A CIRCUIT FOR DETECTING SIGNAL CHARGES TRANSFERRED IN A CHARGE TRANSFER DEVICE

Abstract: The signal charge detector includes a charge-voltage con­ verter, a pre-charge transistor pre-charging the charge-voltage converter to a first voltage, a first amplifier receiving the voltage at the charge-voltage converter, a clamp circuit having a capacitor coupled with the output of the first amplifier at one end thereof and a gate for supplying a clamp voltage to the other end of the capacitor in response to a clamping pulse and a second amplifier for amplifying the output from said clamp circuit, and a means for producing an output signal in accord­ ance with the output from the second amplifier.

Description: This invention relates to a circuit for detecting signal charges transferred in a charge transfer device such as a charge coupled device (hereinafter, referred to as a CCD), and particularly to an improvement thereof for producing an output signal proportional to a value of signal charges. Description of the Prior Art: A CCD linear image sensor has a line of solid-state photosensors in which charges generated in response to a light irradiated therein are accumulated and two charge shift registers arranged in parallel with and disposed on opposite sides of the line of solid-state photosensors. The charges accumulated in alternate photosensors are applied to one charge shift register and those accumulated in the other photosensors are to the other charge shift register. The charges applied to the charge shift registers are transferred in the shift registers toward one ends thereof at which a signal charge detector is formed. The transferred charges are converted into a signal of voltage form in response to their value by the signal charge detector. The signal charge detector in the prior art has one or two sets of charge-voltage converter and MOS FET circuits. The charge-voltage converter is a P-N junction formed in a semiconductor substrate. The MOS FET circuit is a first stage amplifier receiving the converted voltage, a second stage amplifier amplifying the output of the first stage amplifier, a sampling circuit of the output of the second stage amplifier, an output stage amplifier amplifying the sampled value to output an output signal and a pre-charge FET for charging the charge-voltage converter. Here, it is noted that in the case of using two FET circuits, an output stage amplifier is commonly used for two FET circuits. First, the charge-voltage converter is charged by applying a reset pulse to the pre-charge FET. Thereafter, the transferred charges are introduced to the charge-voltage converter to apply a potential change of the charge-voltage converter to the first stage amplifier and then to the second stage amplifier. The output of the second stage amplifier is sampled in a capacitor in the sampling circuit in response to a sampling pulse. The sampled voltage signal is derived from the output stage amplifier. The output of the signal charge detector in the prior art is affected by the gate capacitance of the pre-charge FET. When the reset pulse is applied, the charge-voltage converter is charged to a predetermined voltage. However, in response to the removal of the reset pulse, the charged voltage at the charge-voltage converter decreases by a value determined by the gate capacitance. Therefore, if two FET circuits are used in a signal charge detector, since the gate capacitances of the pre-charge FET's have a great deviation due to uncontrollable manufacturing condition, the output voltages from two FET circuits differs from each other. IEE Journal of Solid State Circuits Vol. SC-9, No. 1, February 1974, pages 1-13 and Vol. SC-12, No. 6, December 1977, pages 626-632 and US-A-4.355.244 disclose signal charge detectors in which the voltage at the input to a second stage amplifier is clamped in order to control the reference level of the output signal. The output of the signal charge detector in the prior art involves the signal components based on dark current flowing in the CCD shift registers and the solid-state photosensors. This signal components should be subtracted from the output signal to obtain a true output signal which is proportional to signal charges. It is an object of the present invention to provide a signal charge detector of solid-stage imaging device which produces an output signal having a reference voltage such that the output signal is free from dark current component of the solid-state photosensors. According to the present