method, the signal current extracted from the photoelectric conversion surface 11 by electron beam scanning is converted into a voltage by the resistor 12, amplified by the amplifier 13, and then superimposed on the reference bias voltage 15 by the adder circuit 14 to control the beam current control electrode. It is added to the (first grid electrode) 2. The following equations are established in the above circuit.

vs = -Is · R (1) v _O = B · vs (2) E _G1 = v _O + V _O1 (3) However, vs: input signal voltage of the amplifier 13, Is: signal current, R:
Resistance, v _O : output signal voltage of amplifier 13, B: amplification factor of amplifier, E _G1 : beam current control electrode voltage, V _O1 : reference bias voltage.

Where the ratio of the beam current control electrode voltage to the beam current is
With gm ₁ , the following equations hold.

I _B = −g _m1 · E _G1 (4) I _O1 = −g _m1 · V _O1 (5) where I _{B is the} scanning beam current and I _{O1 is the} scanning beam current bias value.

When I _B is obtained from the above five equations, the following equation (6) is obtained.

I _B = A · I _S + I _O1 (6) However, A = g _m1 · R · B. Equation (6) above is the eighth
The control characteristics of the scanning beam current of the circuit shown in the figure are shown.
When = 1, ideal ABO operation is possible. But,
The above circuit has two problems. The first problem is the camera tube 10
When the capacitive coupling as shown by 17 exists between the first grid electrode 2 and the photoelectric conversion surface 11, the positive feedback circuit as shown in FIG. 9 is constructed and oscillates. In the above, the polarity (negative polarity) of the signal voltage applied to the first grid electrode 2 is the photoelectric conversion surface.
This is because the signal charges generated in 11 have the same polarity (negative polarity), and thus the signals mixed by capacitive coupling are added in the same polarity on the photoelectric conversion surface 11. The second problem is the eighth
If the beam control circuit shown in the figure enters a beam shortage state during operation, it may cause oscillation depending on circuit conditions. When the beam becomes insufficient, the signal current becomes irrelevant to the light information and increases in proportion to the beam current, and the circuit of FIG. 8 becomes a positive feedback circuit. If the proportional constant of the signal current and the beam current is F, then I _S = F · I _B (7) and Eq. (8) is established from Eqs. (6) and (7).

The oscillation condition of the feedback circuit is determined by the denominator of equation (8), and if AF <1, it does not oscillate. Although F varies depending on the characteristics and state of the photoelectric conversion film 11, it is a value almost close to 1. Therefore, in order to suppress oscillation, it is necessary to satisfy the condition of A <1 or devise not to enter the beam shortage state. is there.

The condition of A <1 can be realized by lowering the amplification factor B of the amplifier 13. However, since the value of g _m generally changes depending on the voltage of the first grid electrode 2, the value of B is set to the maximum value of g _m. You have to set it. As a result, the amplification factor becomes insufficient at the operating point where g _m becomes the minimum value, and the control range becomes small.

[Object of the Invention]

The present invention relates to a method of controlling the beam amount of an image pickup tube which does not oscillate even when capacitive coupling exists between the first grid electrode and the photoelectric conversion surface and when a beam shortage occurs during ABO operation. And control device.

[Outline of Invention]

A method and apparatus for controlling an electron beam amount of an image pickup tube according to the present invention include a cathode that emits electrons, a first grid electrode having an opening that is arranged in a subsequent stage of the cathode, and a minute grid arranged in a subsequent stage of the first grid electrode. In an electron beam amount control method and apparatus for an image pickup tube having a second grid electrode having an opening and a photoconductive target, the first grid electrode is operated at a positive voltage with respect to a cathode, By applying a voltage signal corresponding to the return beam to the second grid electrode of the image pickup tube, a stable ABO operation is performed and the scanning electron beam amount of the image pickup tube is controlled.

Example of Invention

Next, embodiments of the present invention will be described with reference to the drawings. First
FIG. 1 shows a first embodiment of an electron beam amount controller for an image pickup tube according to the present invention.
FIG. 2 is a circuit diagram showing an embodiment, FIG. 2 is a circuit diagram showing a second embodiment of the present invention, FIG. 3 is a diagram showing a relationship between a second grid electrode voltage and beam current, and FIG. FIG. 5 is a circuit diagram showing a third embodiment, and FIG. 5 is a circuit diagram showing a fourth embodiment of the present invention.
In FIG. 1, most of the beam current extracted from the cathode 1 of the image pickup tube 10 is absorbed by the first grid electrode 2 and the second grid electrode 3, but a part thereof becomes the scanning beam current I _B. On the photoelectric conversion surface 11, a part of the scanning beam current I _B becomes a signal current, but the remaining electrons become a return beam current I _R and are absorbed by the second lattice electrode 3. Therefore, the return beam current I _R is expressed by equation (9).

