JPS59126379A - Signal output circuit of solid-state image pickup element - Google Patents

Abstract

PURPOSE:To offer an output circuit with small size and having general-purpose application without deteriorating the S/N by detecting a signal charge applied from a reading section of an image sensor to a charge detecting section by a means whose sensitivity is made variable. CONSTITUTION:A signal charge group converted photoelectrically and stored by a photo sensing section PT is transferred in the lump to a charge transfer section by a transfer gate TG, passes beneath an output gate OG in time series by a charge transfer pulse and is applied to a floating diffusion FD. MOS TRQ5, Q6 are connected to a gate electrode Dg of the upper face of the FD and when a gate voltage Vg of the TRQ5 is changed, the equivalent static capacitance C'FD including a depletion layer capacitance of the FD is changed according to Equation C'FDproportional Vg1/2. Through the constitution above, the C'FD is set by the adjustment of the gate potential Vg of a source follower amplifier Q5 so as to obtain an optional value as a gate voltage of a TRQ1 to the same signal charge. The charge applied to the FD is eliminated by a VRD power supply via a reset TRQR. The output is outputted from a terminal T0 via the detecting amplifier TRQ1.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a signal output circuit that can be integrated on the same semiconductor substrate as a light receiving section.

CCD image sensor is CCD (charge coupled device)
This is a solid-state imaging device with a functional VLSI structure that integrates photoelectric conversion, charge storage, charge transfer, and charge detection functions on the same semiconductor substrate. Due to its compact size and high reliability, it has been widely used in television cameras, Noakunmiri two-character recognition devices, and the like. In recent years, as its application has become more widespread.

The performance required of CCD image sensors continues to diversify, and if things continue as they are, there are concerns that they will be forced to produce a wide variety of products in small quantities, and that it will be difficult to achieve low costs. For this reason,
Increase productivity by making the CCD image sensor a more versatile functional element.

It is desired to reduce costs. On the other hand, this is coupled with the demand for smaller devices and higher functionality.

As an LSI element with more integrated elements, it integrates not only the imaging function in the device but also the peripheral circuits necessary to operate it and the peripheral function circuits related to the system on the same semiconductor substrate (on-chip). By,
There is a need to reduce the total cost and downsize the 7-stem system.

Here, a conventional example of an output circuit in a CCD image sensor, which is required to be improved as described above, will be described with reference to the circuit diagram shown in FIG.

In the figure, the photosensitive part (photodiode area) PT is.

It has the role of reading each horizontal row of the illuminated imaging target 1, such as characters on a form, by photoelectric conversion, and the signal charges generated and accumulated in the odd-numbered and even-numbered photosensitive cells are transferred. The respective readout sections SR arranged above and below the photosensitive section are activated by the operation of the gate TG.
A. Configure SRB.

For example, the data is transferred all at once to each bit in a two-phase drive type CCD shift register. These two signal charge groups are transferred in the directions of arrows A and B through the transfer paths of the not registers SRA and SRR by charge transfer pulses applied to the terminals φl and φ2, respectively.

The output signal is supplied to a floating diffusion node (hereinafter referred to as FD) which is connected to a MOS (MOS) lannostar QlO gate for charge detection by passing alternately directly under the OG (output circuit). Here, the equivalent capacitance CFD exists between the FD and the substrate as an inherent value, and this FD
The signal charge Qs ig transferred to is approximately.

ΔvFG = Qsig / CFD
---(1) appears at the gate of the MOS transistor Q+. This ΔVFD
MOS in a fast follower amplifier) modulates the current I, of the lannostar Q2, and outputs the output signal V. Extracted as UT.

