CLAMPING CIRCUIT

Abstract: A clamping circuit suitable for an image processing is disclosed. This clamping circuit includes a differential output circuit (13) to which an input signal is applied, series-connected two MOS type gate circuits (14,15) respectively having gate electrodes to which an output signal from the differential output circuit to be clamped and a reference voltage (V REF ) of a target value are applied, and reset circuit (18) including a holding capacitor (17) connected to the drain electrode side of either of the MOS type gate circuits (14,15) to discharge charge in the holding capacitor (17) at the time of beginning of clamping operation. The clamping circuit further includes a current source (16) connected to the junction between the MOS type gate circuits (14,15), charge current flow control circuit (19) for accumulating charge in the holding capacitor (17), and feedback circuit (20) for feeding back a potential of the holding capacitor (17) to the differential output circuit (13). Thus, a small-sized clamping circuit which can perform stable operation is provided.

Description: BACKGROUND OF THE INVENTION This invention relates to a clamping circuit or a clamper suitable for image processing. For setting an analog output level to a desired external set voltage as in the case of giving a reference black level to a charge coupled device (CCD) output, a clamping circuit is used. In a typical prior art clamping circuit, two differential amplifiers connected in a loop and a holding capacitor provided at the junction between the differential amplifiers are used. Such a device is described in US-A-4 513 321 and is shown schematically in Fig. 1. Since a differential amplifier used in such a circuit is required to have a high gain, the circuit construction becomes complicated and the occupation area on a semiconductor device is also large, thereby preventing miniaturization of the device. In addition, an overshoot is produced during the clamping operation because the differential amplifier has a high gain, so that the entire system is likely to become unstable. Depending upon circumstances, defective phenomena such as oscillation, etc. may occur. SUMMARY OF THE INVENTION Therefore, an object of this invention is to provide a clamping circuit which can have a small size because of a simple construction, and which can perform a stable operation. In accordance with this invention, there is provided a clamping circuit comprising: a differential amplifier having a first input terminal to which an input signal is applied; characterized in that said clamping circuit further comprises two MOS type gate circuits connected in series, means for applying an output signal from said differential amplifier to be clamped and a reference voltage which is a clamping target value to respective gate electrodes of said MOS type gate circuits; a holding capacitor and reset means connected to a drain side of one of said MOS type gate circuits to charge or discharge charges in said holding capacitor respectively at a time when a clamping operation is initiated; a current source connected to the junction between said MOS type gate circuits; charge flow control means for allowing the accumulation of charge flowing from said current source into said holding capacitor; and feedback means for feeding back the potential of said holding capacitor to a second input terminal of said differential amplifier; said charge flow control means operating by allowing charge to flow into said holding capacitor until equilibrium is reached between said two MOS type gate circuits after a reset operation by said reset means has been completed. In such a clamping circuit, the fact that charge moves toward a deeper side of potential wells below two MOS type gate circuits connected in series having gate electrodes to which an output from the differential output circuit and a reference voltage are input, respectively is utilized to provide a feedback of the charge, thus to hold the charge when the depths of the potential wells below both MOS type gate circuits are equal to each other. Thus, the circuit construction becomes simple. This is advantageous to miniaturization of a semiconductor device. In addition, since the differential output circuit is not required to have high sensitivity, an unstable operation such as oscillation is eliminated. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG. 1 is a circuit diagram of a conventional clamping circuit; FIG. 2 is an output waveform diagram showing the operation of the conventional clamping circuit shown in FIG. 1; FIG. 3 is a circuit diagram showing an embodiment of a clamping circuit according to this invention; FIG. 4 is an output waveform diagram showing the operation of the clamping circuit shown in FIG. 3; FIG. 5 is a circuit diagram showing a modification of the clamping circuit shown in FIG. 3; FIG. 6 is a circuit diagram showing another embodiment of a clamping circuit according to this invention; FIG. 7 is an output waveform diagram showing the operation of the clamping circuit shown in FIG. 6; and FIG. 8 is a circuit diagram showing a modification of the clamping circuit in FIG. 6. DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to the description of this invention, an outline of the prior art will be described with reference to FIG. 1 showing a typical conventional clamping circuit. This circuit includes two differential amplifiers 3 and 4 which are connected in a loop. A signal line 1 for transmitting a control signal is connected to the plus (+) side of the differential amplifier 3, and an output of the differential amplifier 4 is connected to the minus (-) side of the differential amplifier 3 through a switch 5 shown as a FET switch, and to a holding capacitor 6. An output line 2 of the differential amplifier 3 is connected to the plus (+) side of the differential amplifier 4, and a reference voltage V ref is inputted to the minus (-) side of the differential amplifier 4. The operation of the clamping circuit mentioned above will be described with reference to FIG. 2. For brevity of explanation, it is assumed in this example that an internal signal is a d.c. voltage in this example. When the switch 5 is turned on at time t₁, a difference between an output voltage V out and a reference voltage V ref is fed back to the differential amplifier 3. Thus, the output voltage V out of the differential amplifier is clamped to a definite voltage which is equal to the reference voltage V ref in this case. An output from the differential amplifier 4 at this time is held by the holding capacitor 6. Thus, the clamping state is maintained. Since two differential amplifiers are used in this circuit, the circuit construction becomes bulky, thereby preventing miniaturization of the device. In addition, since the differential amplifiers used have a large gain, unstable operation such as oscillation is likely to occur. Several embodiments according to this invention will now be described in detail with reference to the accompanying drawings. In a circuit shown in FIG. 3, two MOS type gate circuits 14 and 15 are connected in series and a junction therebetween is connected a current source 16. An operation completion gate circuit 19 controlled by a pulse Φ B is connected to the junction. An output line 12 of a differential amplifier 13 is connected to the gate electrode of the MOS type gate circuit 14. A reference voltage V ref is applied to the gate electrode of the MOS type gate circuit 15. In addition, a holding capacitor 17 and a reset gate circuit 18 controlled by a pulse Φ A are connected on the drain electrode side of the MOS type gate circuit 14. The junction among the reset gate circuit 18, the MOS type gate circuit 14 and the holding capacitor 17 is connected to the plus (+) side input terminal of the differential amplifier 13, acting as an output circuit, through a level shifter 20 for making a level adjustment. It is to be noted that the MOS type gate circuits 14 and 15 have substantially the same construction and their characteristics are substantially the same. The operation of the circuit shown in Fig. 3 will be described with reference to FIG. 4 showing the waveforms of applied pulses Φ A and Φ B , and the output waveform V out . For brevity of description, it is here assumed that a d.c. voltage is applied to a control signal line 11. At time t₁₁, pulse Φ A applied to the gate electrode of the reset gate circuit 18 builds up to a high value. For an interval up to the time t₁₂, an electric charge (electrons) in the holding capacitor 17 is discharged, thus bringing the capacitor 17 to a floating state. At this time, pulse Φ B applied to the gate electrode of the operation completion gate circuit 19 is maintained at a high level, e.g., 12 volts. Where a voltage at the junction between the reset gate circuit 18 and the MOS type gate circuit 14 is 12 volts, the output voltage V out is about 8 volts. When it is assumed that the reference voltage is 5 volts, current flows into the drain electrodes of MOS type gate circuits 14 and 15 via the operation completion gate circuit 19 because the potential well below the operation completion gate circuit 19 is the deepest. Since setting can be made such that a relation V out >V ref holds at the time of initiation of comparison, a description will be made assuming such a relation. Then, at time t₁₃, the pulse Φ B builds down. Thus, the charge in capacitor 17 flows toward the MOS type gate 14 having a potential well deeper than that of the MOS type