Amplifiers – With semiconductor amplifying device – Including atomic particle or radiant energy impinging on a...
Reexamination Certificate
2003-04-02
2004-10-12
Choe, Henry (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including atomic particle or radiant energy impinging on a...
C330S069000, C250S2140AG
Reexamination Certificate
active
06803825
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to fiber optic communication devices and more particularly to a transimpedance amplifier in a fiber optic receiver circuit.
2. Description of the Related Art
A fiber optic receiver circuit typically includes a photodiode and a transimpedance amplifier. The photodiode converts a light signal received from a fiber optic cable to an electrical current signal, and the transimpedance amplifier converts the electrical current signal to an electrical voltage signal for further processing. The power of the light signal can vary at an input of the fiber optic receiver circuit. For example, the power of the light signal varies with the distance traveled in the fiber optic cable and the attenuation ratio of the fiber optic cable.
In some applications, the power of the light signal at the input of the fiber optic receiver can range from less than 100 microwatts to over 2 milliwatts. The transfer characteristic of the photodiode is typically 0.75 to 1 watt per ampere. Thus, the electrical current signal generated by the photodiode has a wide range of possible values. The transimpedance amplifier typically requires a wide dynamic range to process the electrical current signal from the photodiode without distortion.
FIG. 1
illustrates a typical fiber optic receiver circuit. The fiber optic receiver circuit includes a single-ended optical detector circuit
100
and a single-ended input transimpedance amplifier
102
. The optical detector circuit
100
includes a photodiode
104
, a filter capacitor
106
, and a bias resistor
108
. The bias resistor
108
is coupled between a cathode of the photodiode
104
and a power source (VCC). The filter capacitor
106
is coupled between the cathode of the photodiode
104
and ground. An anode of the photodiode
104
provides a current as a single-ended output of the singled-ended optical detector circuit
100
.
A light signal (or a light input) at the input of the fiber optic receiver circuit is unipolar (e.g., has two logical states represented by dark or light). In response to the unipolar light signal, the photodiode
104
generates a unipolar electrical current (Iin) flowing in one direction from the cathode to the anode. The amplitude of the unipolar electrical current is proportional to intensity of the light signal. As the intensity of the light signal changes, the amplitude and average value of the unipolar electrical current also change. The average value or direct current (DC) portion of the unipolar electrical current is provided via the bias resistor
108
, and the alternating current (AC) portion of the unipolar electrical current is provided via the filter capacitor
106
. The anode of the photodiode
104
is typically connected to a single-ended input of the transimpedance amplifier
102
.
The transimpedance amplifier
102
includes a first stage amplifier
110
, a second stage amplifier
112
, and a feedback resistor
114
. The unipolar electrical current is provided to an inverting input of the first stage amplifier
110
. A non-inverting input of the first stage amplifier
110
is coupled to ground. The feedback resistor
114
is coupled between the inverting input and an output of the first stage amplifier
110
. The output of the first stage amplifier
110
is coupled to a non-inverting input of the second stage amplifier
112
. A reference voltage is coupled to an inverting input of the second stage amplifier
112
. The second stage amplifier
112
outputs a differential pair of voltages (V+, V−) on a V+ output terminal and a V− output terminal, respectively.
The first stage amplifier
110
uses the feedback resistor
114
to convert the unipolar electrical current (or the input current) to a single-ended voltage. The second stage amplifier
112
uses the reference voltage to convert the single-ended voltage to the differential pair of output voltages (V+, V−), which is the output of the transimpedance amplifier
102
.
The single-ended transimpedance amplifier
102
has a limited dynamic range. For example, relatively high input currents can cause circuit saturation in the first stage amplifier
110
. The circuit saturation affects timing (e.g., extra delay or jitter at data transitions) and can result in degradation of bit error ratio, which is a measure of the system performance.
Furthermore, varying input currents can cause signal distortion at the output of the second stage amplifier
112
. The reference voltage used by the second stage amplifier
112
to convert the single-ended voltage to the differential voltage typically has a fixed level. The level of the reference voltage determines a slice level which differentiates between logic high and logic low. The slice level should be set at approximately 50% of the amplitude (or the DC level) of the single-ended voltage to preserve signal pulse widths (or duty cycle) in the differential voltage. The DC level of the single-ended voltage varies as the amplitude of the input current varies. Thus, the duty cycle (or pulse widths) may be distorted at the output of the transimpedance amplifier
102
.
FIG. 2
illustrates another typical fiber optic receiver circuit which is substantially similar to the fiber optic receiver circuit described in
FIG. 1
with an additional DC cancellation circuit
200
as part of a transimpedance amplifier
202
. The DC cancellation circuit
200
minimizes the signal distortion described above by sensing the DC levels of the differential pair of output voltages (V+, V−) and generating a current sink to remove the DC input current (Iin(DC)) at the input of the transimpedance amplifier
202
.
For example, the DC cancellation circuit
200
includes a low pass filter
204
, an operational amplifier
206
, and a voltage control current source (VCCS)
208
. The low pass filter
204
is coupled across the differential output voltage to sense a difference in the DC levels between V+ and V−. The operational amplifier
206
is coupled to an output of the low pass filter
204
and generates a control voltage indicative of the difference in the DC levels between V+ and V−. The VCCS
208
generates a current sink with a level that is determined by the control voltage, and the current sink is coupled to the input of the transimpedance amplifier
202
.
The DC cancellation circuit
200
is a feedback loop that typically responds slowly (or requires a long time to settle to a final state) due to bandwidth requirements. Thus, the DC cancellation circuit
200
is not suitable for burst mode data communication. Furthermore, the value of capacitors used in the DC cancellation circuit
200
is relatively large and cannot be efficiently implemented in integrated circuits.
SUMMARY OF THE INVENTION
The present invention solves these and other problems by providing a pseudo-differential transimpedance amplifier which improves dynamic range and minimizes signal distortion. The pseudo-differential transimpedance amplifier can be used in an optical receiver circuit which translates a light signal into an electrical voltage signal. An optical detector first senses the light signal and generates an electrical current signal in response. The pseudo-differential transimpedance amplifier uses a differential configuration to convert the electrical current signal to differential voltage signals. In addition to other benefits, the differential configuration can advantageously double a current-to-voltage conversion gain.
In one embodiment, the pseudo-differential transimpedance amplifier includes a pseudo-differential input stage. For example, the pseudo-differential input stage includes a first input amplifier and a second input amplifier. The first input amplifier is AC-coupled to the optical detector and is configured to convert a first input current to a first output voltage (or a first pseudo-differential voltage). The second input amplifier is DC-coupled to the optical detector and is configured to convert a second input curre
Chiou Chii-Fa
Isobe Yuji
Choe Henry
Knobbe Martens Olson & Bear LLP
Microsemi Corporation
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