JP2010177928A - Imaging element and imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To minimize power consumption, and to obtain an image of high picture quality even at photographing with high sensitivity. <P>SOLUTION: The imaging element includes: a plurality of pixels (201) configured to output electric signals obtained through photoelectric conversion; a floating diffusion amplifier configured to amplify the electric signals; an amplifier (211) configured to amplify the electric signals amplified by the floating diffusion amplifier; and a capacitor (212) configured to vary the gain of the amplifier (211). The floating diffusion amplifier includes a constant current source (209) for supplying a current corresponding to the gain varied by the capacitor. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an imaging element, a control method thereof, and an imaging apparatus, and more particularly to an imaging element that amplifies and outputs an image signal, a control method thereof, and an imaging apparatus.

  2. Description of the Related Art Conventionally, imaging devices such as a digital camera and a digital video camera that use a CCD or CMOS APS (CMOS Active Pixel Sensor) as an imaging device and record a photographed image have been sold. As a method of amplifying a signal in these image pickup apparatuses, there are an analog signal amplification mainly performed by an image sensor or an analog front end (AFE), and a digital signal amplification performed by a processing circuit for the image pickup signal.

  By the way, considering the image quality of the image obtained by the imaging device from the viewpoint of noise, the amount of noise appearing in the image differs depending on which stage in the imaging device is amplified. In general, signal amplification is less likely to appear in the image when it is performed at the front stage in the imaging device. This is because if the gain is applied at the rear stage in the imaging device, noise generated between the front stage and the rear stage is also generated. It is because it amplifies. As described above, when signal amplification is performed in the imaging apparatus, noise is mixed in the middle of the image pickup device. Therefore, an image with better image quality is generally obtained by performing signal amplification closer to the photoelectric conversion element. As an image sensor that performs signal amplification on the side close to the photoelectric conversion element, for example, Patent Document 1 discloses an image sensor having an amplifier for each column.

JP 2005-217771 A

  By the way, in recent digital cameras and the like, high gain is realized by applying more gain to a signal obtained from a photoelectric conversion element. However, as the gain is increased, characteristics that have not been a problem in the past become more prominent. For example, shading in the dark occurs remarkably due to insufficient driving capability of a circuit such as an amplifier. These characteristics are improved by increasing the drive capability of a circuit such as an amplifier. As a method for increasing the driving capability of the circuit, it is conceivable to increase the current flowing through the circuit. However, simply increasing the current leads to an increase in power consumption.

  The present invention has been made in view of the above problems, and an object of the present invention is to minimize power consumption and to obtain a high-quality image even in high sensitivity shooting.

  In order to achieve the above object, an image pickup device of the present invention includes a plurality of pixels that output an electrical signal obtained by photoelectric conversion, a first amplification unit that amplifies the electrical signal, and the first amplification unit. A second amplifying means for amplifying the electric signal amplified by the step (a), and a changing means for changing the gain of the second amplifying means, wherein the first amplifying means is changed by the changing means. A first current source for supplying a current corresponding to the gain is included.

  In addition, an imaging apparatus according to the present invention includes the above-described imaging element.

  In addition to minimizing power consumption, high-quality images can be obtained even when shooting at high sensitivity.

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings.

  FIG. 1 is a diagram schematically showing an overall configuration of an image sensor 100 according to the present embodiment. In FIG. 1, reference numeral 101 denotes a pixel portion composed of a plurality of pixels, 102 denotes a vertical selection circuit, 103 denotes a readout circuit, and 104 denotes a horizontal selection circuit. Among the pixels constituting the pixel portion 101, the signal of the row selected by the vertical selection circuit 102 is transferred to the readout circuit 103, and is output from the readout circuit 103 to the outside of the image sensor 100 by driving the horizontal selection circuit 104. .

  FIG. 2 is a circuit diagram illustrating a configuration of one pixel in the pixel unit 101 illustrated in FIG. 1 and a configuration for reading a signal from the pixel in the reading circuit 103.

  Each pixel 201 includes a photodiode (PD) 202, a transfer switch 203, a floating diffusion unit (FD) 204, a reset switch 207, an amplification MOS amplifier 205, and a selection switch 206.

  The PD 202 functions as a photoelectric conversion unit that photoelectrically converts light incident through the optical system. The anode of the PD 202 is grounded, and the cathode is connected to the source of the transfer switch 203. The transfer switch 203 is driven by a transfer pulse φTX input to its gate, and transfers the charge generated in the PD 202 to the FD 204. The FD 204 functions as a charge-voltage conversion unit that temporarily accumulates charges and converts the accumulated charges into a voltage signal.

