Input circuit for CCD delay line

Abstract: The circuit comprises for a charge-coupled device (CCD) delay line a semiconductor substrate (4), a CCD delay line (1) formed on the semiconductor substrate, and first and second registers (2,3) each having substantially the same maximum charge handling capacity as that of the CCD delay line. An input portion of the first register is given the same structure as that of the CCD delay line, and output portions of the first and second registers have substantially the same structure each other. A control circuit acts on the second register (1) so that an output signal from the latter becomes a signal corresponding to the maximum charge handling capacity. A comparator circuit (5) analyzes the output signals of the first and second registers, and produces an output signal which is fed back to an input source of the input portion of the first register (2) so that the output signal from the first register becomes equal to the output signal from the second register. A high level clock signal voltage is supplied to an input gate of the first register, and a voltage of the input source of the first register is supplied to an input source of an input portion of the CCD delay line (1). Thus, an overflow of charge can be avoided satisfactorily.

Description: Field of the invention. The present invention generally relates to a charge-coupled device (i.e., CCD) delay device which is used to delay an analog signal such as a video signal or the like and is more particularly directed to an input circuit for a CCD delay line. Description of the Prior Art . A modern television receiver (or the like) utilizes a CCD delay device formed of a CCD delay line to delay an analog signal such as a video signal. In the CCD delay line of the gate input type, when an input signal of large level (e.g., level slightly higher than the level corresponding to the maximum amount of charge that the CCD delay line can handle) is supplied thereto, an overflow of charge occurs in a charge transfer portion of the CCD delay line, which leads to various disadvantages. In order to avoid the charge overflow, it is proposed in the prior art to limit the level of the input signal by a limiter circuit provided independently of the CCD delay device. However, in accordance with this previously-proposed method in which the limiter circuit is provided independently of the CCD delay line to limit the level of the input signal, the maximum amount of charge handled by the CCD delay line can vary depending on the CCD delay line characteristics, so that the limiting level of the limiting circuit must be adjusted in concert with the CCD delay line, which is very inconvenient. Furthermore, the CCD delay line has a temperature characteristic, so that when the temperature is changed, the level K controlled by the limiting circuit and the maximum acceptable charge amount inputted to the CCD delay line will diverge, so creating a difference. This difference will cause an overflow of charge if the change of temperature is too large. OBJECTS AND SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved input circuit for a CCD delay line in which the aforenoted shortcomings and disadvantages of the prior art can be eliminated. More specifically, it is another object of the present invention to provide an input circuit for a CCD delay line in which an overflow of charge can be avoided satisfactorily. It is still another object of the present invention to provide an input circuit for a CCD delay line with applications in delaying an analog signal such as a video signal or the like. As an aspect of the present invention there is provided a charge-coupled device (CCD) delay line system, comprising: a semiconductor substrate; a CCD delay line formed on said semiconductor substrate; first and second registers formed on said semiconductor substrate, an input portion of said first register being formed to have substantially the same structure as the input portion of said CCD delay line and output portions of said first and second registers being formed to have substantially the same structure as each other, the CCD delay line, first and second registers each having an input source; comparator means for comparing output signals of said first and second registers; means for supplying a high level clock voltage signal to an input gate of said first register; and means for supplying a voltage of the input source of said first register to the input source of said CCD delay line; characterised in that: the first and second registers have substantially the same maximum charge handling capacity as said CCD delay line; and there is provided: control means for controlling the second register so that an output signal from said second register becomes a signal corresponding to said maximum charge handling capacity; means for feeding an output signal of said comparator means back to the input source of said first register so that the output signal from said first register becomes equal to the output signal