JP2803171B2 - Signal output circuit of charge transfer element - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a signal output circuit suitable for a charge transfer element operating at high speed.

[Conventional technology]

In recent years, a solid-state imaging device using a charge transfer device such as a charge-coupled device (hereinafter referred to as a CCD) has a tendency to increase the number of pixels in order to improve resolution. Accordingly, signal reading per pixel must be performed at high speed. For example, in response to the NTSC television standard, the readout speed per pixel of a CCD two-dimensional solid-state imaging device that is about 800 pixels horizontally and about 500 pixels vertically is relatively low, 14 MHz.
However, in response to the HDTV system,
The readout speed per pixel of a CCD two-dimensional solid-state imaging device having 2000 pixels and approximately 1000 pixels arranged vertically is as high as 74 MHz.

Factors that limit the high-speed operation of the CCD are considered to be deterioration in transfer efficiency in the shift register, delay in transfer of signal charges from the shift register to the floating diffusion region for charge detection, and insufficient band of the on-chip output amplifier. The present invention deals with an improvement in a signal output circuit that compensates for a band shortage of an on-chip output amplifier. FIG. 2 is a schematic diagram of a signal output circuit conventionally used to compensate for a band shortage of an on-chip output amplifier, and illustrates a periphery of an output portion of a CCD shift register and a signal output circuit associated therewith. In the case of a CCD two-dimensional solid-state imaging device, as is well known, an imaging unit including a photodiode group and a CCD vertical shift register group is coupled to the upper part of the illustrated CCD shift register, but is not directly related to the present invention. Therefore, it is not shown here. In the figure, 1 is the final charge transfer electrode of the CCD shift register transfer pulse phi _H is applied. Also adjacent to the final charge transfer electrode 1, and the output gate electrode 2 to which a DC voltage V _OG is applied, the floating diffusion region 3, and the reset gate electrode 4 in which the reset pulse phi _R is applied, the DC voltage V _RD is The reset drain 5 to be applied is connected in series. The potential change in the floating diffusion region 3 is converted into a voltage signal via an on-chip output amplifier 6 composed of a two-stage source follower circuit composed of MOS transistors. The feature of this conventional example is that the bipolar transistor 8 is mounted on the same package 7 as the CCD shift register in order to prevent the frequency band from deteriorating due to the increase in the load capacitance of the on-chip output amplifier 6. In this case, an output signal from the on-chip output amplifier 6 is input to the base of the bipolar transistor 8 via the bonding pads 9 and 10 at the shortest distance, thereby minimizing an increase in load capacitance due to stray capacitance of the wiring. Holding down. Here, a bonding pad corresponding to the emitter of the bipolar transistor 8 is provided between the bonding pad 9 and the bonding pad 10.
A normal bonding wire connects between the bonding pad 11 of the package 7 and the bonding pad 13 corresponding to the collector of the bipolar transistor 8 and the bonding pad 12 of the package 7.
Further, a current source 15 is connected to an output terminal from the bonding pad 12 and the bonding pad 14 is grounded, so that the bipolar transistor 8 and the current source 15 are connected.
Thus, an emitter follower circuit is formed, and the bonding pad 12 outputs an impedance-converted voltage signal.

[Problems to be solved by the invention]

However, in the above-described CCD signal output circuit, when a device having a sufficiently large gain bandwidth product is used as the bipolar transistor 8, oscillation easily occurs,
Alternatively, there is a disadvantage that the high-frequency side frequency characteristics abnormally rise and the high-frequency noise of the CCD becomes conspicuous. These phenomena are caused by the formation of a positive feedback loop between the emitter and the base of the bipolar transistor 8. In general discrete circuits, several tens of ohms
The above phenomenon is prevented beforehand by connecting the resistors in series. However, in the CCD signal output circuit, there is no effective means of connecting a resistor in series to the base of the bipolar transistor 8 other than connecting the bonding pads 9 and 10 with a bonding wire having a resistance component. It was a major obstacle.

The present invention eliminates the above-mentioned disadvantages of the related art, and has as its object to provide a signal output circuit suitable for a charge transfer element that operates at high speed.

[Means for solving the problem]

The present invention relates to a signal output circuit of a charge transfer element including an output amplifier formed of a MOS transistor and a bipolar transistor having an output terminal connected to a base, wherein the bipolar transistor is a semiconductor formed with the output amplifier. The amplifier is formed on a semiconductor substrate different from the substrate, and a resistance element formed on the same semiconductor substrate as the amplifier is inserted between the output terminal of the output amplifier and the base of the bipolar transistor.