  The amplification MOS amplifier 205 functions as a source follower, and an electric signal that has been subjected to charge-voltage conversion by the FD 204 is input to its gate. The amplification MOS amplifier 205 has its drain connected to the first power supply line VDD1 that provides the first potential, and its source connected to the selection switch 206. The selection switch 206 is driven by a vertical selection pulse φSEL input to the gate thereof from the vertical selection circuit 102, its drain is connected to the amplification MOS amplifier 205, and its source is connected to the vertical signal line 208. When the vertical selection pulse φSEL becomes an active level (high level), the selection switches 206 of the pixels belonging to the corresponding row of the pixel array are turned on, and the source of the amplification MOS amplifier 205 is connected to the vertical signal line 208.

  The reset switch 207 has a drain connected to a second power supply line VDD2 that provides a second potential (reset potential), and a source connected to the FD 204. Driven by a reset pulse φRES input to the gate of the reset switch 207, the FD 204 is reset to a reset potential to remove accumulated charges.

  In addition to the FD 204 and the amplification MOS amplifier 205, a floating diffusion amplifier (first amplification means) is configured by a constant current source 209 (first current source) that supplies a constant current to the vertical signal line 208. In each pixel constituting the row selected by the selection switch 206, the charge transferred to the FD 204 is converted into a voltage signal by the FD 204 and output to a corresponding circuit in the readout circuit 103 through the floating diffusion amplifier.

  A capacitor 210 is a clamp capacitor for clamping the reset potential of the FD 204 as a reset level signal, and is connected to the vertical signal line 208. In addition, a gain is applied to the signal read from the vertical signal line 208 together with the capacitor 212 and the amplifier 211. Further, the clamp operation is performed by combining the clamp switch 213 and the clamp pulse φC. Specifically, when the clamp pulse φC is on, the potential of the vertical signal line 208 is clamped to the clamp potential Vref.

  The switch 218 is a switch for reading the offset of the amplifier 211 (second amplification means), and is driven by an offset read pulse φTN supplied from the horizontal selection circuit 104. In the offset storage capacitor 219, the offset signal of the amplifier is stored immediately before reading out the pixel signal.

  The switch 220 is a switch for reading a voltage signal corresponding to the difference between the noise signals reduced by the clamping operation from the charge signal generated in the PD 202, and is driven by a signal read pulse φTS supplied from the horizontal selection circuit 104. The signal level storage capacitor 221 stores a signal obtained by reducing noise from the pixel signal when the pixel signal is read.

  The differential amplifier 223 is an amplifier that outputs the difference between the level of the signal stored in the offset storage capacitor 219 and the level of the signal stored in the signal level storage capacitor 221 to the output line 224. The switches 225 and 226 are driven by a horizontal signal selection pulse φHi (i represents a currently driven column) supplied from the horizontal selection circuit 104 and transmit the potentials of the capacitors 219 and 221 to the differential amplifier 223, respectively. To do.

  Here, the common output lines 227a and 227b connected to the input terminals of the differential amplifier 223 are typically driven by horizontal signal selection pulses φH1 to φH (i−1) and φH (i + 1) to φHn. The other rows of switches 225, 226 are connected. That is, the vertical signal line 208 to the switch 226 are configured for each column, and the differential amplifier 223 is configured for the number of read channels of the image sensor 100. Note that n is the number of columns of the pixel unit 101.

  FIG. 3 is a circuit diagram showing a configuration of the constant current source 209. A constant current characteristic is obtained by using a MOS transistor in a saturation characteristic region. A transistor 301 has a drain connected to the vertical signal line 208 and a source connected to the ground potential. A voltage Vi is applied to the gate, and a drain current corresponding to the voltage Vi is used as a constant current.

  FIG. 4 shows a drive pattern of each pixel 201 shown in FIG. In a period t401, the reset pulse φRES and the transfer pulse φTX are applied, the reset switch 207 and the transfer switch 203 are turned on, and the potentials of the PD 202 and the FD 204 are reset to the reset potential. Then, a new exposure period starts with the end of the reset.

  Thereafter, the vertical selection pulse φSEL is applied and the selection switch 206 is turned on to select the readout row. When the reading row is selected, the vertical signal line 208 is charged to a potential corresponding to the reset potential of the FD 204.

  In the period t402, the clamp pulse φC is applied and the clamp switch 213 is turned on, whereby the output of the amplifier 211 is clamped to the clamp potential Vfef via the clamp capacitor 210 according to the reset potential of the FD 204. Further, in a period t403, the offset read pulse φTN is applied and the switch 218 is turned on, whereby the reset level of the FD 204 is multiplied by the gain by the amplifier 211 and written to the capacitor 219.