from said second register; and means for applying a signal to be delayed by the CCD delay line from an input signal source to an input gate electrode of said CCD delay line. Thus, an overflow of charge can be avoided satisfactorily. The preceding and other objects, features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments thereof to be taken in conjunction with the accompanying drawings, in which like reference numerals identify the same or similar elements. BRIEF DESCRIPTION OF THE DRAWINGS The following detailed description, given by way of example and not intended to limit the present invention solely thereto, will best be understood in conjunction with the accompanying drawings in which : Fig. 1 is a cross-sectional view illustrating an embodiment of an input circuit for a CCD delay line according to the present invention; Fig. 2 is a plan view of the input circuit shown in Fig. 1; Fig. 3 is a cross-sectional view illustrating a modified example of a main portion of the input circuit for CCD delay line according to the present invention; and Fig. 4 is a cross-sectional view illustrating a further modified example of the main portion of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention will be described hereinafter with reference to the drawings. Fig. 1 shows a cross-sectional view of a first embodiment of the input circuit for a CCD delay line according to the present invention, and Fig. 2 shows a plan view thereof. In Figs. 1 and 2, reference numeral 4 designates a p-type silicon semiconductor substrate. The p-type silicon semiconductor substrate 4 has formed thereon a CCD delay line 1 through which a signal charge is transferred. The length of the CCD delay line 1 is determined in accordance with a delay time. Further, on the p-type silicon semiconductor substrate 4, there are formed a first register 2 having the same width a as the width a of the CCD delay line 1 and which has a predetermined length , and a second register 3 having the same width a and the same length as the first register 2. In that case, the widths a of the first and second registers 2 and 3 are selected to be equal to the width a of the CCD delay line 1 as mentioned above, and the maximum amounts of charge handled by the first and second registers 2 and 3 and the CCD delay line 1 are made equal. The CCD delay line 1 in this embodiment is formed of an n-channel BCCD (buried-channel CCD) of a double layer electrode structure which is operated in a two-phase system. The CCD delay line 1 and the first and second registers 2 and 3 are each formed of an n -type layer formed on the surface of the p-type silicon semiconductor substrate 4. An input source region 6 of n+ layer is formed on one end of the input portion of the CCD delay line 1. On the CCD delay line 1, there are formed,through an SiO₂ insulating layer 7, first and second input gate electrodes 8 and 9 made of impurity-doped polycrystalline silicon and a number of transfer electrodes 10 forming a first layer and a number of transfer electrodes 11 forming a second layer. The transfer electrodes 10 and 11 forming the first and second layers and the first and second input gate electrodes 8 and 9 are electrically insulated by interlayer insulating layers 12 made of SiO₂. An input portion of the first register 2 is constructed substantially the same as that of the CCD delay line 1. The input portion of the first register 2 has formed at one end thereof an input source region 6a which is identical to the input source region 6 of the CCD delay line 1. The first register 2 has formed thereon through an SiO₂ insulating layer 7, first and second input gate electrodes 8a and 9a, each of which is made of impurity-doped polycrystalline silicon, a transfer electrode 10a forming a first layer and a transfer electrode lla forming a second layer. The transfer electrodes 10a and 11a forming the first and second layers and the first and second input gate electrodes 8a and 9a are electrically insulated by interlayer insulating layers 12 made of SiO₂. The transfer electrodes 10a and 11a of the first and second layers are extended on the second register 3 so as to be usable as transfer electrodes of first and second layers of the second register 3. An output portion of the first register 2 has formed on the other end thereof a floating diffusion layer 13 made of an n+ layer. Further, a pre-charge drain region 14 is formed in the p-type silicon semiconductor substrate 4 at its portion distant from the floating diffusion layer 13 by a predetermined spacing. In addition to the first and second input gate electrodes 8a, 9a and the transfer electrodes 10a, 11a forming the first and second layers, the first register 2 has formed thereon an output gate electrode 15 and a pre-charge gate electrode 16, each of which is made of impurity-doped polycrystalline silicon. The output gate electrode 15 and the pre-charge gate electrode 16 are extended up to the second register 3 so as to be usable as an output gate electrode and a pre-charge gate electrode