invention there is provided a signal charge detector comprising: a charge-voltage converter for receiving charges and producing a signal of voltage form; means for charging said charge-voltage converter to a first predetermined voltage; a first stage amplifier having an input connected to said charge-voltage converter and to said charging means and an output; a second stage amplifier having an input and an output; a clamp circuit connected between said output of said first stage amplifier and said input of said second stage amplifier, said clamp circuit having a capacitor connected between said output of said first stage amplifier and said input of said second stage amplifier, switching means connected between said input of said second stage amplifier and a first terminal for closure in response to a clamping pulse, and means connected to said first terminal for applying a second predetermined voltage to said first terminal; and means for producing an output signal from said second stage amplifier, characterised in that said means for applying said second predetermined voltage produces said second predetermined voltage by comparing a sampled voltage of said output signal with a third predetermined voltage. According to the invention, a signal charge detector has a clamp circuit between first and second stage amplifiers. After the charge-voltage converter is pre-charged to the first predetermined voltage, the voltage at the input of the second stage amplifier is clamped at the second predetermined voltage by the clamp circuit. The voltage across the first capacitor is fixed at a voltage difference between the clamped voltage, i.e. the first predetermined voltage, and the voltage at the input of the first stage amplifier which is determined by the pre-charge voltage, i.e. the second predetermined voltage. The fixed voltage is added to or subtracted from the voltage generated in response to the signal charges to make the reference voltage of the output signal constant. In a case where two sets of the charge-voltage converter, the pre-charging circuit, the first stage amplifier, the clamp circuit and the second stage amplifier are used to combine output signals from two charge coupled devices, the reference voltage levels of the output signals may be made equal. In another case where one signal charge detector is used to derive an output from charge coupled device, the dark current component may be removed from the output by selecting the reference voltage level of the output signal adequately. The invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a circuit diagram of a signal charge detector in the prior art; Fig. 2 is a timing chart for explaining the operation of the signal charge detector shown in Fig. 1; Fig. 3 is a timing chart for explaining a drawback of the signal charge detector shown in Fig. 1; Fig. 4 is a circuit diagram of a signal charge detector according to a first embodiment of the present invention; Fig. 5 is a timing chart for explaining the operation of the signal charge detector according to the first embodiment of the present invention; Fig. 6 is an explanatory view of the solid-state imaging device in the prior art; Figs. 7 and 8 are timing charts for explaining the operation of the solid-state imaging device shown in Fig. 6; Fig. 9 is a circuit diagram of a signal charge detector according to a second embodiment of the present invention; Figs. 10 and 11 are timing charts for explaining the operation of the signal charge detector according to the second embodiment of the present invention; and Fig. 12 is a circuit diagram of a signal charge detector according to a third embodiment of the present invention. DESCRIPTION OF THE INVENTION In a linear solid-state imaging device in the prior art, charges generated in alternate photosensors are applied to two charge shift registers A and B, as shown in Fig. 1. After the charges are transferred through the charge shift registers A and B by applying clock pulses ₁ and ₂ to gate electrodes 32, 33, 22 and 23, they are introduced to diffusion regions 2 and 12 of the conductivity type opposite to the channel region of the shift registers A and B in accordance with pulses V OG applied to output gates 21 and 31 to convert their charge value to voltage form signals. The voltage form signals are amplified and combined by a MOS FET circuit to obtain a series of output signals. The MOS FET circuit has two sets of a transistor (1, 11) for pre-charging the diffusion regions 2 and 12, a first stage