Amplifiers – With semiconductor amplifying device – Including atomic particle or radiant energy impinging on a...
Reexamination Certificate
1999-09-09
2001-05-08
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including atomic particle or radiant energy impinging on a...
C330S053000, C330S292000, C250S2140AG
Reexamination Certificate
active
06229398
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an interface device between a broad-band ultrahigh frequency (hyperfrequency or microwave) optoelectronic sensor and a load.
It applies in particular to the production of ultrahigh frequency monolithic integrated circuits, which are designed in particular for transmission of very broad-band ultrahigh frequency signals on optical fibres.
In general, optoelectronic sensors, such as photodiodes, act as a source of current which is controlled by modulation of the incident light, in which a parasitic capacitor C is mounted in parallel on a resistor with a very high value.
From the point of view of adaptation of impedance, the ideal load (which is also known as the reading resistance) to be submitted to this photodiode would be a resistance R with a high value. Since the cutoff frequency is proportional to the time constant RC, depending on the photodiode used, a load of this type, associated with the parasitic capacitor C, could give rise to a cutoff frequency which is much lower than the maximum frequency of use of the ultrahigh frequency optical connections.
A means for reducing the value of the time constant RC consists of decreasing the resistance R of the load. For example, it is known to decrease this resistance to a value lower than 50 ohms, by connecting the photodiode to an amplifier with a low input impedance. However, a solution of this type has the disadvantage that it decreases greatly the gain of an optical connection chain, and in some cases increases the noise factor, since the said noise factor is a decreasing function of the resistance R, the increase in the noise factor being all the most substantial, the lower the power which is incident on the photodiode.
In addition, in EP-A-801466, the applicant has proposed an assembly known as the bootstrap, in which the negative effects of the parasitic capacitor of the photodiode are compensated for by cancelling out the difference in potential at the terminals of the said parasitic capacitor. In practice, this bootstrap comprises a field-effect transistor mounted in a common drain, the gate of which is connected to one terminal of the sensor, and the source of which is connected to the other terminal of the sensor.
In addition, the bootstrap is completed by an impedance adaptation stage which comprises another transistor mounted in a common drain. The gate of the transistor of the adaptation stage is connected to the source of the transistor of the bootstrap, and the source of the transistor of the adaptation stage is applied to the standardised impedance load of 50 ohms.
An impedance adaptation stage of this type is not entirely satisfactory, in that the useful signal is recuperated in this case on the source resistance of the transistor of the bootstrap. In addition, a connection capacitor is interposed between the source of the transistor of the bootstrap and the gate of the impedance adaptation transistor, and an impact inductor is placed directly in parallel on the load.
As a result, amongst other disadvantages, there is a limitation of recuperation of the useful signal, towards the low frequencies.
SUMMARY OF THE INVENTION
The present invention provides a solution to this problem.
The invention relates to an interface device between a broad-band ultrahigh frequency optoelectronic sensor and a load, the impedance of which is lower than that of the sensor, the said interface device comprising an impedance adaptation stage.
According to a general definition of the invention, the adaptation stage comprises a distributed amplifier with a broad band of frequencies, the input of which is directly connected to a terminal of the sensor, and constitutes an impedance with an ohmic value which is greater than that of the load, and the output of which is connected to the load, the interface device additionally comprising a bootstrap which comprises a field-effect transistor mounted in a common drain, the gate of which is connected to one terminal of the sensor, and the source of which is connected to the other terminal of the sensor, via a capacitor with a selected value.
The high input impedance of the distributed amplifier according to the invention makes it possible to reduce the noise factor of an optical connection chain comprising an interface device of this type, whilst maintaining a high, flat level of gain in a very broad frequency band ranging from a few kHz to a few GHz, whereas the bootstrap according to the invention can compensate for the undesirable effects of the parasitic capacitor of the sensor towards the high frequencies, without any detrimental effects on the low frequencies.
In practice, the distributed amplifier comprises a plurality of amplifier cells, each of which is mounted between a gate line and a drain line, each cell comprising an active circuit which has at least one field-effect transistor mounted in a common source, and passive elements which constitute sections of the gate line and the drain line, one of the ends of the gate line being connected directly to one of the terminals of the sensor, and one of the ends of the drain line being connected to the load.
Preferably, the polarisation of the distributed amplifier according to the invention has saturable loads, which makes it possible to avoid the limitations towards the low frequencies.
Other characteristics and advantages of the invention will become apparent from the following detailed description and the drawings.
REFERENCES:
patent: 4723313 (1988-02-01), Folcke et al.
patent: 5216386 (1993-06-01), Wyatt
patent: 5874861 (1999-03-01), Nagase et al.
patent: 5880640 (1999-03-01), Dueme
patent: 0801466 (1997-10-01), None
patent: 2727585 (1996-05-01), None
patent: 2747523 (1997-10-01), None
Freundorfer, A1 P., et al., “Noise in Distributed MESFET Preamplifiers”, IEEE Journal of Solid-State Circuits, vol. 31, No. 8, pp. 1100-1111, Aug. 1996.
Auric Claude
Dueme Philippe
Choe Henry
Connolly Bove Lodge & Hutz
Pascal Robert
Thomson C&F Detexis
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