Amplifiers – With semiconductor amplifying device – Including atomic particle or radiant energy impinging on a...
Reexamination Certificate
2001-07-25
2003-12-16
Shingleton, Michael B (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including atomic particle or radiant energy impinging on a...
C330S110000, C330S086000
Reexamination Certificate
active
06664858
ABSTRACT:
BACKGROUND OF THE INVENTION
A high-speed optical fiber communication receiver, in one known form, receives a broadband optical signal and converts it to an electric signal. A transimpedance amplifier is used as the front stage of a typical such optical fiber communication receiver. Characteristics of the transimpedance amplifier can affect the overall system performance.
Referring to
FIG. 1
, a schematic diagram illustrates a conventional transimpedance amplifier
10
for use in an optical fiber communication receiver. The transimpedance amplifier
10
is operable to convert an input current to an output voltage. The transimpedance amplifier
10
includes a high-speed operational amplifier U having plus (+) and minus (−) inputs and differential outputs labeled Vo− and Vo+. A photo detector PD is connected between a bias voltage source Vb and the minus (−) input of the operational amplifier U. The plus (+) input of the operational amplifier U is connected to ground. The negative output Vo− comprises a voltage output to a post processing amplifier, which comprises a common voltage amplifier. A feedback resistor Rf is connected between the minus output Vo− and the minus (−) input. A peaking capacitor Cp is connected across the operational amplifier U between the plus output Vo+ and the minus (−) input.
Without the peaking capacitor Cp, the bandwidth of the transimpedance amplifier circuit
10
is defined by
BW=
1/(2*&pgr;*
Rf*Cin*A
)
where Cin is the input capacitance including the amplifier input capacitance and parasitic capacitance of the photo detector PD. A is the open loop gain of the operational amplifier U. &pgr; is the constant 3.1416. The peaking capacitor Cp can balance the input capacitance to widen the bandwidth without sacrifice of the transimpedance gain. For proper operation, it is important to achieve high bandwidth, high gain and low noise.
Since the peaking capacitor Cp forms a positive feedback loop, it has an optimal capacitance value which strongly depends on Cin. If the real peaking capacitance is less than the optimal value, then the transimpedance amplifier
10
can't reach the expected performance level. If the peaking capacitance is more than the optimal value, then stability of the transimpedance amplifier
10
degrades and overshoot or oscillation may occur.
In a typical application, the operational amplifier U, feedback resistor Rf and peaking capacitor Cp are integrated in a dye with the rest of the system, while the photo detector PD is a separate dye, with both dyes being placed in a single package. The parasitic capacitance of the photo detector PD is difficult to control. Photo detector manufacturers specify the maximum parasitic capacitance and the actual parasitic capacitance varies over quite a large range. In this manner, the peaking capacitor Cp has a fixed value while Cin varies. The system yield rate of optical fiber communication receivers using the transimpedance amplifier
10
shown in
FIG. 1
is not stable.
The present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided an optical receiver including a transimpedance amplifier and an adjustable peaking capacitor coupled to the amplifier.
Broadly, there is disclosed herein a transimpedance amplifier comprising an operational amplifier having a current input and developing a voltage output. A variable peaking capacitor is connected across the operational amplifier. A control circuit is operatively coupled to the variable capacitor for controlling capacitance to widen bandwidth of the transimpedance amplifier and reach the optimal peaking capacitance at a real operation environment.
It is a feature of the invention that the operational amplifier has differential outputs and the variable peaking capacitor is connected to the operational amplifier to provide positive feedback.
It is another feature of the invention that the control circuit comprises an interface circuit for receiving an external command representing a desired value of the variable peaking capacitor.
It is a further feature of the invention that the interface circuit receives a data signal representing a desired value of the variable peaking capacitor and further comprising a digital to analog converter connected between the interface circuit and the variable capacitor.
It is still another feature of the invention to provide a measurement circuit selectively connected to the voltage output and the interface circuit for measuring voltage output for determining the desired value of the variable peaking capacitor. In one embodiment, the measurement circuit measures signal amplitude. In another embodiment, the measurement circuit measures pulse rise time. In a still further embodiment, the measurement circuit measures overshoot.
It is still another feature of the invention to provide a photo detector connected to the operational amplifier to provide the current controlled input.
It is another feature of the invention that the variable capacitor comprises a varactor, such as a varactor diode or any voltage control capacitors which can be integrated in an integrated circuit.
There is disclosed in accordance with another aspect of the invention, a high-speed optical receiver including a transimpedance front stage amplifier and a post amplifier. The front stage amplifier includes a photo detector connected to a current controlled input of an operational amplifier developing a voltage output. A variable peaking capacitor is connected across the operational amplifier. A control circuit is operatively coupled to the variable peaking capacitor for controlling capacitance to widen bandwidth of the transimpedance amplifier and reach the optimal peaking capacitance at a real operation environment. The post amplifier is connected to the voltage output.
Further features and advantages of the invention will be readily apparent from the specification and from the drawings.
REFERENCES:
patent: 4868516 (1989-09-01), Henderson
patent: 5146079 (1992-09-01), Lisco
patent: 5257285 (1993-10-01), Thorp
patent: 5373152 (1994-12-01), Domon et al.
patent: 5602511 (1997-02-01), Woolaway
patent: 5786730 (1998-07-01), Hadley
patent: 5933264 (1999-08-01), VanDer Heijden
patent: 403097302 (1991-04-01), None
International Business Machines - Corporation
Shingleton Michael B
Wood Phillips Katz Clark & Mortimer
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