amplifier of transistors 3 and 4 (13 and 14), a second stage amplifier of transistors 5 and 6 (15 and 16) and a sampling transistor 9 (10) and one set of a sampling capacitor C h and an output amplifier of transistors 7 and 8. The operation of the MOS FET circuit will be explained with reference to Figs. 1 and 2. First, the transistor 1 is made conductive by a reset pulse  R ₁ to precharge the diffusion region 2 (timings t₁, t₅) and then is made non-conductive by removal of the reset pulse  R ₁ (timings t₂, t₆). The voltage V₁₀ at the interconnection 41 rises to V OD at the timings t 1 , t 5 . But it drops by the voltage ▵⊘ R1 at the timings t₂, t₆. The gate capacitance C C1 of the transistor 1 causes this voltage drop. Thereafter, charges transferred to the portion under the gate electrode 21 are introduced into the diffusion region 2 in response to the application of the pulse V OG to the output gate 21. The voltage V 10 at the interconnection 41 changes by a value V S1 which is proportional to the charge value introduced into the diffusion region 2, at the timing t₃. The changed voltage V 10 is amplified by the first stage amplifier and then by the second stage amplifier. The output from the second stage amplifier is sampled in the sampling capacitor C h at the timing t₄, in response to the application of sampling pulse ⊘ SH1 . The sampled voltage is derived as an output signal V out from the output stage amplifier. The other set of MOS FET circuit in the signal charge detector operates similarly but with a phase different from the above-explained operation. Therefore, the sampling capacitor alternately samples the outputs from two sets of MOS FET circuits. The output signal V out involves alternately the sampled voltage of the output signals from the two MOS FET circuits. Thus, a series of output signals is obtained. However, one problem arises due to the gate capacitances C C1 and C C2 of the transistors 1 and 11. As explained above, after the removal of the reset pulses ⊘ R1 and ⊘ R2 , the voltages V 10 and V 20 respectively drop by the voltages ▵⊘ R1 and ▵⊘ R2 . These voltage drops are based on voltages necessary to charge the gate capacitances C C1 and C C2 . Those gate capacitances C C1 and C C2 are easy to deviate due to manufacturing error. Therefore, the voltages V 10 and V 20 after the removal of the reset pulses ⊘ R1 and ⊘ R2 differ from each other by the voltage ▵V, as shown in Fig. 3. This differences causes a change of the output signal V out . That is, as shown in Fig. 3, the output signal levels change alternately, even if the signal charges transferred through the shift registers A and B are same. The first embodiment of the present invention shown in Fig. 4 improves this change in output signal level. The same reference numerals are used to the same components as those in the prior art shown in Fig. 1. According to this embodiment, clamp circuits of capacitors C 1 and C₂ and MOS FET's 17 and 18 are added between the first and second stage amplifiers. The capacitor C₁ (C₂) is inserted between the output point 43 (44) of the first stage amplifier and the gate of the MOS FET 5 (15) in the second stage amplifier. A source of the MOS FET 17 (18) is connected to the interconnection of the capacitor C₁ (C₂) and the gate of the MOS FET 5 (15). A clamp voltage V CL is applied to a drain of the MOS FET 17 (18). To gates of the MOS FET's 17 and 18, clamp pulses ⊘ CL1 and ⊘ CL2 are respectively applied. The operation of the first embodiment will be explained with reference to Figs. 4 and 5. After the removal of reset pulse ⊘ R1 (timing t 11 ), the clamp pulse ⊘ CL1 is applied to the gate of the MOS FET 17 to clamp the voltage V 12 at the clamp voltage V CL (timing t₁₂). Since the voltage V 11 at the output point 43 is a voltage level V 11B , the capacitor C₁ is charged by a voltage V 11B - V CL . Thereafter, the charges transferred through the shift register A are introduced to cause a voltage change in the voltage V 11 in accordance with signal charge value. The voltage V₁₂, accordingly, drops from the clamped voltage V CL by the voltage V SH2 which is proportional to the signal charge value. The dropped voltage V₁₂ is then sampled in the sampling capacitor C h by application of the sampling pulse ⊘ SH1 to the transistor 9 (timing t₁₄). The same operation is performed in the other MOS FET circuit with a different phase. In this procedure, although the voltages V 11 and V₁₂ after the removal of the reset pulses ⊘ R1 and ⊘ R2 differ by the difference