I _R = I _B −I _S (9) Since the actual circuit processes with the voltage signal, the voltage signal v _R corresponding to the return beam current I _R is as shown in equation (10).

v _R = v _B −v _S (10) However, v _B is a voltage signal corresponding to the scanning beam current, which is detected by the resistor 20 inserted between the cathode 1 and the ground. Since the cathode current I _C flows through the resistor 20 and the formula (11) is established between I _C and I _B , v _B can be expressed by the formula (12).

I _C = α ・ I _B・ ・ ・ (11) v _B = R _C・ α ・ I _B・ ・ ・ (12) where α is the proportional constant and R _C is the value of the resistor 20 inserted in the cathode 1.

On the other hand, since the signal current I _S flows through the resistor 12, the above (1)
A signal voltage represented by the formula is generated. Therefore, (1),
When v _R is obtained from the equations (10) and (12), the signal shown in the following equation (13) is given.

v _R = α · R _C .I _B −R · I _S (13) The signal of the above formula (13) is easily obtained by adding the voltage signals generated in the resistors 12 and 20 by the adding circuit 21. be able to.

Since the signal of the difference between v _R and the reference voltage 22 shown in (13) is amplified by the differential amplifier 23 and added to the second grid electrode 3, the following expression (14) is established.

E _G2 = B (V ₀₂ −v _R ) ... (14) Here, B is the amplification factor of the differential amplifier, and V ₀₂ is the value of the reference voltage 22.

Further, when the ratio of the cathode current I _C to the voltage of the second grid electrode is g _mc , the equation (15) is established.

Here, the reference voltage V ₀₂ is considered as a current source for convenience, and V ₀₂ =
If I ₀₂ · R ₀ is given, the following equation (16) can be obtained from equations (13), (14) and (15).

Therefore, by selecting R that satisfies R = R ₀ = R _S = α · R _C , the above equation (16) can be rewritten as the equation (17).

However, A ′ = g _mc · B · R · α ⁻¹ . As shown in the equation (17), in the first embodiment shown in FIG. 1 of the present invention, the larger the above A ′, the more ideal the ABO operation can be performed.

In addition, even if the beam shortage occurs during ABO operation and the relation of equation (7) is established between I _S and I _B , and the feedback circuit is configured, oscillation occurs as shown in the following equation (18). The condition does not hold.

Therefore, stable beam control can be performed even if a beam shortage occurs.

Even if capacitive coupling exists between the second grid electrode 3 and the photoelectric conversion surface 11, the polarity of the control signal voltage applied to the second grid electrode 3 becomes positive and the negative electrode generated on the photoelectric conversion surface 11 is generated. Since the polarity is opposite to that of the positive signal charge, the positive feedback circuit is not configured and oscillation does not occur.

In the second embodiment shown in FIG. 2, as in the first embodiment, a voltage signal corresponding to the return beam current is applied to the second grid electrode 3 and 1 / of the control signal applied to the second grid electrode 3 is applied. The characteristic is that the voltage signal of n is generated by the differential amplifier 25 and applied to the first grid electrode 2. 24 is a reference voltage.

As described above, the ABO circuit in FIG. 1 increases the beam current by changing the laminar flow beam state shown by the broken line in FIG. 6 to the concentrated beam state shown by the solid line. On the other hand, in the second embodiment of FIG. 2, a positive control voltage is applied to the second grid electrode 3, and the first grid electrode 2 also has a positive polarity of about 1/10 of the second grid electrode voltage. By applying a voltage, the change to the concentrated beam state is suppressed and the ABO operation is always performed in the laminar beam state. This 2
In the electrode control method, the voltage of the first grid electrode 2 is controlled depending on the voltage of the second grid electrode 2.
The grid electrode 3 can be considered as one electrode, and the relationship between the voltage of the second grid electrode and the beam current is the straight line shown in FIG. As a result, the ratio of the cathode current to the voltage of the second grid electrode g _mc
Can be treated as an almost perfect constant, and more stable beam operation can be performed.

