Circuit arrangement for carrying out light intensity measurements

#The invention relates to a circuit arrangement for carrying out light intensity measurements. The proposed circuit arrangement comprises an input terminal, an output terminal, at least two operational amplifiers, a first and a second photodiode generating output current signals for respective operational amplifiers under influence of an optical light source. The circuit arrangement according to the invention is capable of carrying out differential measurements and offers the possibility of selecting pairs of photodiodes, wherein the emissivity of a light emitting diode depending on the temperature and the variation of the parameters of a photodiode depending on the temperature and time can be compensated.

#The known circuit arrangements for carrying out light intensity measurements applied e.g. for determining small displacements suffer from being sophisticated, comprising a great number of active and passive elements. A simple circuit arrangement constituting a differential light detector is shown in the catalogue of the RCA Corporation (U.S.) in the 1986 edition, issue 1-86, page 368 (drawings nr. 92CM-30009). In this circuit arrangement, however, the drift of the characteristic parameters of the receiving photodiodes, as of their short-circuit current or no-load voltage as the circuit arrangements stands in the catalogue, cannot be compensated. The problem is that some important parameters of the photodiodes show high spread being sometimes as high as 10% in dependency on the temperature and light intensity, further in course of their natural aging. Therefore it has been always necessary to select the photodiodes to forming pairs with regard to the slope of their characteristics and temperature drift values.

#The known differential light detectors are usually applied in sensor arrangements for measuring displacements, different processes resulting in the change of the intensity of the light reaching the photodiodes. The basis of the measurements is the feature that the sum of the short-circuit current of the light receiving photodiodes is regulated to be stable for ensuring alteration of the short-circuit current of any one of the photodiodes, the alteration being proportional to a light intensity change to be measured.

#The invention will be described in more detail by way of example and with reference to some preferred embodiments illustrated in the accompanying drawings. In the drawings

#FIG. 1 is the circuit diagram of a generic embodiment of the circuit arrangement proposed by the invention and

#FIG. 2 shows the circuit diagram of a specific embodiment of the circuit arrangement proposed by the present invention.

#The circuit arrangement as proposed by the invention (FIG. 1) includes an input terminal 5 connected to a supply unit ensuring voltage U.sub.0, connected over current generating means, especially a reference resistor R.sub.s to a common terminal of a bridge circuit coupled with a first operational amplifier 1 and a second operational amplifier 2 and of a third operational amplifier 3. The third operational amplifier 3 is connected to an optical light source S, especially a light emitting diode, if necessary, through a power amplifier 4.

#The second operational amplifier 2 is coupled with a common terminal connected to an output terminal 6 of the proposed arrangement and through a feed-back member with the input of the second operational amplifier 2. The feed-back member is constituted in the embodiment of FIG. 1 by a compensation resistor R.sub.k and the embodiment of FIG. 2 by a compensation resistor R.sub.k, the first operational amplifier 1 and a bridge resistor R.sub.2.

#The essence of the invention lies in applying the bridge circuit having four branches. The common terminal of this bridge circuit connecting the poles of the same sign of a first photodiode D.sub.1 and a second photodiode D.sub.2 is connected over the common terminal to the reference resistor R.sub.s. Oppositely to the two adjacent branches including the first and second photodiodes D.sub.1, D.sub.2 there are two adjacent branches including the first bridge resistor R.sub.1 being an element of the feed-back member according to FIG. 2 and a second bridge resistor R.sub.2. The common terminal of the first photodiode D.sub.1 and the first bridge resistor R.sub.1 is coupled with an input of the first operational amplifier 1 and the junction point of the first and second bridge resistors R.sub.1, R.sub.2 is connected with the output of the first operational amplifier 1. The common terminal of the second photodiode D.sub.2 and the second bridge resistor R.sub.2 is coupled with an input of the second operational amplifier 2.