JPH05244514A - Ccd image pickup element - Google Patents

Abstract

PURPOSE:To suppress fluctuation of an output DC level caused due to a change in a substrate voltage of the CCD image pickup element provided with an electronic shutter function and a sensor section of longitudinal overflow structure to the element. CONSTITUTION:In the CCD image pickup element whose output section 1 is provided with a signal charge amplifier section 4 which is provided with an electronic shutter function in which an electronic shutter pulse is applied to its substrate to discharge the charge of a sensor section to the substrate, a voltage Vgg obtained by dividing a substrate voltage Vsub by resistors R1, R2 is applied to gates of load MOS transistors(TRs) Q2, Q4 of the signal charge amplifier section 4 and the voltage Vgg is varied synchronously with a change in the substrate voltage Vsub.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CCD image pickup device having an electronic shutter function.

[0002]

2. Description of the Related Art Generally, a CCD image pickup device has an output unit 1
In FIG. 4, the signal charge amplification section 4 composed of a source follower as shown in FIG. 4 is built in. That is, in the figure, 6 is a final charge transfer unit of the horizontal shift register of the CCD image pickup device, and 5 is a floating diffusion for charge-voltage conversion, which is connected to the subsequent stage of this final charge transfer unit via a read gate unit. Indicates a part. The signal charge amplifying unit 4 includes a first source follower circuit 2 including a driving MOS transistor Q ₁ and a load MOS transistor Q ₂ .
The first source follower circuit 2 of the first stage is constituted by connecting in cascade a second source follower circuit 3 including a driving MOS transistor Q ₃ and a load MOS transistor Q _4.
The floating diffusion portion 5 is connected to the gate of the driving MOS transistor Q ₁ .

In the signal charge amplifying section 4, the voltage Vdd is applied to the sources of the driving MOS transistors Q ₁ and Q ₃ ,
The sources of the load MOS transistors Q ₂ and Q ₄ are resistors R
_Grounded via _SS . Constant voltage V _gg is applied to the gate of the load MOS transistors Q _2, Q _4, load MOS transistors Q _2, Q ₄ is operated as a current source.

In the CCD image pickup device, the signal charge from each sensor unit is finally transferred to the horizontal shift register for each horizontal line, and the signal from this horizontal shift register (that is, the charge transfer unit 6 at the final stage) is transferred. Electric charges are accumulated in the floating diffusion 5. The voltage change based on the accumulated charge in the floating diffusion portion 5 is supplied to the signal charge amplifying portion 4 constituted by the source follower, and is output as the output signal V _out from the output terminal t ₁ .

The MOS transistor Q _{1 of the} output section ₁
Q ₄ is required to operate stably against any disturbance. This is because when the operation of the MOS transistors Q _{1 to} Q ₄ of the output section 1 changes, the current I _DS flowing from the drain side of the voltage V _dd to the source side of the voltage V _ss changes, and the MOS transistors Q _{1 to} Q ₄ change. This is because the operating point of will change.

The fact that the operating points of the MOS transistors Q _{1 to} Q ₄ change means that the operation of the signal charge amplifying section 4 changes, that is, the output signal changes. Particularly, when such a change in the output signal occurs at the time of outputting the effective video signal, it is expected that the change will appear on the screen, which is a problem in the operation of the CCD image pickup device.

On the other hand, recently, a CCD image pickup device having a so-called vertical overflow structure sensor section in which a substrate serves as an overflow drain has become mainstream. In order to select an arbitrary exposure time, such a CCD image pickup device is generally provided with an electronic shutter function for discharging the signal charges accumulated in the sensor section toward the substrate.

In order to discharge the signal charges of the sensor portion to the substrate side, a voltage higher than the normal substrate voltage must be applied to the substrate portion. Such a change in the substrate voltage naturally affects the signal charge amplification unit 4 of the output unit 1 formed in the same substrate.

Generally, a change in substrate voltage causes a substrate bias effect (so-called back gate effect), which affects the operation of a MOS transistor formed in the substrate. Moreover, the MOS transistors formed in the signal charge amplifying section 4 have different internal structures due to the difference in their roles on the drive side and the load side, and the threshold voltage V _th is also different. That is, the substrate bias effect is the driving MOS transistor Q.
₁ and Q ₃ and load MOS transistors Q ₂ and Q ₄ do not work in the same way. As a result, the operating point changes in each MOS transistor, and the output DC (direct current component) value changes.