gate circuit 15, thus allowing its potential well to become shallower. The charge also flows into the holding capacitor 17, thus lowering its potential. This potential is fed back to the differential amplifier 13 through the level shifter 20. As long as the output V out of the differential amplifier 13 is larger than V ref , feedback action is continued so that V out is decreased. Such a feedback is stopped when a relation V out =V ref holds, i.e., the depths of the potential wells below the MOS gate circuits 14 and 15 are substantially equal to each other thus manifesting an equilibrium state. Since the above described action occurs irrespective of a quantity of charge in the holding capacitor, as a principle, the differential amplifier 13 is not required to have any gain. After reaching this equilibrium state, the charge from the current source 16 discharges to the drain electrodes of the MOS type gate circuits via the potential well below the MOS type gate circuit 15. Thus, clamping operation is completed. Thereafter, when the pulse Φ B builds up to the high level at time t₁₄, the charge discharges to the drain electrodes of MOS type gate circuits 14 and 15 via the operation completion gate circuit 19, so that no change in the holding capacitor occurs even if any change in V out occurs. Thus, stable clamping operation is ensured. This clamping operation has a speed of the order of 10 µs. As previously described, the feedback operation is conducted only when the relation V out > V ref holds. Any feedback operation is not caused under the condition of V out ≦V ref , so that any oscillation is not produced. It is to be noted that, strictly speaking, the point at which flow of the charge is stopped is not the point where the relationship of V out =V ref holds, but has an error as described below. Namely, as described above, flow of charges in the holding capacitor is stopped when the charge has been completely discharged to the drain electrode through the potential well below the MOS gate circuit 15. Denoting the width and the length of the gate electrode of the MOS type gate circuit 15 by W and L, respectively, the ratio therebetween is expressed by W/L, and when the modulation factor of the potential well and a current value of the current source are denoted by m and I, respectively, the relationship holds: I = A·(W/L)·(mΔV)² where A is a constant. When V out is reduced by ΔV, viz., when the following equation holds, the flow of the charge will be stopped: V out = V ref - ΔV. An error ΔV in this case is expressed as follows: ΔV = 1/m√ I/A(W/L) . An actual stop point is shifted by this error ΔV. To reduce such an error to a value permissible to the system, it is sufficient to set I to a small value W/L to a large value. FIG. 5 shows a modification of FIG. 3 wherein the charge flowing into the holding capacitor 17 is controlled by the operation completion gate circuit 21 on-off controlled by pulse Φ B . Namely, when the operation completion gate circuit 21 is turned on even in a case wherein the potential well directly below the MOS type gate circuit 14 is deeper than the potential well directly below the MOS gate circuit 15, the comparison operation is completed. A clamping value corresponding to the charge in holding capacitor at that time will be outputted. FIG. 6 is a circuit diagram showing another embodiment of this invention. In this figure, the connection relationship of the MOS type gate circuits 14 and 15, the constant current source 16, and the operation completion gate 19 is the same as that in the case shown in FIG. 3. This embodiment differs from that shown in FIG. 3 in that MOS type gate circuit 15 having a gate electrode to which a reference voltage is applied and a reset gate circuit 18 are connected in series, and holding capacitor 17 and level shifter 20 connected to the minus (-) side of the output differential amplifier 13 are connected to the junction between gate circuits 15 and 18 and that a control signal is applied to the plus (+) side of the differential amplifier 13. FIG. 7 is a waveform diagram showing the clamping operation of the circuit shown in FIG. 6. Before time t₂₁, the holding capacitor 17 is in a charge accumulated state. Since a voltage corresponding to the charge is applied to the minus (-) side of the differential amplifier 13, the output voltage V out is maintained at a potential higher than V ref . During an interval between time t₂₁ and t₂₂, when reset pulse Φ A is applied to the reset gate circuit 18, the charge discharges into the drain electrode of gate circuit 14 via gate circuit 15. Thus, the potential of V out is rapidly lowered, resulting in a state where the relation V out <V ref holds. Assuming now that the pulse Φ B applied to the operation completion gate