of the second register 3. An input source region 17 of an n+ layer is formed on the input portion at one end of the second register 3. A floating diffusion region 18, which is identical to the floating diffusion region 13 of the first register 2, is formed on the output portion at the other end of the second register 3. A pre-charge drain region 19, which is identical to the pre-charge drain region 14 of the first register 2, is formed away from the floating diffusion region 18 by a predetermined spacing. In addition to the transfer electrodes 10a, 11a forming the first and second layers, the output gate electrode 15 and the pre-charge gate electrode 16, the second register 3 has formed thereon transfer electrodes 20 and 21 of first and second layers made of impurity-doped polycrystalline silicon at the positions corresponding to the first and second input gate electrodes 8a and 9a of the first register 2. The pre-charge drain regions 14 and 19 of the first and second registers 2 and 3 are used to accumulate charges of the floating diffusion regions 13 and 18 in association with the opening and closing operation of switches 25 and 30 (which will be described later). The accumulation of charges is controlled by the pre-charge gate electrode 16. The floating diffusion region 13 of the first register 2 is connected through a source follower transistor 24 formed of metal-oxide-semiconductor field-effect transistors (i.e., MOSFETs) 22 and 23 and the switch 25 to an inverting input terminal (-) of an operational amplifier circuit 5 which forms a comparing circuit. Similarly, the floating diffusion region 18 of the second register 3 is connected through a source follower transistor 29 formed of MOSFETs 27 and 28 and the switch 30 to a non-inverting input terminal (+) of the operational amplifier circuit 5 which forms the comparator circuit. An output terminal of the operational amplifier circuit 5 is grounded via a series circuit comprising a resistor 31 and a capacitor 32. A junction between the resistor 31 and the capacitor 32 is coupled to the input source region 6a of the first register 2. Thus, a compared output signal from the operational amplifier circuit 5 is fed back to the input source region 6a of the first register 2. In this embodiment, an input signal source 33 is connected to the second input gate electrode 9 of the CCD delay line 1, and the second input gate electrode 9a of the first register 2 is supplied with a voltage provided when a clock signal is at high level, for example, a power source voltage V DD of 5 Volts. Further, in this embodiment, the input source region 17 of the second register 3 is coupled with a battery 34 which generates a dc voltage. The voltage of the battery 34 is selected to be such that charge equal to the maximum charge handling capacity of the second register 3, - i.e., charge equal to the maximum charge handling capacity of the CCD delay line 1 - can always be transferred to the second register 3. The charge transfers of the CCD delay line and the first and second registers 2 and 3 are performed by supplying predetermined two-phase clock signals 1 and 2 to the pairs of the transfer electrodes 10, 10a and 20 of the first layer and the transfer electrodes 11, 11a and 21 of the second layer in a predetermined sequential order. The transfer direction of charge is determined by changing a concentration of impurities in the semiconductor substrate 4 beneath the transfer electrodes 10, 10a, 20 of the first layer and the transfer electrodes 11, 11a, 21 of the second layer to form asymmetric potential wells. In this embodiment, the voltage supplied to the input source region 6a of the first register 2 is supplied to the input source region 6 of the CCD delay line 1. The first input gate electrodes 8 and 8a of CCD delay line 1 and the first register 2 are supplied with the clock signal 2 whose phase is different from that of the clock signal 1 supplied to the transfer electrodes 11, 10, 11a and 10a of the output side of the second input gate electrodes 9 and 9a. The input circuit for the CCD delay line of this embodiment is constructed as set out above. According to this embodiment, a charge transferred through the second register 3 and arriving at the floating diffusion region 18 is converted into a voltage by the source follower transistor 29 and is then sampled and held by the switch 30. As described above, the operational amplifier circuit 5 is supplied at the non-inverting input terminal (+) thereof with the voltage of magnitude corresponding to the maximum charge handling capacity of the second register 3, i.e. the voltage of magnitude corresponding to the maximum charge handling capacity of the CCD delay line 1. In the early stage of operation, no charge exists in the first register 2 so that the voltage supplied to the inverting input terminal (-) of the differential amplifying circuit 5 goes high in level. A feedback voltage of magnitude corresponding to the difference between the voltage supplied to the non-inverting