1. Field of the Invention
The present invention relates to charge-coupled devices; and more particularly, to a circuit and method for increasing the direct injection bandwidth of a CCD.
2. Description of the Prior Art
In the conventional CCD device having an input section consisting of an input diffusion diode and adjacent electrodes, one method of injecting a signal from a signal source into a CCD is to apply the signal to the input diffusion diode. This is the direct current injection method. When the direct injection method is used, the input section of the CCD can be analyzed as though it were a common gate MOSFET (Metal-Oxide-Semiconductor-Field-Effect-Transistor), that is, the input section of a CCD consisting of an input diffusion diode, input electrode and subsequent electrodes which make up the first stage of the CCD function similarly to the source, gate, and drain of an MOSFET. The bandwidth of a signal injected into the CCD by the direct injection method is called the direct injection bandwidth.
When the direct injection mode of operation is used, the direct injection bandwidth of the CCD is proportional to the level of direct current provided by the signal source. Where the signal source is a photodiode, for example, the level of current provided by the photodiode is a function of the photon flux level of the radiation impinging upon the photodiode. At the low flux levels and contrast ratios in the useful IR bands, it is difficult to achieve sufficient signal bandwidth for applications requiring high signal bandwidth such as for TV data rate display in forward-looking-infrared (FLIR) systems, for example. In the past, a boost amplifier was used at the output of the CCD in order to compensate for the low bandwidth produced by low flux levels. But, the use of boost amplifiers in this manner increases the noise levels at the CCD output consequently degrading the performance of the device.
It is desired that a simple method and circuit be provided to increase the direct injection bandwidth of a CCD without thereby increasing noise levels and degrading device performance.