In order to avoid the above problem, conventionally, the substrate voltage is changed during the horizontal line retrace period during which the effective video signal period is not affected. That is, when the electronic shutter operation is performed, there is no problem even if the output DC value fluctuates by adding the electronic shutter pulse to the horizontal blanking period.

[0011]

However, even if the period during which the substrate voltage is changed is the horizontal retrace line period, the fluctuation of the output value is too large. This is because the circuit that receives the output signal from the CCD image pickup device has a large fluctuation. This may cause a malfunction and may also affect the normal video signal section.

In such a case, there is a difference in the video signal between the line where the electronic shutter pulse is standing and the line where the electronic shutter pulse is not standing, and the upper part and the lower part of the screen are distinguished on the screen. A line like this appears. This is called a shutter step.

Further, since the period for applying the electronic shutter pulse is limited to the horizontal blanking period, the so-called shutter speed can only be a multiple of one horizontal scanning period, and an arbitrary shutter speed cannot be selected. There was also.

In view of the above points, the present invention provides a CCD image pickup device capable of preventing the output DC value from varying due to an electronic shutter pulse and selecting an arbitrary shutter speed.

[0015]

According to the present invention, the output section 1 has a signal charge amplifying section 4 composed of a source follower, and an electronic shutter pulse is applied to the substrate to discharge the charge of the sensor section to the substrate side. In the CCD image pickup device having the electronic shutter function, the load transistor Q ₂ , of the signal charge amplification unit 4,
A control voltage synchronized with the electronic shutter pulse is applied to the gate of Q ₄ .

[0016]

According to the present invention, the control voltage synchronized with the electronic shutter pulse applied to the substrate is applied to the gates of the load transistors Q ₂ and Q ₄ of the signal charge amplifying section 4, that is, the substrate voltage V _sub By changing the voltage V _gg applied to the gates of the load transistors Q ₂ and Q ₄ in synchronism with the change, the substrate bias effect received by the load transistors Q ₂ and Q ₄ is compensated and the fluctuation of the output DC value is suppressed.

Further, since the fluctuation of the output DC value is suppressed, the electronic shutter pulse can be applied during the period other than the horizontal retrace line period, and the arbitrary shutter speed can be selected.

[0018]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of the present invention. C according to this example
The CD image pickup device has a so-called vertical overflow structure sensor unit in which the semiconductor substrate is an overflow drain, and changes the substrate voltage V _sub , that is, applies an electronic shutter pulse to the substrate and stores it in the sensor unit. The electronic shutter function is provided for discharging the generated signal charges to the substrate side.

In FIG. 1, 6 is a final charge transfer section of the horizontal shift register of the CCD image pickup device, and 5 is a floating diffusion section connected to the subsequent stage of the final charge transfer section 4 via a read gate section. Further, 1 indicates an output unit. This output unit 1 has a first source follower circuit 2 including a driving MOS transistor Q ₁ and a load MOS transistor Q ₂ and a second source follower circuit 2 including a driving MOS transistor Q ₃ and a load MOS transistor Q ₄ , as described above. Incorporating a signal charge amplifying section 4 which is connected in cascade with the source follower circuit 3 of FIG.

The floating diffusion section 5 is connected to the gate of the driving MOS transistor Q ₁ of the first source follower circuit 2 in the signal charge amplification section 4, and the voltage V _dd is applied to the drains of the driving MOS transistors Q ₁ and Q _3.
Is applied to the sources of the load MOS transistors Q ₂ and Q ₄ and is grounded via a resistor R _ss .

Thus, in this example, the substrate is provided.
Voltage, that is, substrate voltage V_subResistance R ₁And R₂By minutes
The voltage V_ggLoad MOS transistor
Q ₂, Q_FourIt is applied to the gate of.

In such a structure, the normal substrate voltage V is applied to the substrate.
_{When sub} is given, the predetermined voltage V _gg similar to that described in FIG. 4 is applied to the gates of the load MOS transistors Q ₁ and Q ₃ . Then, when the electronic shutter is operated, a voltage higher than the normal substrate voltage, that is, an electronic shutter pulse is applied to the substrate, and when the so-called substrate voltage V _sub changes, the voltage is _divided according to the change of the substrate voltage V _sub. The voltage V _gg also changes.

Therefore, the voltage V _gg changes in synchronism with the electronic shutter pulse, so that the load MOS transistors Q ₂ , Q of the output section 1 are _driven by the electronic shutter pulse.
_The substrate bias effect received by ₄ is compensated, the current I _DS is adjusted, and the fluctuation of the DC component of the output signal from the output terminal t _{1, that} is, the output DC value can be suppressed.