of gate capacitances C C1 and C C2 of the transistors 1 and 11, the voltages V₁₂ and V₂₂ drops from the constant clamp voltage V CL by voltages proportional to the signal charge value. Therefore, the obtained output signal V out has a constant reference level. In other words, when the signal charges are constant, the output signal V out does not change in level and is not affected by the difference of gate capacitances of the transistors 1 and 11. Fig. 6 is another solid-state imaging device in the prior art in which one MOS FET circuit is used as the signal charge detector. A plurality of photosensing elements P 1 , P 2 .... P₂₀₄₈ and D₁, D₂ .... D 6 are disposed in a line. The photosensitive element D₁, D₂ .... D₄ are shaded to detect a dark current component. Charges accumulated in alternate photosensitive elements are applied to CCD shift register 51 in response to an application of transfer pulse ⊘ TG to a transfer gate 53. Charges accumulated in the other photosensitive element are similarly applied to another CCD shift register 52. Charges applied to the CCD shift registers 51 and 52 are transferred therethrough by the clock pulses ⊘ 1 and ⊘ 2 . Charges arrived at an output gate 56 are introduced into a diffusion region 54 having a conductivity type opposite to the channel region of the CCD shift registers 51 and 52, in response to the application of a pulse ⊘ OG to the output gate 56, to convert the charge value to a voltage form signal. The voltage form signal is further applied to a MOS FET circuit 55 to derive an output signal V out . This MOS FET circuit 55 has a transistor Q `1 receiving a reset pulse ⊘ R , a first stage amplifier of transistors Q 2 and Q 3 , a second stage amplifier of transistors Q₄ and Q 5 , a sampling circuit of a transistor Q₆ receiving a sampling pulse ⊘ SH and a sampling capacitor C₁ and an output stage amplifier of transistors Q 7 and Q₈, and is similar to the circuit in Fig. 1 consisting of the transistors 1, 3, 4, 5, 6, 7, 8, 9 and the capacitor C h , except for using depletion type FET's as the transistors Q 1 , Q₃, Q₅ and Q₈. The explanation of basic operation is omitted by showing the timing chart of clock pulses ⊘ 1 and ⊘ 2 , reset pulse ⊘ R , voltage V₁ at the gate of the transistor Q 2 , sampling pulse ⊘ SH and the voltage V₂ at the output terminal, in Fig. 7. The problem accompanied with the imaging device in the prior art shown in Fig. 6 is based on dark currents flowing in the CCD shift registers 51 and 52 and in the photosensitive elements P 1 .... P₂₀₄₈ and D₁ .... D₆. This will be explained with refference to Fig. 8. The voltage V₂ is highest during the time period from t₁ to t₄ when the diffusion region 54 is pre-charged by the reset pulse ⊘ R . After the removal of the reset pulse ⊘ R , the shift registers 51 and 52 are operated with the clock pulses ⊘ 1 and ⊘ 2 . Thus, charges based the dark current flowing are introduced into the diffusion region 54 to cause a voltage drop V R in the voltage V₂ (period from the timing t₄ to timing t₅). Following the timing t 5 , charges accumulated in the shaded photosensitive elements D₁ .... D₆ are introduced into the diffusion region 54. The charges from the shaded photosensitive elements D₁ .... D₆ are accumulated in all the photosensitive elements irrespectively of the light radiator, i.e. dark current component and cause a further voltage drop V S in the voltage V₂. From the timing t₆ charges accumulated in the photosensitive elements P 1 .... P₂₀₄₈ are introduced into the diffusion region 54 to cause output V' out at the output terminal. This output V' out involves the voltages V R and V S in addition to the voltage V 0 based on the signal charges generated in the photosensitive elements P₁ .... P₂₀₄₈. The voltage components V R and V S must be subtracted from the output V' out by the additional circuit to derive an out V 0 . However, since the voltages V R and V S are based on dark currents which have temperature dependences, the additional circuit must have a very complicated circuit structure. For example, the voltages V R and V S become double in response to the temperature increment of 8°C. The MOS FET circuit 61 shown in Fig. 9 as a second embodiment of the present invention derives an output signal V out (V₃₀) free from the dark current component. The circuit 61 has additional circuit of a clamp circuit 62 and a clamp level control circuit 63, compared to the circuit 55 in the prior art shown in Fig. 6. The gate of the transistor Q 2 is connected with