  In the period t404, the transfer pulse φTX is applied. When the transfer pulse φTX is applied, the transfer switch 203 is turned on, the charge accumulated in the PD 202 is transferred to the FD 204, and the vertical signal line 208 is charged to a potential corresponding to the potential of the FD 204. Here, the potential on the output side (the side not connected to the vertical line) of the clamp capacitor 210 is a potential corresponding to the signals of Vfef and PD202. That is, the clamp circuit removes fixed pattern noise of the amplification MOS amplifier 205 for each pixel 201 and reset noise of the reset switch 207.

  Further, in a period t405, a signal is written in the capacitor 221 by applying the signal read pulse φTS and turning on the switch 220.

  When the pulse φHi is applied in the period t406, the switch 225 and the switch 226 are turned on. As a result, the difference between the signal stored in the signal level storage capacitor 221 and the signal stored in the offset storage capacitor 219 is amplified by the differential amplifier 223 and output to the output line 224. By adopting such a configuration, the difference between the signal level and the reset level is amplified and output, so that fixed pattern noise of the image sensor can be reduced, and noise caused by variations in the reset switch of the pixel can be reduced. it can.

  The gain of the amplifier 211 is determined by the ratio of the capacitor 210 and the capacitor 212. Although not shown, the capacitor 212 (changing means) is composed of a plurality of selectively connectable capacitors, and the gain applied by the amplifier 211 can be changed by switching the capacitance value. When a plurality of capacitors are connected and the capacitance value of the capacitor 212 is large, the gain is low. Conversely, when the connected capacitance is small and the capacitance value is small, the gain is high.

  When the high gain is applied, the dark characteristic is also multiplied by the gain, so the voltage Vi is increased to increase the capacity of the constant current source 209.

  The voltage Vi can be generated externally, but may be generated using a current bias circuit inside the apparatus. Here, a case where the voltage Vi is generated using a current bias circuit will be described with reference to FIG.

  FIG. 5 shows an example of a circuit that copies a constant current to the vertical signal line 208 using a resistor. The pulse ΦR + is at a negative level (low level) so that the switch 501 is turned off when the gain is low, and the voltage Vi that determines the constant current capability is finally determined by the current I controlled by the resistor 502. Here, when photographing at a high gain, the current I is determined by the resistors 502 and 503 by setting ΦR + to an active level (high level). That is, the current I is increased by reducing the resistance value by connecting resistors in parallel. Since the voltage Vi increases as the current I increases, the constant current capability increases.

  FIG. 6 is a table showing an example of the current amount of the constant current source 209 that is changed according to the gain of the amplifier 211. When the gain of the amplifier 211 is 1 to 8 times, the constant current source 209 passes 20 μA, when it is 16 times, it flows 30 μA, and when it is 32 times, it flows 40 μA. Thus, by increasing the current only at the time of high gain, it is possible to improve the dark characteristic.

  By the way, the characteristic at the time of darkness fluctuates also with the capability of the amplifier 211. FIG. 7 is a circuit diagram showing a basic differential amplifier. The amplifier 211 can be configured by such a circuit. As indicated by reference numeral 701, the amplifier 211 also has a constant current source (second current source). The capacity of the amplifier 211 can be improved by increasing the current of the constant current source 701 only at high gain.

  FIG. 8 is a table showing an example of the relationship between the current amount of the constant current source 209 changed according to the gain of the amplifier 211 and the current amount of the constant current source 701 of the amplifier 211. When the gain of the amplifier 211 is 1 to 8 times, the constant current source 209 passes 20 μA, when it is 16 times, it flows 30 μA, and when it is 32 times, it flows 40 μA. At the same time, the constant current source 701 passes 30 μA when the gain of the amplifier 211 is 1 to 4 times, and 40 μA when 8 to 32 times.

  In this way, the current amount of the constant current source 701 of the amplifier 211 and the current amount of the constant current source 209 are switched separately. By controlling in this way, it is possible to improve the dark time characteristics due to the insufficient capacity of the constant current source 209 and the dark time characteristics due to the insufficient capacity of the current source 701 of the amplifier 211 as necessary.

  The relationship between the gain and the current amount shown in FIGS. 6 and 8 is an example, and can be changed as appropriate according to the capabilities of the amplifier 211, the constant current source 209, and the constant current source 701.

  Next, with reference to FIG. 5, an example in which the imaging device driven by the above-described driving method is mounted on a digital camera that is an imaging device will be described.