That is, the voltage signals appearing in the two readout sections SRA and SRB are sequentially detected by the Ml)S) transistor Q+, but the signal charge generated in one photosensitive cell flowing into the FD is detected by the MOS) transistor Q1. If the charge still remains after the charge is removed, it will be mixed with the inflow charge that should be detected secondary. To avoid this,
Every time one charge detection is completed, the residual charge must be removed by the reset transistor QR (5) driven by the reset charge φ□. In this way, as a basic functional circuit of a solid-state image sensor, the output circuit of a CCD image sensor is connected to a floating diffuser F for charge detection.
D, and an amplifier Q+”-Q4 for the voltage signal appearing on FD.
and a reset transistor QR. In such a conventional CCD image sensor, the signal amplitude of the output voUT at the To terminal has a fixed amplification factor in the output circuit.
It is directly proportional to the product of the incident light illuminance LEL on the surface and the cycle TTo of operating the transfer case, that is, the exposure amount LF, L×TTo, and is expressed as the quadratic equation % (2). As is clear from this formula, C
The output signal V of the CD image sensor increases as the cycle of operating the transport cart 8 becomes shorter, that is, as the number of times signals are taken out per unit time increases. It can be seen that the amplitude of UT becomes correspondingly smaller.

Hiko, today, the most required performance of image sensor (6) One of these is the speed at which the subject is read.

The speed required for the same element is 1. For example, there is a growing demand for rapid response to rationalization of office work.

In a facsimile machine that incorporates a copying function, one requirement is that the ratio between the speed of copying and the speed required of an image sensor for facsimile transmission is as much as 40:1. This means that while higher speeds are required for the copying function, for facsimile IJ, the frequency band of subscriber telephone lines used as the transmission medium is 30.
It is limited to 0 to 3400 Hz. Although band compression technology has improved in recent years, there is a limit to how high the transmission speed can be increased. in this way.

Despite wide-ranging demands for speed, conventional solid-state image sensors are not capable of handling 2 For example, when high-speed reading of an object is required, the exposure time naturally shortens, resulting in a signal charge generated in the optical sensor due to photoelectric conversion. ■ The output signal decreases. The UT also has a small amplitude, causing a problem that the signal-to-noise ratio (hereinafter referred to as S/N) deteriorates. To compensate for this, it is necessary to increase the illuminance on the subject side (intensify the light source) or to switch to a more sensitive image sensor. However, if this is done forcibly, the device will become larger and more expensive.On the other hand, when reading objects at low speeds, the output will become saturated if the intensity of light from the light source remains strong, so light shielding measures will be required. This is an inconvenience. In this way, in conventional technology,
This has a major drawback in that the performance that can be achieved as an image sensor is extremely limited.

An object of the present invention is to detect the signal charge supplied from the readout section of the image sensor to the charge detection section through a means that can vary the sensitivity, thereby eliminating the drawbacks of the conventional method and improving the S/N ratio.
3. According to the present invention, it is an object of the present invention to provide a signal output circuit for a solid-state image sensor that is compact and can be used in a wide range of applications without deteriorating the image. In a solid-state image sensor configured with a photosensitive section consisting of cells and a readout section that transfers signal charges of the photosensitive section and outputs them in time series, the readout section includes a floating diffusion that receives the transferred charges; a reset means for removing residual charge on the output side of the floating diffusion; a means for amplifying the signal voltage obtained at the output side of the floating diffusion; A signal output circuit for a solid-state image pickup device is obtained, which is characterized in that a variable means for setting the amplitude of the signal voltage to a preferable value after converting the signal voltage into a signal voltage wave is connected thereto.

Next, a signal output circuit for a CCD image sensor according to the present invention will be explained with reference to the drawings.

1. Before explaining the present invention, I will cite an example of a reading device using a CCD image sensor, and 2. I will explain the characteristics that are the basis for suggesting the present invention by referring to the perspective view in Fig. 2 and the time chart in Fig. 3. Refer to and explain. In Figure 2, ■ is a manuscript.

The image of the reading surface illuminated by the light source 2 is collected by the lens 3 and projected onto the photosensitive section (9) of the image sensor 4. The signal converted into an electrical signal by the photosensitive section is a photoelectric conversion signal V. Extracted as UT. Now, with the brightness of the light source 2 constant, the exposure time is as shown in the time chart (a) of FIG.

