Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1998-07-21
2001-10-16
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C333S018000, C333S02800T, C330S059000
Reexamination Certificate
active
06303922
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical receivers, and more particularly, to a ranges witching optical receiver that exhibits high sensitivity and wide dynamic range.
BACKGROUND OF THE INVENTION
Front-end amplifiers for optical receivers are traditionally realized in the form of transimpedance amplifiers which provide a good compromise between high sensitivity and wide dynamic range. However, to achieve the best possible performance from new transistor technologies and low capacitance photodiodes, a standard transimpedance amplifier configuration is often not adequate. It would be desirable to have an amplifier that allows very high overload combined with sensitivity that is limited only by the intrinsic noise of the input transistor, thus reaching the theoretical limit for PIN/FET receiver performance.
It is well known that to achieve minimum sensitivity, it is necessary to minimize the input capacitance of an optical receiver, and to maximize the value of feedback resistance in the transimpedance amplifier. The difficulty in accomplishing this in practice comes from the large values of feedback resistors needed. Large feedback resistors cause at least three separate problems:
Overload capability is inversely proportional to the feedback resistor value, hence large resistors reduce dynamic range; and
Transimpedance amplifier bandwidth is also inversely proportional to feedback resistor value, hence large resistors reduce bandwidth. Bandwidth can be improved by increasing the open loop gain of the transimpedance amplifier, but the increase in gain is limited by increasing amplifier phase shift which gives rise to frequency response peaking and associated sensitivity degradation; and
Shunt capacitance in parallel with the feedback resistor can eventually limit the transimpedance amplifier bandwidth as the feedback resistor value grows.
In the current art, the problem of dynamic range is sometimes overcome by the use of FETs acting as voltage-variable resistors to shunt photocurrent away from the transimpedance amplifier input when high optical signal levels are received. A FET shunt can be configured either a) in parallel with the feedback resistor, or b) as a shunt from the transimpedance amplifier input to ground. With an extremely large feedback resistor and very small input capacitance, the utility of the shunt FET approach is limited. A FET in parallel with the feedback resistor adds shunt capacitance and limits the bandwidth. Bandwidth could potentially be restored by equalization, but nonlinearity of the FET would limit the equalizer's effectiveness. As the shunt is turned on with increasing signal levels, it would be necessary to reduce the open-loop gain of the transimpedance amplifier to prevent peaking. Because of the high open-loop gain needed with a large feedback resistor, the circuit would already be at or beyond the phase shift limit where peaking becomes a problem. The difficulties of matching the shunt FET resistance and open loop gain over temperature, optical power range, and process variation would be extremely formidable.
Alternatively, a shunt FET to ground might be used. However, the ON resistance of this FET would have to be much lower than that of an FET used as a feedback resistor shunt, necessitating a much larger device size. A large FET would add considerable parasitic input capacitance, hence reducing the receiver sensitivity. Calculations have shown an expected degradation of 1.5 dB, which would significantly reduce the range of application of the receiver.
SUMMARY OF THE INVENTION
To overcome these difficulties, the present invention presents a new approach to optical receiver design. In one embodiment, a transimpedance amplifier with very low input capacitance and a very large feedback resistor is used to obtain input-FET-limited sensitivity. Rather than relying on high open-loop gain in the transimpedance amplifier to obtain bandwidth, equalization is used. This avoids frequency response peaking and associated sensitivity degradation. The above combination, usually referred to as a high impedance receiver, is capable of providing the theoretically best sensitivity.
To allow the equalized receiver concept to achieve wide dynamic range, which is the traditional drawback to the high impedance approach, the equalization is split into a number of stages, which reduces the noise added in the equalization process and also improves the overload capability. Feedback resistor and equalizer range switching is also implemented to accommodate very high optical input power levels, hence allowing the receiver to have an overload capability comparable to or better than the best shunt FET transimpedance amplifier designs.
REFERENCES:
patent: 4723313 (1988-02-01), Folcke et al.
patent: 5477370 (1995-12-01), Little et al.
patent: 5565672 (1996-10-01), Siegel et al.
Christie Parker & Hale LLP
Le Que T.
Luu Thanh X.
Ortel Corporation
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