JPH0759053B2 - Solid-state imaging device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a solid-state imaging device, and more particularly to a solid-state imaging device suitable for low noise and low power consumption.

[Conventional technology]

A solid-state image pickup device using a semiconductor technology has been put into practical use as an image pickup means for obtaining a video signal of a television or the like. The solid-state imaging device can be reduced in size and weight, and the captured signal can be directly taken out by a wire.
It has many advantages over the one using an image pickup tube and tends to be used more and more in the future.

Although various types of solid-state image pickup devices have been proposed, MOS type image pickup devices and CCD type image pickup devices are now mainstream.

In the conventional MOS type image pickup device, for example, as described in Japanese Patent Application Laid-Open No. 57-46592, the output of the MOS type image pickup device and the input of the preamplifier are directly connected, that is, DC-coupled. The reason will be described with reference to FIGS. 7 and 8.

FIG. 7 is an explanatory diagram in the case where the output of the solid-state image sensor and the input of the preamplifier are AC-coupled. 1 is an image sensor, 2 is a preamplifier, 3 is a capacitor, 4 is a signal output terminal of the image sensor, 5
Is an output terminal of the preamplifier, 6 is a resistor, 7 is a power source, and 12 is a feedback resistor. In the figure, when the image pickup device 1 and the preamplifier 2 are AC-coupled via a capacitance C _c , a DC bias is applied to the image pickup device (this is usually called a video voltage V _v ).
Must be supplied, and this DC bias is supplied by the series circuit of the power supply 7 and the resistor R _v . Then this resistance
Thermal noise is generated in R _v , and sensitivity is reduced. To reduce such thermal noise, the resistance R _v may be increased, but the voltage of the power supply 7 must be increased accordingly, which is not realistic.

For this reason, if the method in which the image sensor and the preamplifier are DC-coupled and read out is adopted as described in the above publication, there is a problem that the power supply voltage of the preamplifier becomes high. This problem will be described in detail with reference to FIG.

FIG. 8 is a schematic configuration diagram of a preamplifier that is DC-coupled to the output of the solid-state image pickup device. For the detailed circuit, refer to FIG. 7 of the above publication. In FIG. 8, 5 is the output of the preamplifier, 8 and 9 are power supplies, 10 is a junction field effect transistor (JFET) that constitutes the first stage amplifier, 11 is a load resistor, 12 is a feedback resistor, and 13 is an amplifier together with the JFET10. Make up. Further, 14 is an input terminal of the preamplifier.

In the figure, the noise characteristics of the preamplifier are particularly important, and a junction field effect transistor (JFET) 10 is used for the first-stage amplifier as shown. JFET10 to reduce noise
It is known that it is better to increase the drain-source voltage V _ds , the drain-source current I _ds , and the load resistance R _l of the above. As a rough guide, if the conditions of V _ds > 2V, I _ds > 6mA, R _l > 400Ω (1) are observed, noise characteristics that are not much different from the ideal conditions can be obtained.

When the DC bias V _v of the image sensor is 3 _v , the gate voltage V _g is 3 v and the source voltage V _s is about 3.5 v and the drain voltage is V _ds when the condition of the equation (1) is to be observed. in addition
It will be 5.5v. If a voltage drop of 2.4 _l is added to R _l , the power supply voltage V _cc needs to be approximately 8 v or higher. Generally, V _cc V _g + 5v ……………… (2) is one guide.

[Problems to be solved by the invention]

As described above, in the conventional MOS image pickup device, if the image pickup element and the preamplifier are simply DC-coupled, there is a problem that the power supply voltage of the preamplifier becomes high, which is one of the obstacles to lowering the power consumption of the image pickup device. Was there. Also,
When an AC coupled preamplifier is used to reduce the power supply voltage, there is a problem that thermal noise increases and sensitivity decreases.

An object of the present invention is to provide a solid-state imaging device capable of reducing the power supply voltage of the preamplifier without lowering the sensitivity.

[Means for solving problems]

The purpose is to (1) AC-couple the signal output of the image sensor and the input of the preamplifier, and (2) provide a DC bias voltage supply terminal on the image sensor and connect it to the signal output line via a switch to perform a blanking period. It is achieved by providing a DC bias without increasing noise by making the switch conductive.

