The present invention relates to an iris control circuit for a video camera.
In a video camera, the exposure setting must be correct at all times in order to correctly reproduce a wide variety of scenes with high fidelity. In order to meet this requirement, recent video cameras have been provided with a so-called "automatic iris mechanism" which automatically controls the degree of opening of video camera iris to hence determine the exposure setting.
An example of a control circuit used in a conventional automatic iris mechanism of this type is shown in FIG. 1. In this circuit, an iris control signal, which may, for instance, be derived from a luminance signal component of a composite video signal, is applied to the input terminal P. In response thereto, a drive coil L.sub.1 connected to the output terminal of an operational amplifier A.sub.0 drives a video camera iris (not shown). In order to smoothly drive the iris, a brake coil L.sub.2 is provided which is wound on the core (not shown) of the drive coil L.sub.1. The brake coil L.sub.2 is connected to an input terminal of the operational amplifier A.sub.0. With this arrangement, the core operates in association with the driving of the iris in such a manner that a brake torque is applied to the iris which varies according to the driving speed of the iris.
The conventional circuit, because it requires both a drive coil and a brake coil, takes up a relatively large amount of mounting space. Accordingly, the lens mounting structure where the iris is positioned is necessarily large, which makes it difficult to miniaturize the video camera.
As described above, the coils L.sub.1 and L.sub.2 are mounted on a common core. In general, the core should be a flat, plate-shaped core which is rotatably supported, and the driving coil L.sub.1 and the braking coil L.sub.2 are wound on opposite end portions of the core. The resulting structure is a movable-magnet type structure in which the outputs of the coils L.sub.1 and L.sub.2 are nonlinear with respect to the movemet of the core. Accordingly, when it is attempted to perform all the various necessary control operations of the servo system utilizing the outputs of the coils L.sub.1 and L.sub.2, various difficulties occur, with the result that the manufacturing cost is accordingly increased.
In view of the foregoing, an object of the invention is to provide a video camera iris control circuit which is so designed that a video camera can be easily miniaturized and in which the output of an iris driving coil has a linear characteristic.
USPTO Backgrounds
USPTO Backgrounds