circuit 19 builds down to a low level at time t₂₃, the junction between the MOS type gate circuits 14 and 15 is brought into a floating state. Because the relation V out <V ref holds as described above, so that charge flows into the holding capacitor 17 via the well below the MOS gate circuit 15, thus lowering its potential. Since this potential is applied to the minus (-) side of the differential amplifier 13 through the level shifter 20, the output V out gradually increases. The increase of the output V out is stopped at the time when an equation V out =V ref holds. By the the charge in the holding capacitor at this time, the output voltage V out is kept at a fixed value. Also in this case, the circuit becomes operative only when the output is lower than V ref . Thus, the possibility of oscillation, etc. is completely eliminated. FIG. 8 is a circuit diagram showing another modification of the embodiment shown in FIG. 6. In this embodiment, an operation completion gate circuit 22 controlled by pulse Φ B is serially inserted between MOS type gate circuits 15 and the holding capacitor 17. Only when the operation completion gate circuit 22 is in an ON state, comparison between an output V out and a reference voltage V ref is made. As just described, the operation completion gate circuit 22 is required only to control the charge flowing into the holding capacitor 17. For this reason, as shown in Figs. 5 and 8, the operation completion gate circuit 22 can be connected between one of two MOS type gate circuits 14 and 15 and the holding capacitor 17, so that, excellent operation can be performed. In addition, by giving a value nearly equal to the clamping value of the holding capacitor to a drain electrode of the MOS type gate circuit 15 for application of a reference voltage in the arrangements shown in FIGS. 3 and 5, a high speed operation can be realized. While a reference voltage applied to one of MOS type gate circuits employs a target value as it is in the above-mentioned embodiments, a voltage expressed as (V ref -ΔV) may be used as a reference voltage by taking an error in operation into account, thus making it possible to further improve the clamping accuracy.

Claims: A clamping circuit comprising: a differential amplifier (13) having a first input terminal to which an input signal is applied; characterized in that said clamping circuit further comprises two MOS type gate circuits (14,15) connected in series, means for applying an output signal from said differential amplifier (13) to be clamped and a reference voltage which is a clamping target value to respective gate electrodes of said MOS type gate circuits; a holding capacitor (17) and reset means (18) connected to a drain side of one of said MOS type gate circuits to charge or discharge charges in said holding capacitor respectively at a time when a clamping operation is initiated; a current source connected to the junction between said MOS type gate circuits (14,15); charge flow control means (19,21,22) for allowing the accumulation of charge flowing from said current source into said holding capacitor; and feedback means for feeding back the potential of said holding capacitor to a second input terminal of said differential amplifier (13); said charge flow control means operating by allowing charge to flow into said holding capacitor (17) until equilibrium is reached between said two MOS type gate circuits after a reset operation by said reset means has been completed. A clamping circuit as set forth in claim 1, wherein said feedback means includes a level shifter (20) for adjusting the level of the feedback potential. A clamping circuit as set forth in claim 1, wherein said reference voltage is selected so that it is equal to a value obtained by calculating the value of, and correcting for an error in operation. A clamping circuit as set forth in claim 1, wherein said two MOS type gate circuits (14,15) connected in series have the same construction. A clamping circuit as set forth in claim 1, wherein said charge flow control means (19,21,22) comprises an operation completion MOS type gate circuit (19) with its source electrode connected to said junction, and which operates to drain charge from said junction after completion of clamping. A clamping circuit as set forth in claim 1, wherein said charge flow control means (19,21,22) comprises a further MOS type gate circuit (21,22) connected in series between said reset means and said two MOS type gate circuits (14,15) connected in series. A clamping circuit as set forth in claim 1, wherein said charge flow control means (19,21,22) comprises a MOS gate circuit (19) connected to said junction.

Inventors: GOTO, HIROSHIGE
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: H04N5
IPC codes: H04N5, H03K5