[Work]

With the configuration of (1) above, the input DC potential V _g of the preamplifier
Can be lowered, and the power supply voltage V _{cc of the} preamplifier can be lowered. That is, equation (2), in Figure 8, for example, V _g = In 1 v V _cc = 6v, it is possible to design V _g = 0 v in V _cc = 5 v. In addition, under the condition that V _{cc is} constant, the noise of the preamplifier can be reduced in principle although it is slight.

Further, according to the above configuration (2), since the switch interposed between the DC bias voltage supply terminal and the signal output line is open during the scanning period, thermal noise does not change as compared with the conventional example. The switch is turned on during the horizontal blanking period to apply a DC voltage to the signal output line. The potential of the signal output line decreases in accordance with the signal charge amount as the signal is read during the scanning period, but this problem can be easily solved by increasing the value of the coupling capacitance between the signal output line and the preamplifier. The voltage drop in the on resistance of the switch cannot be avoided, and the DC potential of the signal output line becomes a value slightly smaller than the value of the voltage source. This relationship is expressed as follows: DC bias voltage applied to the signal output line is V _v , DC potential of the signal output line is V _v ′, ON resistance of the switch is R _on , average signal current is I _av , and time of the switch is on. If the ratio is expressed as α, it becomes as follows.

ΔV≡V _v −V _v ′ = I _av · R _on / α (3) According to the present invention, since the thermal noise does not increase as described above, the value of the on-resistance R _on of the switch is changed. Therefore, the voltage difference ΔV can be reduced. For example, in the design example of R _on = 10 kΩ and α = 0.1, I _av = 0.1 μA
At that time, ΔV = 10 mV, and the change in the saturation signal level due to this does not pose any problem. In addition, Δ
Regarding the point where V gradually changes from zero to several tens of millivolts, periodic direct current regeneration (usually 1
It can be dealt with by clamping the optical black part to a constant potential every horizontal period), and does not cause the black level to fluctuate.

〔Example〕

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

1A and 1B are a circuit configuration diagram and a timing chart, respectively, showing a first embodiment of a solid-state image pickup device according to the present invention. In this embodiment, a switch 40 is added to the ordinary MOS type image pickup device described in the above-mentioned Japanese Patent Laid-Open No. 57-46592, and the basic operation is as described in the above-mentioned known example.

In the figure (a), 2 is a preamplifier, 3 is a coupling capacitance (C _c ), 4 is a signal output terminal, 5 is a preamplifier output terminal,
12 is a feedback resistor, 21 is a horizontal scanning circuit, 22 is a vertical scanning circuit,
23 is a signal output line, 25 is a vertical signal line, 27 and 28 are MOS transistors that are switching elements, 32 is a photodiode that is a photoelectric conversion element, 33 is a gate line, 40 is a MOS transistor that is a switch, and 41 is a reset pulse. , 42 is a DC voltage input terminal for applying a DC bias, h _{1 to} h ₃ are scanning circuit output pulses of the horizontal scanning circuit 21, and v _{1 to} v ₃ are scanning circuit output pulses of the vertical scanning circuit 22. ,Ten
Reference numeral 0 is an image sensor. Further, in FIG. 2B, v _{1 to} v ₂ ,
h ₁ to h ₃ is the scanning circuit output pulse, rp is reset pulses.

The operation of the circuit shown in FIG. 1A will be described with reference to the timing chart shown in FIG.

During the horizontal blanking period, the signal charges on the photodiodes 32 of the row selected by the vertical scanning pulses v _{1 to} v ₂ are transferred to the vertical signal line 25, and the reset pulse rp from the terminal 41 is set.
Thus, the switch 40 is turned on to reset the potential of the signal output line 23 to the V _v '. In the horizontal scanning period, the horizontal MOS transistors 27 are sequentially turned on by the horizontal scanning pulses h _{1 to} h ₃ to read out the signal, but since the switch 40 is open at this time, the thermal noise by the switch 40 does not increase. 'To be reset to the potential of photodiode 32 to be selected next horizontal blanking period is approximately V _v at the same time as the signal read' signals read simultaneously with the vertical signal line 25 is substantially V _v is reset to.