That is, °°1 of the operating pulse φTG of the transfer case TG.
Let Tl be the period from ``to 1'', and T2 be twice that period, then the photocharges accumulated during the exposure time T1 will start to be released at the same time as a new exposure starts. ,
As shown in (b), it is extracted as a signal with an output amplitude υ1B. Also, the signal during the exposure time T2 has an output amplitude τ
Obtained as 2B. This amplitude v2B becomes υ2g=2Xvio from the above-mentioned relationship of equation (2). Furthermore, (b)
shows the relationship between the output signals when the sensor is open, but in a dark state, that is, in a state where there is no incident light on the photosensitive section, there should be no signal output. However, the CCD image sensor.

Its basic operation is performed in a thermally non-equilibrium state, and at room temperature some dark output signal is observed due to thermoelectrons and the like. (C) is an enlarged view of the black signal level. It shows the dark output (10) signal, and the accumulation of thermoelectrons also takes place during the φTO period Tl or T2 shown in (a). Therefore, similar to the output signal when open, the dark output amplitude τ1 during the period T1
Between 0 and the dark output amplitude τ2D of the period T2, l′u2
There is a relationship of D=2XυID. Here.

The ratio of the open output amplitude to the dark output amplitude is the so-called S/N ratio, and under constant illuminance.

v + a/l' 10-v 2B/'' 2D. This S/N ratio is characterized in that it does not change even if the exposure time changes.

The reason for this is that in a CCD image sensor, the dark output signal is a large noise factor, and the detection section and amplification section are on-chip in the sensor section.

This is because it is less susceptible to external noise. The present invention
Based on this characteristic, the reduction in the amplitude of the output signal due to shortening the exposure time is amplified by the on-chip output circuit in the sensor section, so that the sensor can always be used at the optimal output signal level. .

FIG. 4 shows a circuit diagram of a first embodiment according to the present invention. In this figure, the parts indicated by broken lines indicate additional elements that are different from the conventional example in Fig. 1, and other parts are indicated by the same reference numerals (understood as having the same functions as the conventional example). sea bream. Therefore, photosensitive part PT, transfer key) TG, shift register SR
A, C composed of SRB and output OG
The function of the CD image sensor is the same as that of the conventional example shown in FIG. 1, and the signal charge group photoelectrically converted and accumulated in the photosensitive part PT is transferred all at once to the charge transfer part by the operation of the transfer gate TG. , time-sequential output due to charge transfer and leakage (-) Passes directly under OG and becomes a floating differential.

- Supplied to Noyon FD. Note that there is a gate electrode Dg on the top surface of this FD, and a gate electrode Dg is connected to the gate electrode Dg.
O8) Transistors Q5 and Q6 are connected. Then, when the r-gate voltage Vg of the MOS transistor Q5 is changed, the equivalent capacitance, including the depletion layer capacitance of the FD, changes according to the change in the potential of the gate electrode Dg, as expressed by the well-known relational expression. It can be changed by C, :Doc<. This date electrode Dg is used for shift registers SRA and SR in the manufacturing process of the CCD image sensor.
It is configured similarly to B, and also.

MOS) Lannostar Q5, Q6 is Ql lQ2
+Qa +Q4 MOS) It is constructed in the same way as a transistor.

According to such a configuration, the equivalent capacitance CFD is set by adjusting the dart potential vg of the source follower amplifier Q5, and even for the same signal charge amount, the dart voltage of the MOS transistor Qt is determined from the relationship of equation (1). It is possible to obtain any value as . The charge supplied to the FD is set at a reset voltage φ to detect the signal charge supplied to the primary
, the output of the reset MOS transistor Q+ is driven by the source follower amplifier MO8) transistor Q3.
, Q4, and then the output terminal T. obtained from.