the diffusion region 54 of Fig. 6. The clamp circuit 62 is inserted between the first stage amplifier of transistors Q 2 and Q₃ and the second stage amplifier of transistors Q 4 and Q₅ and is made of a capacitor C 11 and a transistor Q₁₄. The transistor Q 14 has a source connected to the gate of the transistor Q 4 and the capacitor, a gate receiving a clamp pulse ⊘ CL and a drain connected to the output of the clamp level control circuit 63. The clamp level control circuit 63 has a transistor Q 20 having a source connected to the output of the output stage amplifier of transistors Q₇ and Q₈, a gate receiving a pulse ⊘ B , a first differential amplifier A₁ having a noninverting input connected to a drain of the transistor Q 20 and an inverting input and an output connected with each other, a second differential amplifier A₂ having an inverting input connected with the output of the first differential amplifier A₁ through a resistor R₁, a noninverting input receiving a divided voltage of the clamp voltage V CL by the resistors R 2 and R₃ and an output connected with the inventing input thereof through a parallel circuit of a resistor R 4 and a capacitor C₁₃ and a third differential amplifier having a noninverting input connected with the output of the second differential amplifier A₂ and an inverting input and an output connected with each other. The clamp voltage V CL is applied at the terminal 65. All the differential amplifiers A₁, A₂ and A₃, all the resistors R 1 , R₂, R₃ and R₄, the transistor Q₂₀ and the capacitor C₁₃ are formed in a semiconductor integrated circuit. A capacitor C 14 is added to the integrated circuit through the terminal 64 and connected to the noninverting input of the first differential amplifier A₁. The operation will be next explained with reference to Fig. 10. The diffusion region 54 (Fig. 6) is precharged at the timing t₁. The charges accumulated in photosensitive elements P 1 .... P₂₀₄₈ and D₁ .... D₆ are applied to the CCD shift registers 51 and 52 in the period from the timing t₂ to the timing t₃. The pre-charging terminates at the timing t₄. At she same time, the charge transfer through the CCD shift registers 51 and 52 starts. In the time period until the charges from the shaded photosensitive elements D 1 .... D₆ are introduced into the diffusion region 54, the voltage V₃₀ at the output terminal drops the voltage V R due to the dark currents flowing through the CCD shift registers 51 and 52. The charges from the shaded photosensitive elements begin to be introduced into the diffusion region 54 at the timing t₅ and the voltage V₃₀ further drops by the voltage V S due to the dark current in the photosensitive elements. The pulse ⊘ B is applied to the transistor Q 20 in the time period from the timing t d1 to the timing t d2 to charge the capacitor C₁₄. Here, the operation of the clamp level control circuit 63 will be explained with reference to Fig. 11. As a result of the application of the pulse ⊘ B , the voltage V₄₀ at the terminal 64 becomes the same voltage as the voltage V₃₀. Since the output and the inverting input of the first differential amplifier A₁ is directly connected, the voltage V₅₀ at the output thereof becomes the same voltage as the voltage V₄₀. That is, the voltages V₃₀ and V₅₀ become same. The voltage V₅₀ is compared with the clamp voltage V CL by the first differential amplifier A2. If the voltage V₅₀, i.e. the voltage V₃₀, is lower than the clamp voltage V CL , the voltage V₆₀ at the output of the second differential amplifier A₂ rises. The same voltage appears at the output of the third differential amplifier A₃ as the voltage V₇₀. In response to the clamp pulse ⊘ CL , the voltage V₃₀ is clamped at the voltage V₇₀. Due to the clamp, the voltage V₃₀ is rised at the time of next sampling pulse ⊘ SH . If the rised voltage V₃₀ is still lower than the clamp voltage V CL , the above operation is repeated until the voltages V₃₀ and V CL become equal. The gain of the second differential amplifier A₂ is designed so that the voltages V₃₀ and V CL may become equal within the time period from the timing t d1 to the timing t d2 which equals the pulse width of the pulse ⊘ B . Such gain control is easily achieved by controlling the resistance of the resistor R₄. On the contrary, if the voltage V 30 is higher than the clamp voltage V CL , the voltage V₇₀ has a lower voltage and the voltage V₂₀ is clamped to this lower voltage V₇₀. As a result, the voltage V₃₀ decreases and finally becomes the same voltage as the clamp