input terminal (+) and the inverting input terminal (-) of the operational amplifier circuit 5 is supplied to the input source region 6a of the first register 2 from the output terminal of the operational amplifier circuit 5, whereby the charge quantity of the input source region 6a is determined by this feedback voltage. Under this condition, the clock signal 2 is supplied to the first input gate electrode 8a of the first register 2 and the voltage V DD , provided when the clock signals 1 and 2 are at high level, and is supplied to the second input gate electrode 9a, whereby charge is transferred to the first register 2 formed under the transfer electrodes 10a and 11a of the first and second layers. The charge thus transferred is finally brought to the floating diffusion region 13 via the transfer electrodes 10a and 11a forming the first and second layers. Since the transfer electrodes 10a and 11a of the first and second layers and the output gate electrode 15 are made common to the first and second registers 2 and 3, the charge corresponding to the maximum charge handling capacity of the second register 3 is transferred to the floating diffusion region 18 of the second register 3 at the same cycle as when the charge is transferred to the floating diffusion region 13 of the first register 2. The charges respectively transferred to the floating diffusion region 13 of the first register 2 and the floating diffusion region 18 of the second register 3 are converted into voltages by the source follower transistors 24 and 29, respectively, and are sampled and then held at the same time slot so that the operational amplifier circuit 5 is supplied at its inverting input terminal (-) with the voltage of magnitude corresponding to the amount of charge transferred through the first register 2 and at its non-inverting input terminal (+) with the voltage of magnitude corresponding to the maximum charge handling capacity of the second register 3. The operational amplifier circuit 5 generates at its output terminal a feedback voltage corresponding to a difference between the voltages supplied to the inverting input terminal (-) and the non-inverting input terminal (+). The amount of charge accumulated in the input source region 6a of the first register 2 is controlled by the feedback voltage from the output terminal of the operational amplifier circuit 5. As described above, the feedback voltage is generated from the output terminal of the operational amplifier circuit 5 so that the amount of charge transferred through the first register 2 becomes equal to the amount of charge transferred through the second register 3. This feedback voltage is then supplied to the input source region 6a of the first register 2. Therefore, in the stationary state, the amount of charge transferred through the first register 2 is always kept equal to the maximum amount of charge transferred through the second register 3. The voltage supplied to the input source region 6a of the first register 2 is supplied to the input source region 6 of the CCD delay line 1, whereby the voltage of the input source region 6 of the CCD delay line 1 is usable as a source for providing the charge quantity corresponding to the maximum charge handling capacity of the first register 2. Thus, regardless of the magnitude of the input signal supplied to the second input gate electrode 9 of the CCD delay line 1, charge in amounts exceeding the charge handling capacity is inhibited from being supplied to the charge transfer portion of the CCD delay line 1, thereby avoiding the overflow of charge in the charge transfer portion of the CCD delay line 1. Further, the input structures of the CCD delay line 1 and the first register 2 are made the same and the output structures of the first and second registers 2 and 3 are also made the same on the common semiconductor substrate 4. Thus, when the temperature is changed, the input portion of the CCD delay line 1 and the input portion of the first register 2 are both affected in the same fashion, and similarly the output portion of the first register 2 and the output portion of the second register 3 are both affected in the same fashion. Therefore, it is possible to prevent the charges of the charge transfer portion of the CCD delay line 1 from overflowing as a result of the change of temperature. In the above-described embodiment, the charges transferred to the floating diffusion regions 13 and 18 of the first and second registers 2 and 3 are converted into the voltages by the source follower transistors 24 and 29 and these voltages are compared in level by the operational amplifier circuit 5, which can be modified as follows. Fig. 3 shows a modified example of Fig. 1, and in Fig. 3, like parts corresponding to those of Fig. 2 are marked with the same references and shall therefore not be described in detail. As shown in Fig. 3, floating gate electrodes 36 and 37 made of polycrystalline silicon are respectively formed on the first and second registers 2 and 3 via the insulating layer 