CN109474795A

CN109474795A - A kind of low-noise pixel circuit structure based on transconductance cell

Info

Publication number: CN109474795A
Application number: CN201811282037.2A
Authority: CN
Inventors: 高静; 张德志; 聂凯明; 徐江涛; 史再峰
Original assignee: Tianjin University
Current assignee: Tianjin University
Priority date: 2018-10-31
Filing date: 2018-10-31
Publication date: 2019-03-15

Abstract

The present invention discloses a kind of low-noise pixel circuit structure based on transconductance cell, including transistor M1-M4, transistor M1 drain terminal connect with transistor M2 source, and transistor M1 source connects with the one end photodiode PPD, and transistor M2 drain terminal meets FD node reset level V_res, transistor M3 grid connects in FD node and source terminates at power supply V_DDOne Gm unit of upper formation；Transistor M1 is TG pipe, transfer of the control exposure charge to FD node, transistor M2 is the reset transistor of FD node, transistor M3 leaks termination transistor M4 source, transistor M4 is line EAC SEL, grid connects switching signal, and transistor M4 drain terminal connects one end of switch SH, and another termination of switch SH samples holding capacitor C_sOne end, sample holding capacitor C_sOther end ground connection, and capacitor reset level V is connect by switch RST_RST, voltage output V is connect by switch READ_pix.One aspect of the present invention realizes Pixel-level amplification, can reduce input reference noise, on the other hand can also be achieved gain regulation.

Description

A kind of low-noise pixel circuit structure based on transconductance cell

Technical field

The present invention relates to cmos image sensor technical field of integrated circuits, and in particular to a kind of based on the low of transconductance cell Noise pixel circuit structure.

Background technique

In cmos image sensor field, the source level follower (source follower, SF) of pixel, column grade are related double Sampling (correlated double sampling, CDS), column grade analog-digital converter have contributed much overall noise, therefore It will limit the imaging performance of sensor.In order to solve this problem, high-gain column grade amplifier or minimum are primarily now used Change the capacitor of floating diffusion (floating diffusion, FD) node in pixel to realize.However in voltage domain or charge-domain The middle deterioration that dynamic range is inevitably led to using fixed high-gain amplifier.

Summary of the invention

The purpose of the present invention is being directed to input reference noise existing for cmos image sensor in the prior art, and provide one Low-noise pixel circuit structure of the kind based on transconductance cell.

The technical solution adopted to achieve the purpose of the present invention is:

A kind of low-noise pixel circuit structure based on transconductance cell, comprising:

The source phase of the drain terminal of transistor M1, transistor M2, transistor M3, transistor M4, transistor M1 and transistor M2 It connects, the source of transistor M1 connects with one end of photodiode PPD, photodiode PPD other end ground connection, transistor M2's Drain terminal meets FD node reset level V_res, the grid of transistor M3 connects in FD node and source terminates at power supply V_DDOn, form a Gm Unit；Transistor M1 is TG pipe, and transfer of the control exposure charge to FD node, transistor M2 is the reset transistor of FD node, The leakage of transistor M3 terminates at the source of transistor M4, and transistor M4 is line EAC SEL, and grid meets switching signal, transistor M4 Leakage terminates at one end of switch SH, and another termination of switch SH samples holding capacitor C_sOne end, sample holding capacitor C_sIt is another One end ground connection, and capacitor reset level V is connect by switch RST_RST, voltage output V is connect by switch READ_pix。

The transistor M1 and transistor M2 is NMOS tube, and the transistor M3 and transistor M4 are PMOS tube.

Low-noise pixel circuit structure proposed by the present invention based on transconductance cell, is reduced using amplification method in pixel It inputs reference noise, and can also realize variable gain.

On the one hand low-noise pixel structure proposed by the present invention realizes Pixel-level amplification, can reduce input reference noise, separately On the one hand it can also be achieved gain regulation.

Detailed description of the invention

Fig. 1 is the schematic diagram of the low-noise pixel circuit structure based on transconductance cell；

Fig. 2 is that the principle schematic diagram after CDS is integrated in the low-noise pixel structure based on Gm unit.

Specific embodiment

The present invention is described in further detail below in conjunction with the drawings and specific embodiments.It should be appreciated that described herein Specific embodiment be only used to explain the present invention, be not intended to limit the present invention.

Referring to shown in Fig. 1-2, a kind of low-noise pixel circuit structure based on transconductance cell, including transistor M1-M4, electricity Hold C_s, photodiode PPD, current source I_bias, switch SH, READ, RST, M1-M4 and photodiode PPD constitute pixel list Member, wherein transistor M1-M2 is NMOS tube, and M3-M4 is PMOS tube.The source of one end access transistor M1 of photodiode PPD End, transistor M1 are TG pipe, transfer of the control exposure charge to FD node, the source phase of the drain terminal and transistor M2 of transistor M1 It connects, transistor M2 is the reset transistor of FD node, and the drain terminal of transistor M2 meets FD point reset level V_res, the grid of transistor M3 Pole connects in FD node, and source terminates at power supply V_DDOn, a Gm unit is formed, the leakage of transistor M3 terminates at the source of transistor M4 End, transistor M4 are line EAC SEL, and grid connects switching signal, and transistor M4 leaks the one end for terminating at switch SH, the other end of SH Meet sampling holding capacitor C_s, and capacitor reset level V is connect by switch RST_RST, voltage output V is connect by switch READ_pix。

The course of work are as follows: when the pixel is selected, line EAC conducting, Gm unit is activated, and generates and FD node voltage V_FDProportional electric current, the short time turn on the switch RST after turn off, will sample holding capacitor C_sVoltage amplitude be V_RST, next Turn on the switch SH, electric current is from original levels V_RSTStart to C_sCharging, charging time T_ch, by adjust the charging time, it can be achieved that Variable gain, charging finish, and turn on the switch READ, by C_sOn voltage read, wherein I_biasIt is pixel for column bias current sources Output line provides electric current, can be realized by current-mirror structure.