FIG. 5 shows a circuit diagram of a second embodiment according to the present invention. In this figure, parts indicated by broken lines indicate elements that are different from the conventional example shown in Fig. 1, and other parts are indicated by the same reference numerals. (13) Please understand that Newly added Q7 +Q8
, Q9 is MOS) transistor Q+ + Q2
+ Qa + MOS formed in the same process as Q4
) transistor, configured as a modified common source amplifier. A variable voltage Vg is externally applied to the date of the transistor Q7 to adjust the amplification gain.

As a result, even when the signal charge supplied from the charge transfer section to the floating diffusion FD is small and the output of the first stage detection amplifier is transmitted to the dart of the transistor Q8 with a small amplitude, the operating resistance R9 of the trough star Q9 Set the external voltage vg connected to the dart of transistor Q7 so that the operating resistance R7 of transistor Q7 is sufficiently large than As a result, the gain is multiplied by R7/R9, and a signal output with an appropriately large amplitude can be obtained.

As is clear from the above description, according to the present invention, the output circuit is configured such that the output signal amplitude versus exposure amount, that is, the sensitivity, of the solid-state image sensor can be set to an arbitrary value (14) from the outside of the solid-state image sensor. By configuring this structure, it can be used for a wide range of applications from high-speed to low-speed reading of objects even when the light source illuminance is constant, without deteriorating the S/N ratio, and it also increases versatility. By further increasing this, the productivity of the solid-state image sensor can be increased and the cost can be reduced, which is a great effect.

In addition, in a reading device that uses a lens like the one shown in Figure 2, problems such as those shown in Waveform distortion of the output signal with center symmetry,
Generally, so-called shading occurs,
It is also possible to correct the noeding waveform signal to a uniform average waveform by adding gain to the output signals at both ends, and the cost savings are significant compared to current correction methods. be. Furthermore, reading devices, especially OCR
Countermeasures against fluctuations in the amount of light from the light source are an important issue in computer and facsimile machines, and when photographing subjects with various contrast conditions, automatic variable gain control ( AGC
) Since the circuit can be configured, great effects can be expected, such as not only improving the quality of two images, but also improving reliability and downsizing the device.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a conventional example of a signal output circuit in an image sensor, FIG. 2 is a perspective view showing a schematic structure of a reading device using a CCD image sensor, which is the basis for suggesting the present invention, and FIG. The diagram is. A time chart for explaining the operation of the device shown in FIG. 2,
FIG. 4 is a circuit diagram of a first embodiment of the invention, and FIG. 5 is a circuit diagram of a second embodiment of the invention. In the figure, PT is the photosensitive part and TG is the transport dart. SRA and SRB are soft registers, OG is output 8-
to. FD is floating diffusion + C, Dha F
Equivalent capacitance of D + Q + ~ Q g + Q R is MOS
) Lens-4 [

Claims (1)

[Claims] 1. Consisting of a photosensitive section consisting of a plurality of light receiving cells that generate signal charges through photoelectric conversion, and a readout section that transfers the signal charges of the photosensitive section and outputs them in time series. In the solid-state image sensor, the readout section includes a floating diffusion receiving the transferred charge, a reset means for removing residual charge on the output side of the floating diffusion, and a signal voltage obtained on the output side of the floating diffusion. and a variable means for setting the amplitude of the signal voltage to a preferable value when or after converting the signal charge supplied to the floating diffuser into a signal voltage wave. A signal output circuit for a solid-state image sensor, characterized in that: 2. The signal output circuit according to claim 1, wherein the variable means is provided close to the floating diffusion. It consists of an amplifier circuit that drives the gate, and a terminal that provides a driving input to the amplifier circuit, and by varying the DC voltage that is applied to the terminal. A signal output circuit for a solid-state image sensor, characterized in that the equivalent capacitance of the floating diffusion is controlled and set via the gate. 3. The signal output circuit according to claim 1, wherein the variable means is connected to at least the first stage of the amplification means and a circuit for amplifying the signal voltage, and a gain control input to the amplification circuit. 1. A signal output circuit for a solid-state image sensor, characterized in that the gain of the amplifier circuit is controlled and set by varying the DC voltage applied to the terminal.