Since the image sensor 100 and the preamplifier 2 are AC-coupled,
The power supply voltage of the preamplifier 2 can be lowered as compared with the conventional one.

FIG. 2 shows (a) a circuit configuration diagram and (b) a timing chart showing a second embodiment of the solid-state image pickup device according to the present invention, for example, a "horizontal read" MOS described in JP-A-59-144278. The switch according to the present invention is added to the image pickup device.

In the figure, 24 is a signal output line, 26 is a horizontal signal line, 101
Is an image sensor, and the same parts as those in FIG. 1 are designated by the same reference numerals.

In the figure, the reset pulse is applied during the horizontal blanking period.
The switch 40 is made conductive by rp, and the signal output line 24 and the horizontal signal line 26 are reset to V _v '. In the scanning period, the vertical scanning pulses v ₁ and v ₂ turn on the vertical MOS transistors 29 and 30 in the selected row, and the horizontal scanning pulses h _{1 to} h ₃ cause the horizontal M
By sequentially turning on the OS transistor 31, the signal on the photodiode 32 of that row is read. Simultaneously with this signal reading, the photodiode 32 is reset to the potential of approximately V _v '.

Since the image pickup device 101 and the preamplifier 2 are AC-coupled, the power supply voltage of the preamplifier can be lowered as compared with the conventional case.

As a result of studies by the inventors, a driving condition for keeping the charge pump current accompanying horizontal scanning below the allowable limit has been found.When using this condition, the installation location of the horizontal MOS transistor 31 and the vertical MOS transistor 30 is found. Can be replaced.

FIG. 3 is a circuit configuration diagram showing a third embodiment of the solid-state image pickup device according to the present invention, in which 102 is an image pickup element, and the same parts as those in FIG.

In this embodiment, the horizontal and vertical MOS transistors 31, 30 in the embodiment shown in FIG. 2 are replaced with each other, and the switch 40 is provided only on the signal output line 24.
It is a simplified example excluding the MOS transistor between 26 and the terminal 42.

In the embodiment shown in FIG. 2, unless the horizontal signal line 26 is reset every horizontal period, the potential drop of the horizontal signal line 26 due to photoelectric conversion is transmitted to the capacitance between the MOS switching elements of the MOS transistors 30 and 31. The so-called "smear" phenomenon occurs, but in the embodiment of FIG. 3, this problem does not occur and the number of switches 40 can be reduced. The drive timing chart of this embodiment may be the same as that shown in FIG.

The embodiments of the solid-state imaging device according to the present invention have been described above. The point of these embodiments is to supply a DC potential to the signal output line through the switch during the blanking period. In the above description, an example in which the number of signal output lines is one has been described for simplification, but it goes without saying that the present invention is also applicable to an image sensor having a plurality of signal output lines.

In the case of the preamplifier, the reduction of the power supply voltage can be achieved by the circuit configuration shown in FIG. 8 and by the constant design in which V _g , V _s , V _d and V _cc are lowered in parallel. However, the V _g in the circuit arrangement of Figure 8
The dynamic range in the output section becomes difficult when lowered near 0 _v . Hereinafter, a preamplifier that solves this problem will be described with reference to FIG.

FIG. 4 is a circuit diagram showing an example of a preamplifier which solves the problem of the dynamic range in the output section.
Are capacitors and 51 to 53 are resistors, and the same parts as those in FIG.

In the figure, the output DC voltage at the output terminal 5 can be made higher than the gate voltage V _{g of the} JFET 10 by attenuating the DC feedback amount of the amplifier with the resistors 53 and 51, which solves the dynamic range problem. be able to. Resistance 53
A design without is also conceivable, in which case the output DC voltage can be freely selected with V _g = 0 _v .

Some of the embodiments of the invention described above work well with a typical home video camera. However, it is true that the effective video bias V _v ′ will decrease slightly during the horizontal scanning period, and it is also true that V _v ′ changes depending on the amount of incident light. There may be cases where

FIG. 5 is a block diagram (a) showing a fourth embodiment of the present invention configured to reduce the change in effective video bias.
(B) A characteristic diagram showing the relationship between the average incident light amount and the voltage conversion circuit output, wherein 60 is a video output terminal, 61 is a signal processing circuit, 62 is a voltage conversion circuit, 63 is a timing generation circuit, 64 is a resistor, and 65 is a resistor. Is a diode and 103 is an image sensor.

