Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1985-11-08
1987-02-10
Mullins, James B.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
330 59, 330 85, H01J 4014
Patent
active
046424539
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to apparatus for increasing the dynamic range in an input stage of integrating type in an optoelectric receiver.
Two usual optical input stages are a transimpedance stage and an integrating stage. See, for example, the book Semiconductor Devices for Optical Communications, H. Kressel, Springer-Verlag, Berlin, Heidelberg, New York 1980. Integrating stages are better from the sensitivity aspect, since less noise is generated than in transimpedance stages. Since integrating stages easily become over driven they have, however, poorer dynamic properties than transimpedance stages.
To obtain high sensitivity in a receiver with an integrating input stage, it is sought to obtain low capacitance on its amplifier input. Low capacitance causes a high voltage on the amplifier input since the input signal is integrated. The input stage may therefore be over driven already at a moderate level of the input signal. An integrating stage is very sensitive for overamplification, since the signal information is in the derivative of the integrated signal.
Attempting to connect in extra capacitance electrically on the amplifier input to increase the dynamic range results in deteriorated sensitivity, due to unavoidable stray capacitances in the apparatus used for the connection.
SUMMARY OF THE INVENTION
An object of the invention is to achieve an integrating input step with increased dynamic range and retained sensitivity so that both weak and strong input signals may be detected correctly. The problem is solved by a special feed-back arrangement.
An apparatus in accordance with the invention is thus characterized as is apparent from the accompanying claims.
BRIEF DESCRIPTION OF DRAWING
The invention will now be described in detail with reference to the accompanying drawings, where
FIG. 1 illustrates a schematic diagram of a conventional integrating input stage, and
FIG. 2 illustrates an integrating input stage improved in accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the already known input stage according to FIG. 1, an optoelectric converter D, e.g. a photo diode, and a resistor R having a high resistance are connected in series in a supply circuit between a voltage source V and earth. The input to a high impedance amplifier F1 with the gain -A1 is connected to a suction point p between the photo diode D and the resistor R. C.sub.d and C.sub.i are capacitances unavoidably associated with the photo diode D and amplifier F1, respectively, these capacitances resulting in an integration operation. The amplifier F1 generates a signal constituting the output singal of the stage.
When light L is incident on the photo diode D, a current proportional to the luminance is generated, this current constituting the electrical input signal of the stage. The resistor R gives the photo diode D an operating point. The resistance is selected as high as possible, which gives little thermal noise and low downward boundary frequency. However, the stage has poor dynamic properties, since, due to the low capacitance, it will be easily over driven on integration.
The transfer function of the stage can be written: ##EQU1## where s denotes the Laplace variable.
The improved input stage depicted in FIG. 2 differs from the one in the prior art by the use of a feed-back circuit F2 with the transfer function A2, connected between the output stage and the pole of the photo diode D facing away from the input of the amplifier F1. The circuit F2 is shown as a controllable amplifier but may also be a controllable voltage divider. The gain may be regulatable continuously or stepwise, and is controlled by the output signal from a transducer G, the output signal being proportional to the luminance and the transducer being connected in the supply circuit between the resistor R and earth. The transducer may also be connected at other places in the input stage where it generates a signal proportional to the luminance e.g. in another place at the supply circuit or after the
REFERENCES:
patent: 4574249 (1968-03-01), Williams
Nordqvist Glenn O.
Svenson Lars-Gote
Mullins James B.
Telefonaktiebolaget LM Ericsson
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