voltage V CL . Under the condition where the voltage V₃₀ is clamped to the clamp voltage V CL , the charges from the photosensitive elements P 1 .... P₂₀₄₈ are introduced into the diffusion region 54 from the timing t₆. Therefore, the voltage V₃₀ changes from the clamp voltage V CL in accordance with the charges from the photosensitive elements P₁ .... P₂₀₄₈. The voltage difference of the voltage V₃₀ from the clamp voltage V CL is proportional to the signal charge value, irrespective of the voltages V R and V S due to dark currents. The third embodiment of the present invention shown in Fig. 12 does not clamp the output voltage V₃₀ to the clamp voltage V CL , but clamps the voltage V₂₀ at the gate of the transistor Q 4 in the second stage amplifier to the clamp voltage V CL . The clamp circuit 71 is added to the circuit in the prior art shown in Fig. 6. Compared to the second embodiment shown in Fig. 9, the clamp level control circuit 63 is omitted. The clamp circuit 71 has a capacitor C₁₁ and a transistor Q 14 . The clamp voltage V CL is applied to the drain of the transistor Q 14 . The clamp pulse ⊘ CL is applied to the gate of the transistor Q 14 at the time period from the timing t CL1 to the timing t CL2 as shown in Fig. 10. As a result, the voltage V₂₀ is clamped at the clamp voltage V CL . Following such voltage clamp, the charges from the photosensitive elements P 1 .... P₂₀₄₈ are introduced into the diffusion region 54 (Fig. 6). The voltage V₂₀ changes from the above clamped voltage V CL in accordance with the signal charges accumulated in the photosensitive elements P₁ .... P₂₀₄₈. Therefore, the output voltage V₃₀ changes from some voltage corresponding to the clamp voltage V CL by a value corresponding to the signal charge value. According to the third embodiment, the output signal having a fixed reference level is obtained with a simple circuit. The fixed reference level may be adjusted by controlling the clamp voltage V CL . Therefore, the circuit is preferably applicable to control the black level in facsimile equipment. Although the above-explained embodiments are examples applying the present invention to solid-state linear imaging devices, the present invention is equally applicable to solid-state area imaging devices. The transistor elements may be either N-type or P-type.

Claims: A signal charge detector comprising: a charge-voltage converter for receiving charges and producing a signal of voltage form; means for charging said charge-voltage converter to a first predetermined voltage; a first stage amplifier having an input connected to said charge-voltage converter and to said charging means and an output; a second stage amplifier having an input and an output; a clamp circuit connected between said output of said first stage amplifier and said input of said second stage amplifier, said clamp circuit having a capacitor connected between said output of said first stage amplifier and said input of said second stage amplifier, switching means connected between said input of said second stage amplifier and a first terminal for closure in response to a clamping pulse, and means connected to said first terminal for applying a second predetermined voltage to said first terminal; and means for producing an output signal from said second stage amplifier, characterised in that said means for applying said second predetermined voltage produces said second predetermined voltage by comparing a sampled voltage of said output signal with a third predetermined voltage. A signal charge detector as claimed in claim 2, characterised in that said switching means is a transistor having a control electrode connected to a second terminal to receive said clamping pulse and providing an electrical path between said input of said second stage amplifier and said first terminal, the conductivity of said electrical path being controlled by said clamping pulse. A signal charge detector as claimed in claim 1 or 2, characterised in that said means for applying said second predetermined voltage comprises means for sampling said output signal to produce said sampled voltage, means for producing said third predetermined voltage, means for comparing said sampled voltage with said third predetermined voltage, and means for producing said second predetermined voltage in accordance with the output from said comparing means.

Inventors: MIWADA, KAZUO C/O NEC CORPORATION
Applicants: NEC CORPORATION
Application year: 1984
Main classification: G11C19
IPC codes: H03K5, G11C19