A third embodiment shown in FIG. 4 is a modification of the circuit of the first embodiment, in which a signal corresponding to the return beam current is amplified by an amplifier 26 and then a bias power supply 27 using a DC clamp circuit 28.
ABO operation is the same as that in the first embodiment.

The fourth embodiment shown in FIG. 5 is a modification of the second embodiment described above, in which the first grid electrode 2 and the second grid electrode 3 are controlled simultaneously. A signal corresponding to the return beam current is amplified by the amplifier 26 and applied to the second grid electrode 3 by being superposed on the bias power supply 27 by the DC clamp circuit 28. In addition, the first grid electrode 2 has a second
A signal of 1 / n of the control signal applied to the grid electrode 3 is produced by the amplifier 29, and is superimposed and applied to the bias power supply 31 for the first grid electrode by the DC clamp circuit 30. The circuit of the fourth embodiment is also an ABO
The operation is similar to that of the second embodiment shown in FIG.

〔The invention's effect〕

As described above, the method and apparatus for controlling the electron beam amount of the image pickup tube according to the present invention include a cathode that emits electrons, a first grid electrode having an opening arranged at a stage subsequent to the cathode, and the first grid electrode.
In a method and an apparatus for controlling an electron beam amount of an image pickup tube having a second grid electrode having a minute aperture arranged in a subsequent stage of the grid electrode and a photoconductive target, the first grid electrode is operated at a positive voltage with respect to a cathode. By controlling the amount of scanning electron beam by the signal corresponding to the return beam from the photoconductive target, capacitive coupling exists between the first grid electrode and the photoelectric conversion surface, and a beam shortage state occurs during ABO operation. Even if it rushes into, without causing oscillation,
It is possible to stably control the electron beam amount of the image pickup tube.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of an electron beam amount control device for an image pickup tube according to the present invention, FIG. 2 is a circuit diagram showing a second embodiment of the present invention, and FIG. 3 is a second grid electrode voltage. FIG. 4 is a circuit diagram showing a third embodiment of the present invention, FIG. 5 is a circuit diagram showing a fourth embodiment of the present invention, and FIG. 6 is a bipolar electron gun. 7 is a diagram showing the relationship between the first grid electrode voltage and the beam current, FIG. 8 is a conventional electron beam amount control circuit diagram, and FIG. 9 is a diagram when capacitance coupling occurs. It is an equivalent circuit diagram. 1 ... Cathode, 2 ... First grid electrode 3 ... Second grid electrode 11 ... Photoconductive target (photoelectric conversion surface) 21 ... Addition circuit, 23, 25 ... Differential amplifier 26, 29 ... Amplifier 28, 31 …… DC clamp circuit

Front page continuation (72) Inventor Toru Mimura 1-280, Higashi Koigakubo, Kokubunji, Tokyo Inside Central Research Laboratory, Hitachi, Ltd. (72) Inventor, Shusaku Nagahara 1-280, Higashi Koigakubo, Kokubunji, Tokyo Hitachi Central Research Co., Ltd. In-house (56) References Japanese Patent Publication Sho 52-36648 (JP, B1)

Claims (2)

[Claims] 1. A method for controlling an electron beam amount of an image pickup tube for controlling an amount of electrons emitted from a cathode of the image pickup tube toward a photoconductive target, comprising: A positive polarity voltage signal based on the return beam from the photoconductive target is applied to a second lattice electrode having a minute aperture arranged at a stage subsequent to the first lattice electrode, and a positive polarity voltage applied to the first lattice electrode is applied to the second lattice electrode. An electron beam amount control method for an image pickup tube, comprising controlling the amount of scanning electron beams for scanning the photoconductive target by making the positive electrode positive voltage applied to the lattice electrode smaller. 2. A cathode that emits electrons, a first grid electrode having an opening arranged in a stage subsequent to the cathode, and a second opening having a minute opening arranged in a stage subsequent to the first grid electrode.
The first grid electrode and the second grid based on a grid electrode, a photoconductive target that the electrons reach through the second grid electrode, and an output signal of the photoconductive target and a signal input to the cathode. And a means for controlling the voltage applied to the first grid electrode to be smaller than the second grid electrode signal as a positive electrode, and controlling the scanning electron beam amount of the image pickup tube. Tube electron beam control device.