FIG. 2 shows another example of the present invention. In the figure, parts corresponding to those in FIG.

In the present example, the gates of the load MOS transistors Q ₂ and Q ₄ of the signal charge amplifier 4 are selectively supplied with different voltages V through the switching circuit 8 driven by the voltage V _sub applied to the substrate. _gg1 or voltage V _gg2 is applied.

V _gg1 is a voltage applied to the gate when a normal substrate voltage is applied to the substrate, and V _gg2
Is a voltage applied to the gate when an electronic shutter pulse is applied to the substrate.

In this structure, when the normal substrate voltage V _sub is applied to the substrate, the switching circuit 8 is switched to the contact a side so that the load MOS transistor Q ₂ ,
The voltage V _gg1 is applied to the gate of Q ₄ . When an electronic shutter pulse is applied to the substrate and the substrate voltage V _sub changes significantly, the switching circuit 8 is switched to the contact b side by this voltage, and the voltage V _gg2 is applied to the gates of the load MOS transistors Q ₂ and Q _4. Is applied.

In this way, the switching circuit 8 is driven according to the change of the voltage V _sub applied to the substrate, and the voltage applied to the load MOS transistors Q ₂ and Q ₄ is changed to V _gg1 or V _gg2. Q ₂ , Q
_It is possible to compensate for the substrate bias effect received by ₄ and suppress fluctuations in the output DC value.

The voltage V _sub applied to the substrate as described above
In addition to the method of directly driving the switching circuit 8, the switching circuit 8 may be driven by supplying a switching pulse from a clock signal supplied to a driving circuit unit that supplies the substrate voltage V _sub .

Alternatively, a dedicated pulse may be generated by a timing generator which supplies a timing pulse for driving the CCD register, and the switching circuit 8 may be driven using this pulse.

It is necessary to _apply the voltage V _gg2 to the gates of the load MOS transistors Q ₂ and Q ₄ in synchronization with the fluctuation of the substrate voltage V _sub . In this case, if there is a delay in the rise of the substrate voltage when the electronic shutter pulse is applied to the substrate, a circuit for adjusting the phase, for example, a delay circuit 9 is provided in front of the switching circuit 8 as shown in FIG. It can be inserted into

According to the above embodiment, it is possible to prevent the fluctuation of the output DC value caused by the electronic shutter pulse,
A so-called shutter step can be prevented. Further, since unnecessary fluctuations in the output DC value can be suppressed, the margin can be widened and the degree of freedom in design can be increased with respect to the circuit that receives the output signal from the CCD image pickup device.

Further, since the fluctuation of the output DC value can be suppressed, it becomes possible to apply the electronic shutter pulse during a period other than the horizontal retrace line period, so that an arbitrary shutter speed can be selected.

As described above, the present invention is useful for a CCD image pickup device that performs an electronic shutter operation for discharging the signal charges accumulated in the sensor section by changing the substrate voltage, but other The image pickup device also has a signal charge amplifying unit formed of a source follower in the device, and is also useful for an image pickup device that changes the substrate voltage in operation.

[0036]

According to the present invention, it is possible to prevent the fluctuation of the output DC value caused by the electronic shutter pulse and prevent the so-called shutter step. Further, since unnecessary fluctuation of the output DC value is suppressed, the margin is widened for the circuit receiving the output signal from the CCD image pickup element, and the degree of freedom in design can be increased. Furthermore, since the electronic shutter pulse can be given during a period other than the horizontal blanking period, it is possible to completely select an arbitrary shutter speed.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an example of an output unit of a CCD image pickup device of the present invention.

FIG. 2 is a circuit diagram showing another example of the output section of the CCD image pickup device of the present invention.

FIG. 3 is a circuit diagram showing still another example of the output section of the CCD image pickup device of the present invention.

FIG. 4 is a circuit diagram of an output section of a conventional CCD image sensor.

[Explanation of symbols]

1 output section 2, 3 source follower circuit 4 signal charge amplification section 5 floating diffusion section 8 switching circuit 9 delay circuit Q ₁ , Q ₃ drive MOS transistor Q ₂ , Q ₄ load MOS transistor V _sub substrate voltage

Claims (1)

[Claims] 1. A CCD image pickup device having an electronic shutter function for applying an electronic shutter pulse to a substrate to discharge an electric charge of a sensor unit to a substrate side, the output unit having a signal charge amplifying unit composed of a source follower. A CCD image pickup device characterized in that a control voltage synchronized with the electronic shutter pulse is applied to a gate of a load transistor of the signal charge amplification section.