1. Field of the Invention
This invention relates to an automatic gain control circuit (hereunder referred to simply as an AGC circuit) and more particularly to an AGC circuit adapted for an apparatus for measuring a physical quantity relating to energy of light or the like such as a reflectivity (or reflectance) and transmissivity (or transmittance) of an object.
2. Description of the Related Art
FIG. 5 shows an AGC circuit of related art adapted for, specially, a sensing circuit for measuring a reflectivity and transmissivity of an object. In the circuit of this figure, when a light receiving element 1 generates a signal corresponding to a monitored quantity of light, an optical-to-electric (O/E) conversion circuit 2 converts the generated signal into a monitoring signal representing the monitored quantity of light. On the other hand, when a light receiving element 3 produces a signal indicating the measured quantity of light, an optical-to-electric conversion circuit 4 converts this signal into another signal of which magnitude is proportional to the measured or observed quantity of light. The output signal of the optical-to-electric conversion circuit 4 is inputted into an amplifier 5, an output of which is in turn used to calculate the reflectivity and transmissivity of the object. The amplifier 5 is provided with a CdS cell 6 as means for regulating a gain of the AGC circuit. In this case, the CdS cell 6 comprises a heating element 6a and a temperature detecting resistor 6b. This resistor 6b is connected between input and output terminal of the optical-to-electric conversion circuit 2. Further, an output of the optical-to-electric conversion circuit 2 is inputted into an amplifier 7, the amplified output of which is arranged to be applied to the heating element 6a of the CdS cell 6.
In the arrangement shown in this figure, if, for example, the monitored quantity of light is constant, the gain of the amplifier 5 remains constant and further a signal, of which magnitude is proportional to the measured quantity of light, is obtained by the amplifier 5. In a case where the observed quantity of light reflected by or transmitted through an object is reduced from the previously observed quantity of light reflected by or transmitted through another object of the same reflectivity or transmissivity due to change in quantity of light emitted from a source (not shown), an output of the optical-to-electric conversion circuit 4, as well as an output of the optical-to-electric conversion circuit 2, is also reduced. Further, the reduction of the output of the optical-to-electric conversion circuit 2 causes reduction of an output of the amplifier 7 as well as reduction of electric current flowing through the heating element 6a of the CdS cell 6, resulting in increase of the resistance of the temperature detecting resistor 6b. Thereby, the gain of the amplifier 5 increases to make up an amount of the reduction in the output of the optical-to-electric conversion circuit 4.
Thus, even if quantity of light radiated from the source of light varies, the change in the observed quantity of light can be automatically compensated.
As above described, the AGC circuit of the related art comprises the amplifier 5 having the CdS cell 6 to which a signal (hereunder sometimes referred to simply as a measuring signal) representing the observed quantity of light and further comprises another amplifier 7. A signal (hereunder sometimes referred to simply as a monitoring signal) representing the monitored quantity of light is applied to the amplifier 7 and thereafter an output of the amplifier 7 is applied to the CdS cell 6, whereby the above described AGC circuit of the related art can control the gain of the amplifier 5 to prevent the change in magnitude of the monitoring signal from effecting an output of the amplifier 5. Thus, the AGC circuit of the related art has been widely used in not only measurement of a reflectivity and transmissivity of an object but also evaluation of a ratio of quantity of a certain kind of energy issued from a source to quantity of another kind of energy converted therefrom. In the case of the above described AGC circuit of the related art, it is necessary but very difficult to equalize the amplifiers 5 and 7 with each other in all characteristics thereof to improve the precision of measurement. Practically, it is impossible to equalize the amplifiers in temperature coefficient and drift. Thus, the AGC circuit of the related art has encountered a problem that the precision of measurement is limited below a certain undesirable value due to the variation in characteristics of the amplifiers.
The present invention is accomplished to resolve the above described problem of the related art.
It is therefore an object of the present invention to provide an AGC circuit which can significantly improve the precision of measurement.