Detection circuit having a variable impedance circuit

Details Detection circuit having a variable impedance circuit Detection circuit having a variable impedance circuit

1999-11-15

2001-10-23

Le, Que T. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C250S2140RC

Reexamination Certificate

active

06307196

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
This invention relates to a detection circuit, in particular a circuit which produces a voltage signal which depends on the intensity of light in an incident optical signal.
BACKGROUND OF THE INVENTION
Detection circuits are known, which include a photodiode detector, which produces a current proportional to the intensity of incident light. The circuits further include a transimpedance amplifier circuit, which converts this current into an output voltage. For example, U.S. Pat. No. 5,030,925 shows a detection circuit of this type.
A recognised problem with detection circuits of this type is that the input light signal intensity, and hence the photodiode current, which is proportional thereto, can vary over a very wide dynamic range. For example, it may be necessary to handle input signals which vary over several orders of magnitude. Without special attention to the design values of the impedances at the amplifier input, the wide dynamic range of inputs to the amplifier would cause problems in producing output signals which are sufficiently large to be useful at low signal levels, while still producing output signals for large input signal levels which the circuit can handle without saturating or limiting.
U.S. Pat. No. 5,708,392 discloses a detection circuit with a limiting transimpedance amplifier (TIA) circuit, in which the amplifier circuit has a feedback resistor with a diode coupled across the resistor. Thus, at high signal levels, the diode conducts the input current, and limits the output voltage to a level below that which would cause saturation of the output stage of the amplifier. TIA amplifiers which use diodes across their input/output must handle the overload current in their output stage, and this requires larger transistors with more capacitance. Thus these designs are more complex and require more silicon area when fabricated in an integrated circuit.
Further, EP-A-0745868 discloses a detection circuit having a photodiode detector and a transimpedance amplifier. In order to be able to handle excessive input currents during an initial detection phase, the optodiode is connected to ground through a further diode, and is also connected to a positive voltage supply through an RC network.
SUMMARY OF THE INVENTION
The present invention relates to a detection circuit which, in preferred embodiments, can handle high overload currents, and in particular can handle persistent high overload currents.
Advantageously, the detection circuit of the present invention may include an overload protection diode, connected in series with a detection photodiode, and connected to a supply voltage by means of a low AC impedance, flat frequency response connection. The connection point between the overload protection diode and detection photodiode feeds a transimpedance amplifier, and the input impedance of the transimpedance amplifier is preferably adjusted to match the impedance of the overload protection diode at a cutoff signal current. For signal currents lower than the cutoff, most of the current is applied to the transimpedance amplifier, whereas, for signal currents higher than the cutoff, most of the current is diverted away from the transimpedance amplifier through the overload protection diode.
This has the advantage that the amplifier design may be simplified.


REFERENCES:
patent: 4623786 (1986-11-01), Rodwell
patent: 4808810 (1989-02-01), Van Doorn
patent: 4819071 (1989-04-01), Nakamura
patent: 5130667 (1992-07-01), Chang et al.
patent: 5532471 (1996-07-01), Khorramabadi et al.
patent: 5592124 (1997-01-01), Mullins et al.
patent: 5606282 (1997-02-01), Yoshida
patent: 5679953 (1997-10-01), Ananth et al.
patent: 3218 439 A1 (1983-12-01), None
patent: 0 402 044 A2 (1990-12-01), None
patent: 0433646 A (1991-06-01), None
patent: 0643496 A1 (1995-03-01), None
patent: 720 311 (1996-07-01), None
patent: 745 868 (1996-12-01), None
patent: 2 233 525 A (1991-01-01), None
patent: 2 247 798 A (1992-03-01), None
patent: WO 95 02278 (1995-01-01), None

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