FIG. 5 shows, for example, when the value of the coupling capacitance C _c is designed to be small in order to enable imaging immediately after power-on, fluctuation of the effective video bias V _v ′ is reduced within one horizontal scanning period. It is a structural example. The point of this configuration is to change the low level of the rp pulse according to the amount of incident light, and allow a little increase in thermal noise when the amount of incident light is large.
The point is that 40 is made conductive with a somewhat high resistance during the scanning period. Since the switch 40 is made conductive during the scanning period, the fluctuation of V _v ′ during the horizontal scanning period can be suppressed to be small. In the figure, V ₂ is a voltage such as an AGC control voltage or an iris control voltage that changes according to the amount of incident light, and is converted by the voltage conversion circuit 62 into a voltage as shown in FIG. When the amount of incident light is large, the output of the voltage conversion circuit 62 is the voltage V ₁ , and the voltage obtained by subtracting the voltage drop of the diode 65 from this is the voltage of the terminal 41 during the scanning period. Voltage
V ₁ is set within the range where the increase in noise is not noticeable when the amount of incident light is large. The image sensor 103 may be any of the image sensors 100 to 102 in the above-mentioned embodiment.

FIG. 6 is a block diagram showing a fifth embodiment of the solid-state image pickup device according to the present invention for reducing the change of the effective video bias V _v ′ depending on the amount of incident light. The voltage drop due to the on-resistance R _on of the switch 40 is shown in FIG. In advance,
It is configured to change the value of the video bias V _v according to the light amount.

〔The invention's effect〕

As described above, according to the present invention, since a DC bias can be supplied to the image pickup device without increasing thermal noise, the output of the image pickup device can be AC-coupled to the preamplifier without lowering the sensitivity, The power supply voltage of the preamplifier can be lowered. For example, it is possible to reduce the power supply voltage of the preamplifier, which was required in the past to 8v, to 5v by using the present invention, and it is possible to provide a solid-state imaging device having an excellent function excluding the above-mentioned drawbacks of the prior art. .

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing a first embodiment of a solid-state image pickup device according to the present invention, (a) a timing chart, and (2).
FIG. 3A is a circuit configuration diagram showing a second embodiment of a solid-state imaging device according to the present invention, FIG. 3B is a timing chart, and FIG. 3 is a circuit configuration diagram showing a third embodiment of the solid-state imaging device according to the present invention. FIG. 4 is a circuit diagram showing an example of a preamplifier that solves the problem of dynamic range in the output section, and FIG. 5 shows a fourth embodiment of the present invention configured to reduce the change in effective video bias ( a) a block diagram, (b) a characteristic diagram showing the relationship between the average incident light amount and the output of the voltage conversion circuit, and FIG. 6 shows a fifth embodiment of the present invention in which the change of the effective video bias due to the incident light amount is reduced. FIG. 7 is a block diagram showing the output of the solid-state image sensor and AC input of the preamplifier. FIG. 8 is a schematic configuration diagram of the preamplifier DC-coupled to the output of the solid-state image sensor. 2 ... Preamplifier, 21 ... Horizontal scanning circuit, 22 ... Vertical scanning circuit, 23 ... Signal output line, 25 ... Vertical signal line, 40 ...
Switch 41 reset pulse input terminal 42 DC voltage input terminal 100 image sensor.

Claims (1)

[Claims] 1. A solid-state imaging device including an image pickup device having a plurality of photoelectric conversion elements, a single or a plurality of signal output lines, and a plurality of switching elements connecting the photoelectric conversion elements and the signal output lines. In the device, the signal output line is coupled to a preamplifier via a capacitor, and a DC bias voltage is supplied to the signal output line via a switch that conducts during a period other than a period for extracting a signal from the signal output line, A solid-state image pickup device, wherein a period for supplying a DC bias voltage to the image pickup device is set to a period other than a period for extracting a signal from the signal output line.