JPS6162280A - Removal circuit of fixed pattern noise - Google Patents

Abstract

PURPOSE:To obtain a video signal with an excellent S/N ratio by connecting a capacitor to a voltage/current convertion circuit output and a switch in parallel with a capacitor and further connecting the input of the capacitor to that of a peak detector circuit and opening and closing the switch synchronous to the horizontal drive signal of an image pickup element. CONSTITUTION:The output of a MOS solid-state image pickup element 1 is amplified by a video amplifier circuit 2, and an NPN transistor S is converted into a current by a voltage/current converter circuit constituted with a resistor 4 and charged to a capacitor 7 by a current mirror circuit constituted with a diode 5 and a PNP transistor 6. On the other hand, a gate pulse is impressed to the gate pulse input terminal 9 on an NPN transistor 8, and the capacitor 7 is initialized. Even if the amplitude of a spike noise is different, a charge amount charged for one period of the gate pulse is constant, and the peak voltage of the capacitor 7 becomes always constant. The voltage of the capacitor 7 is peak-detected by a peak detector circuit constituted with NPN transistors 10 and 11.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a television camera using an MO3 type image sensor.

[Conventional technology]

Television cameras using solid-state image sensors are roughly divided into MOS type and CCD type depending on the image sensor.The oMO5 type solid-state image sensor has a wider dynamic range than the CCD type. The problem with this is that it produces fixed pattern noise that looks like streaks in the vertical direction of the TV screen. Moreover, this fixed pattern noise remains fixed on the screen even when the Sf television camera is moved, and is very noticeable.

In Fig. 2, which shows the basic configuration of an MO3O3 type image sensor, 18 is a vertical shift register, 19 is a horizontal shift register, 20 is a photodiode, and 21 is a vertical MO
The S switching transistor 22 is a horizontal MOS switching transistor. The basic operation is as follows.

When the top row of vertical shift register 18 is selected,
The -th row horizontal MOS switching transistor 21 becomes conductive, and the signal from the photodiode is read out to each vertical line. When the horizontal shift register 19 receives a signal from the left side, the horizontal MOS switching transistor 22 becomes conductive one after another. , becomes non-conductive, and the signal of each vertical line is read out via the horizontal MOS switching transistor 22. Further, by shifting the signal of the vertical shift register 18 downward and reading it out in the horizontal direction as described above, video signals can be sequentially obtained.

FIG. 6 shows an equivalent circuit around the horizontal MOS switching transistor of the MO3 solid-state image sensor.

In the figures, the same numbers as in FIG. 2 indicate the same parts and perform the same operations. 23 is the capacitance of the vertical line, 24 is the capacitance of the horizontal line, and 25 is the gate-drain capacitance of the horizontal MOS switching transistor. When the horizontal drive pulse shown in FIG. 4(α) is applied to the gate of the horizontal MOS switching transistor 25, the output signal shown in FIG. 4(b) is obtained. However, the horizontal MOS switching transistor 22
If there are variations in the capacitance of the gate drain capacitance ff12s, variations in the amplitude of the gate pulse, etc., an output signal with unbalanced amplitude etc. as shown in FIG. 4(1) is generated.

This is the cause of the fixed pattern, which cannot be removed by a reduction filter.

FIG. 5 shows a conventional fixed pattern noise removal circuit. 1 is an MO3 type solid-state image sensor, 2 is a video amplification circuit,
3 is an N P N ) transistor, 4 is a resistor, 7 is a capacitor, 28 is a switch, 29 is a resistor, and 30 is a power supply. The operation will be explained using FIG. The output of the MOS type solid-state image sensor 1 is amplified by a video amplifier 2. A video signal is shown in FIG. 6(cL). The switch 28 is the sixth
It is opened and closed according to the gate pulse synchronized with the horizontal drive pulse shown in FIG. 6. When the switch 28 is closed, the capacitor 7 is charged to the voltage of the power supply 30 via the resistor 29. When the switch 28 is opened, the capacitor 7 is charged to the voltage of the power supply 30.
N) A voltage-current conversion path consisting of a transistor 3 and a resistor 4 causes writing and discharging with a current according to the video signal voltage. Figure 6(d) shows the video signal output of the resistor 29.As can be seen from the example in Figure 6, the peak of the spike noise changes between positive and negative during one period of the gate pulse (&). If the values of are the same, the discharge current will only increase or decrease during that one cycle, and the amount of discharge charge will be the same.Therefore, the video signal output of the resistor 29 will be uniform as shown in FIG. 6(d). Therefore, fixed pattern noise can be removed.