7, and these floating gate electrodes 36 and 37 are connected to the source follower transistors 24 and 29, respectively. In that case, the charges transferred through the first and second registers 2 and 3 can be detected as image charges generated in the floating gate electrodes 36 and 37. A MOSFET 38 is employed as a reset switch, and the potential of the floating gate electrode 36 is controlled by turning ON or OFF this reset switch. The charges in the floating gate electrodes 36 and 37 are converted into the voltages by the source follower transistors 24 and 29 and the two voltages are compared in level by the operational amplifier circuit 5 similarly to the manner described above. The main portion of this invention may be further modified. As an example, shown in Fig. 4, resistors 39 and 40 are respectively connected between the pre-charge drain regions 14, 19 and the voltage source V DD and voltage drops thereacross are compared by the operational amplifier circuit 5. While in the above-mentioned embodiment the CCD delay line is formed of the n-channel BCCD of double-layer structure which is operated in two-phase system, the present invention is not limited thereto. It is needless to say that a CCD delay line formed of a surface channel CCD or a CCD delay line formed of three-phase or multi-phase structure CCD delay line can be employed. Further, it is possible to employ a CCD delay line of single electrode structure or a CCD delay line formed of multi-layer electrode structure of more than three-layer. According to the present invention, since the voltage supplied to the input source of the CCD delay line 1 serves as the source for supplying an amount of charge corresponding to the maximum charge handling capacity of the transfer portion of the CCD delay line 1, the upper limit of charge transferred to the transfer portion of the CCD delay line 1 corresponds to the maximum charge handling capacity. There is then the substantial advantage that, even when the large input signal is supplied, the overflow of charge can be avoided. Furthermore, since the first and second registers 2 and 3 for determining the voltage of the input source of the CCD delay line 1 are formed on the same semiconductor substrate 4 as used for the CCD delay line 1, the CCD delay line 1 and the first and second registers 2 and 3 are affected by changes in temperature in the same fashion. Therefore, regardless of the change of temperature, the overflow of charge can be avoided in the charge transfer portion of the CCD delay line 1.

Claims: A charge-coupled device (CCD) delay line system, comprising: a semiconductor substrate (4); a CCD delay line (1) formed on said semiconductor substrate; first and second registers (2,3) formed on said semiconductor substrate (4), an input portion of said first register (2) being formed to have substantially the same structure as the input portion of said CCD delay line (1) and output portions of said first (2) and second (3) registers being formed to have substantially the same structure as each other, the CCD delay line (1), first (2) and second (3) registers each having an input source (6,6a,17); comparator means (5) for comparing output signals of said first (2) and second (3) registers; means for supplying a high level clock voltage signal to an input gate (8a) of said first register (2); and means for supplying a voltage of the input source of said first register (2) to the input source of said CCD delay line (1); characterised in that: the first (2) and second (3) registers have substantially the same maximum charge handling capacity as said CCD delay line (1); and there is provided control means (34) for controlling the second register (3) so that an output signal from said second register (3) becomes a signal corresponding to said maximum charge handling capacity; means (31) for feeding an output signal of said comparator means (5) back to the input source (6a) of said first register (2) so that the output signal from said first register (2) becomes equal to the output signal from said second register (3); and means for applying a signal to be delayed by the CCD delay line from an input signal source (33) to an input gate electrode (9) of said CCD delay line (1). The CCD delay line system according to claim 1, wherein said semiconductor substrate (4) is a p-type silicon semiconductor substrate. The CCD delay line system according to claim 1, wherein a width (a) of said CCD delay line is the same as the widths of said first (2) and second (3) registers. The CCD delay line system according to claim 1, wherein said CCD delay line (1) is an n-channel buried CCD of double layer electrode structure. The CCD delay line system according to claim 1, wherein said comparing means (5) is an operational amplifier circuit.

Inventors: MIURA HISANORI C O SONY CORPOR, NARABU TADAKUNI C O SONY CORPO, MIURA, HISANORI, C/O SONY CORPORATION, NARABU, TADAKUNI, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: G11C27
IPC codes: G11C27, G11C19, H01L21, H01L29