Dot structure proposed by the present invention is based on Gm unit, Gm unit and sampling holding capacitor C_sGm-C integrator is formed, The voltage signal of pixel FD node is mainly converted into current signal using Gm unit to export, the output current integration of Gm unit Holding capacitor C is sampled in column grade_sUpper generation voltage output.Ignore the non-ideal effects of circuit, time domain output voltage is writeable are as follows:

Wherein, g_mIt is the transconductance value of Gm unit, C_sIt is sampling holding capacitor, n is integer, T_chIt is to sample filling for holding capacitor Electric time, T_sIt is the sampling period, t is C_sThe time of integration, q is C_sCharge, I is the output signal electric current of Gm unit, and dt is micro- Divide symbol, V_FDIt is the voltage of FD node.

In the following, the CDS function to pixel circuit of the invention is illustrated:

1, in order to ensure not related between two neighbouring sample operations, by connecting RST before next new sampling Carry out reset samples holding capacitor.

2 provide enough Gm for Pixel-level, need relatively large-sized transistor M3.

3, the sampling holding capacitor device in each column will consider time constant and gain requirement, while be ensured that relevant KT/C noise is not dominant.

Fig. 2 shows CDS (column grade correlated-double-sampling is integrated in the low-noise pixel structure based on Gm unit (correlated double sampling, CDS)) function schematic diagram.

As shown in Fig. 2, the reset signal of pixel reads as V by reset signal access_reset+V_RST, pixel exposure signal is logical Overexposure photo-signal channel reads as V_signal+V_RST, two voltages are subtracted each other by voltage subtraction, obtain signal output V_pix= V_signal-V_reset, partial noise can also be reduced by CDS function.

Low-noise pixel structure proposed by the present invention based on Gm unit replaces the SF of conventional pixel by using Gm unit For structure, it can be achieved that enlarging function inside pixel, reducing input reference noise will not be in pixel without changing reading out structure Inside introduces extra resistance and capacitor；In addition, by adjusting capacitor charging time T_ch, variable gain may be implemented.

The above is only a preferred embodiment of the present invention, it is noted that for the common skill of the art For art personnel, various improvements and modifications may be made without departing from the principle of the present invention, these improvements and modifications Also it should be regarded as protection scope of the present invention.

Claims

1. a kind of low-noise pixel circuit structure based on transconductance cell characterized by comprising

The drain terminal of transistor M1, transistor M2, transistor M3, transistor M4, transistor M1 connect with the source of transistor M2, brilliant The source of body pipe M1 connects with one end of photodiode PPD, photodiode PPD other end ground connection, the drain terminal of transistor M2 Meet FD node reset level V_res, the grid of transistor M3 connects in FD node and source terminates at power supply V_DDOn, it is mono- to form a Gm Member；Transistor M1 is TG pipe, and transfer of the control exposure charge to FD node, transistor M2 is the reset transistor of FD node, brilliant The leakage of body pipe M3 terminates at the source of transistor M4, and transistor M4 is line EAC SEL, and grid connects switching signal, transistor M4 leakage One end of switch SH is terminated at, another termination of switch SH samples holding capacitor C_sOne end, sample holding capacitor C_sIt is another End ground connection, and capacitor reset level V is connect by switch RST_RST, voltage output V is connect by switch READ_pix。

2. the low-noise pixel circuit structure based on transconductance cell according to claim 1, which is characterized in that the crystal Pipe M1 and transistor M2 is NMOS tube, and the transistor M3 and transistor M4 are PMOS tube.

3. a kind of low-noise pixel circuit structure based on transconductance cell characterized by comprising transistor M1, transistor M2, The drain terminal of transistor M3, transistor M4, transistor M1 connect with the source of transistor M2, the source of transistor M1 and two pole of photoelectricity One end of pipe PPD connects, and photodiode PPD other end ground connection, the drain terminal of transistor M2 meets FD node reset level V_res, brilliant The grid of body pipe M3 connects in FD node and source terminates at power supply V_DDOn, form a Gm unit；Transistor M1 is TG pipe, and control exposes Transfer of the optical charge to FD node, transistor M2 are the reset transistor of FD node, and the leakage of transistor M3 terminates at transistor M4 Source, transistor M4 is line EAC SEL, and grid connects switching signal, and transistor M4 leakage terminates at reset signal access and exposure One end of signal path；Reset signal access is identical as the circuit of exposure signal access, one end of other end voltage subtraction circuit Another termination voltage output V of subtraction circuit is most pressed in connection_pix, reset signal access and exposure signal access include and crystal Another termination of the one end for the switch SH that the source of pipe M4 connects, switch SH samples holding capacitor C_sOne end, sampling keep electricity Hold C_sOther end ground connection, and capacitor reset level V is connect by switch RST_RST, voltage subtraction circuit is connect by switch READ One end.

4. the low-noise pixel circuit structure based on transconductance cell according to claim 3, which is characterized in that the crystal Pipe M1 and transistor M2 is NMOS tube, and the transistor M3 and transistor M4 are PMOS tube.