  In FIG. 9, reference numeral 901 denotes a lens unit that forms an optical image of a subject on the image sensor 100, and zoom control, focus control, aperture control, and the like are performed by a lens driving circuit 902. A mechanical shutter 903 is controlled by a shutter control circuit 904. The image sensor 100 for capturing the subject imaged by the lens unit 901 as an image signal has the above-described configuration. An image signal processing circuit 906 performs various corrections on the image signal output from the image sensor 100 and compresses data. A timing generation unit 907 is a driving unit that outputs various timing signals to the imaging device 100 and the imaging signal processing circuit 906. An overall control / calculation unit 909 controls various calculations and the entire imaging apparatus. Reference numeral 908 denotes a memory for temporarily storing image data, reference numeral 910 denotes a recording medium control interface (I / F) for recording or reading on the recording medium, and reference numeral 911 denotes a semiconductor memory for recording or reading image data. It is a removable recording medium. Reference numeral 912 denotes a display unit that displays various information and captured images, 913 denotes a photometric unit (photometric means), and 914 denotes a focus detection unit.

  Next, the operation at the time of shooting in the digital camera having the above configuration will be described.

  When the main power supply is turned on, the power supply for the control system is turned on, and the power supply for the image pickup system circuit such as the image pickup signal processing circuit 906 is turned on.

  Thereafter, when a release button (not shown) is pressed, a high frequency component is extracted based on a signal output from the focus detection unit 914, and the focus state of the subject is detected by the overall control / calculation unit 909. Thereafter, the lens drive circuit 902 drives the lens unit 901 to determine whether or not the lens is in focus based on the signal output from the focus detection unit 914. The unit 901 is driven to determine the in-focus state.

  The shooting operation starts after the in-focus state is confirmed. When starting the shooting operation, the gain of the image sensor 100 is set according to the sensitivity setting set by the camera. The sensitivity setting may be obtained from a photometric value obtained from the photometric result of the photometric unit 913, or may be set by a sensitivity setting button (not shown) (sensitivity setting means).

  When the photographing operation is completed, the image signal output from the image sensor 100 is image-processed by the image-capturing signal processing circuit 906 and written to the memory by the overall control / calculation unit 909. The data stored in the memory 908 is recorded on the recording medium 911 through the recording medium control I / F unit 910 under the control of the overall control / arithmetic unit 909.

  Further, the image may be processed by directly inputting to a computer or the like through an external I / F unit (not shown).

  As described above, according to the present embodiment, the amount of current supplied to the amplifier 211 is changed according to the gain set in the amplifier 211, so that power consumption is minimized and high image quality is achieved even when shooting at high sensitivity. Can be obtained.

1 is a diagram schematically showing an overall configuration of an image sensor in an embodiment of the present invention. FIG. 2 is a circuit diagram illustrating a configuration of one pixel and a configuration for reading a signal from the pixel in the image sensor according to the embodiment of the present invention. It is a circuit diagram which shows the constant current source in embodiment of this invention. FIG. 4 is a timing diagram illustrating a method for driving an image sensor according to an embodiment of the present invention. It is a circuit diagram which shows the structure which produces | generates the voltage supplied to the constant current source in embodiment of this invention. It is a figure showing the relationship between the gain setting of the amplifier 211 in embodiment of this invention, and electric current amount. It is a circuit diagram which shows the structure of the amplifier 211 in embodiment of this invention. It is a figure showing the relationship between the gain setting of the amplifier 211 in embodiment of this invention, and electric current amount. It is a block diagram which shows the structure of the imaging device in embodiment of this invention.

201 pixels 202 photodiode (PD)
203 Transfer switch 204 Floating diffusion part (FD)
205 Amplifying MOS Amplifier 206 Selection Switch 207 Reset Switch 208 Vertical Signal Line 210 Capacitance 211 Amplifier 212 Capacitor 213 Clamp Switch

Claims (6)

A plurality of pixels that output electrical signals obtained by photoelectric conversion;
First amplification means for amplifying the electrical signal;
Second amplifying means for amplifying the electrical signal amplified by the first amplifying means;
Changing means for changing the gain of the second amplifying means;
The imaging device according to claim 1, wherein the first amplifying unit includes a first current source that supplies a current corresponding to the gain changed by the changing unit.   The imaging device according to claim 1, wherein the second amplifying unit includes a second current source.   The imaging device according to claim 1, wherein the first amplifying unit is configured for each of the plurality of pixels.   An imaging apparatus comprising the imaging device according to claim 1. Photometric means;
The imaging apparatus according to claim 4, further comprising a control unit that causes the change unit to change a gain based on a photometric result of the photometry unit. Sensitivity setting means for accepting sensitivity settings from the outside;
The imaging apparatus according to claim 5, further comprising: a control unit that causes the changing unit to change a gain based on the sensitivity set by the sensitivity setting unit.

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