[Action]

By connecting a resistance element in series to the base of the transistor and selecting its resistance value to an appropriate value, usually several tens Ω,
It is difficult to form a positive feedback loop between the emitter and the base of the bipolar transistor.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram showing the periphery of an output section of a CCD shift register and a signal output circuit associated therewith according to an embodiment of the present invention. 2, the same reference numerals as in FIG. 2 denote the same components, and the structure and operation of each component are the same as those in the conventional example shown in FIG. The difference between the present embodiment and the conventional example is that a resistor 16 formed on the same semiconductor substrate as the CCD is connected in series to the output terminal of the on-chip output amplifier 6. That is, the output terminal of the on-chip output amplifier 6 and one end of the resistance element 16 are connected by an electrode wiring such as an aluminum layer. The other end of the resistance element 16 is also connected to the bonding pad 9 by an electrode wiring. Therefore, by connecting the bonding pads 9 and 10 with normal bonding wires, the resistance element 16 is connected in series to the base of the bipolar transistor 8. Here, by selecting the resistance value of the resistance element 16 to an appropriate value, usually several tens of ohms, it becomes difficult to form a positive feedback loop between the emitter and the base of the bipolar transistor 8, and the signal output including the bipolar transistor 8 It is possible to prevent an increase in wide-area noise caused by an oscillation phenomenon in the circuit and an abnormal rise on the high frequency side of the frequency characteristic. Further, the wiring length from the output terminal of the on-chip output amplifier 6 to the base of the bipolar transistor 8 via the resistance element 16 can be reduced because the resistance value is as described above, and the bipolar transistor 8 has a sufficient gain bandwidth product. Since a large device can be selected, the entire signal output circuit can be sufficiently widened. The mounting of the bipolar transistor 8 can be made of a normal mounting material such as silver paste, and the bonding pads 9, 10, 11.
The wiring between the wirings 12 and 13 and 13 and 14 can be made by ordinary bonding wires, so that the number of assembling steps is reduced and the reliability is excellent as in the conventional example.

Next, although various types of resistance elements 16 are conceivable, a diffusion resistance formed by diffusing impurities of a conductivity type opposite to the conductivity type of the silicon substrate, or a film resistance such as a polysilicon film may be used. Can be done. Note that the film resistance is somewhat advantageous since the parasitic capacitance can be reduced.

〔The invention's effect〕

As described above, according to the present invention, by connecting an on-chip resistive element to the output terminal of the on-chip output amplifier of the charge transfer element, the bipolar device mounted on the same package as the charge transfer element is connected. Oscillation phenomena and high-frequency noise in signal output circuits, including transistors, can be prevented, and devices with large gain-bandwidth products can be selected as transistors mounted on the same package. Become. In this embodiment, the case where the buffer transistor is mounted on the same package as the charge transfer element has been described as an example. However, the present invention is also effective when this transistor is not mounted. That is, by setting the resistance value of the resistive element to the same value as the characteristic impedance of the cable or the like connected to the output terminal of the resistive element, it is possible to drive with impedance matching.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a CCD shift register output section and a signal output circuit according to an embodiment of the present invention, and FIG. 2 is a conventional CCD shift register.
FIG. 3 is a schematic diagram illustrating a shift register output unit and a signal output circuit. 1 ... The last charge transfer electrode of the CCD shift register, 2 ...
Output gate electrode, 3 ... Floating diffusion region, 4 ... Reset gate electrode, 5 ... Reset drain, 6 ... On-chip output amplifier, 7 ... CCD shift register package, 8 ... Mounted on package 7 Bipolar transistors, 9-14 …… bonding pads, 16 ……
A resistance component formed on the same substrate as the CCD shift register and the on-chip output amplifier 6.

Claims (3)

(57) [Claims] 1. A signal output circuit for a charge transfer device comprising: an output amplifier comprising a MOS transistor; and a bipolar transistor having an output terminal connected to a base, wherein the bipolar transistor is formed of the output amplifier. A resistor formed on a semiconductor substrate different from the semiconductor substrate, and a resistance element formed on the semiconductor substrate of the output amplifier is inserted between an output terminal of the output amplifier and a base of the bipolar transistor. Signal output circuit of the charge transfer element. 2. The signal output circuit according to claim 1, wherein the resistance element is a diffusion resistance. 3. The signal output circuit according to claim 1, wherein the resistance element is a film resistance.