[Problem that the invention seeks to solve]

However, as can be seen from FIG. 6, in the prior art described above,
First of all, the gate pulse (6) must be in a stable region free from the influence of spike noise in the video signal (,). Second, the width Tc of the gate pulse in (b) is such that the width Tc of the gate pulse in FIG. It has to be set several times the determined time constant. Therefore, the permissible value of the gate pulse in the left and right direction in (b) is TB in the figure. For example, if the gate pulse frequency is about 8MHz, TB is about 10~2Q
? It becomes L8ec, which is a small allowable value. In addition, as can be seen from the video signal output in (d), in addition to the actual video signal, it contains spike noise synchronized with the gate pulse, and the S/N ratio of the signal is not good. have

Therefore, the present invention is intended to solve these problems, and its purpose is to provide an MO3 type solid-state that has sufficient tolerance in the horizontal direction of the gate pulse and that can obtain a video signal with a good S/N tt. The present invention provides a fixed pattern noise removal circuit for an image sensor. Connect the output of the video amplification circuit connected to the voltage-current conversion circuit to the voltage-current conversion circuit, connect a capacitor to the output of the voltage-current conversion circuit,
The present invention is characterized in that a switch is connected in parallel with the capacitor, the capacitor is connected to the input of the peak detection circuit, and the switch is opened and closed in synchronization with the horizontal drive signal of the image sensor.

[For production]

According to the above configuration of the present invention, fixed pattern noise is removed by the integral effect of the capacitor and the voltage-current conversion circuit. Further, by providing a switch in parallel with the capacitor, the capacitor can be initialized in a short time, and the permissible value of the gate pulse in the left and right direction can be increased. A peak detection circuit reduces spike noise synchronized with the gate pulse to obtain a video signal with a good S/N t t.

〔Example〕

FIG. 1 shows an embodiment of a fixed pattern noise removal circuit for an MO3i solid-state image sensor according to the present invention.
1 is an MO5 type solid-state image sensor, 2 is a video amplification circuit, 3,
8, '10, 15 are NPN) transistors, 4, 12
, 13.14 is a resistor, 5 is a diode, 6 is a PNP
) transistors, 7 and 11 are capacitors, 9 is a gate pulse input terminal, 16 is a video signal output terminal, and 17 is a power supply.

The operation will be explained using FIG. 7. The output of the MO3 type solid-state image sensor 1 is amplified by a video amplification circuit 2.

The amplified video signal is shown in FIG. 7 (α). This video signal is converted into a current by a voltage-current conversion circuit comprising an NPN transistor 3 and a resistor 4, and is charged into a capacitor 7 by a current mirror circuit comprising a diode 5 and a PNP transistor. On the other hand, NP
N) The gate pulse input terminal 9 of the transistor 8 has a seventh
The gate pulse shown in figure (b) is applied, and the capacitor 7
Initialize. The voltage across the capacitor 7 is shown in FIG. 7(1). As can be seen from the figure, even if the amplitude of one spike noise is different, the amount of charge charged during one period of the gate pulse is constant, and the peak voltage of capacitor 7 is always constant 0. 7 voltage is NPN)
Run rasters 10 & 2) Peak detection is performed by a peak detection circuit composed of a capacitor 11. Resistor 12, 13, 14
The function of the constant current source composed of transistors 15 (NPN and NPN) is to prevent distortion due to the lack of a discharge route for capacitor 1 when the video signal output becomes small.
A certain amount of charge is discharged from the capacitor 11.

The peak-detected waveform of the video signal output terminal 16 is shown in FIG. 7(d). As can be seen from FIG. 7, even if the amplitude of the spike nozzle synchronized with the gate pulse is different, the peak voltage of the capacitor 7 remains constant. Fixed pattern noise is something that can be removed.

〔Effect of the invention〕

As described above, according to the present invention, since the initialization of the integrating capacitor 7 shown in FIG. )Ran resistor 8
Since the ON resistance of is as small as several ohms or less, the gate pulse width Tc in FIG. 7 can be made sufficiently small. Therefore, the time TII, which is the period TA minus the gate pulse width Tc when there is no influence of spike noise, can have a large tolerance value in the horizontal direction of the gate pulse, which increases the tolerance value for parts errors, etc., and improves the mass production effect. goes up.

Furthermore, as can be seen from the video signal output in Figure 7(d), the spike noise synchronized with the gate pulse, which was suppressed in the past, has been significantly reduced, making it possible to obtain a video signal with a good s/N of 11. 0 that can be obtained

[Brief explanation of the drawing]

FIG. 1 is a fixed pattern noise removal circuit diagram of an MO3 type solid-state image sensor according to the present invention. Figure 2 is a basic configuration diagram of an MO3 type solid-state image sensor. FIG. 3 is an equivalent circuit diagram of an MO8 type solid-state image sensor. FIGS. 4(α) to (c) are diagrams showing the operation of the solid-state image sensor. FIG. 5 is a fixed pattern noise removal circuit diagram of an MO3 type image sensor according to the prior art. Figures 6(α) to (d) are diagrams showing the operation of the fixed pattern noise removal circuit according to the prior art. Figures 7(cL) to (d) are diagrams showing the operation of the fixed pattern noise removal circuit according to the present invention◇ 1...MO9O9 type image sensor 2...Video amplifier circuit 18...Vertical shift register 19...Horizontal shift register 20...Photodiode Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] (1) In a television camera using a MOS solid-state image sensor, the output of a video amplification circuit connected to the image sensor is connected to a voltage-current conversion circuit, a capacitor is connected to the output of the voltage-current conversion circuit, and the capacitor Connect a switch in parallel with the capacitor and the input of the peak detection circuit,
A fixed pattern noise removal circuit characterized in that the switch is opened and closed in synchronization with a horizontal drive signal of the image sensor.