Amplifiers – With semiconductor amplifying device – Including atomic particle or radiant energy impinging on a...
Reexamination Certificate
1999-07-28
2001-10-02
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including atomic particle or radiant energy impinging on a...
C330S282000, C250S2140AG
Reexamination Certificate
active
06297701
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a wide dynamic range transimpedance amplifier which finds application in optical receivers. The invention particularly relates to a bipolar IC transimpedance amplifier.
BACKGROUND OF THE INVENTION
The ever increasing demands for high capacity communications systems has seen the wide spread employment of optical fibre networks across the world. A fundamental component for such systems is a means of converting optical pulses comprising a digital bit stream into electrical signals. This component of such a system is commonly known as an optical receiver.
The operational requirements of such a receiver are very demanding. The receiver is required to exhibit a very low noise characteristic, such that it is capable of detecting very low levels of optical input in systems employing maximum optical fibre lengths, thus requiring high gain amplification for maximum sensitivity, but is conversely required to cope with high levels of optical input in systems employing short fibre lengths, thus requiring low gain amplification. As such, the optical receiver is required to have a wide dynamic range which can only be practically achieved with some form of automatic gain control (AGC). A typical integrated circuit (IC) optical receiver
10
is illustrated in block schematic form in FIG.
1
. This comprises an IC (denoted by broken line
12
) including a transimpedance amplifier stage (denoted by broken line
14
) with an integrator in a control loop providing AGC.
As illustrated by
FIG. 1
, optical input power OP
IN
is converted into an electrical current I
IN
by a PIN diode photodetector
16
. This current I
IN
is applied as an input to the IC optical receiver
10
. The input current I
IN
is amplified by a transimpedance amplifier (Tz Amp)
18
which converts the input current I
IN
into an amplified voltage output signal V
OUT
. To meet the requirement of wide dynamic range the output voltage V
OUT
of the Tz Amp
18
which is in the form of a broadband data signal and may be considered as an ac, multi-frequency signal, is rectified or peak detected by a rectifier/peak detector
20
to provide a dc signal level V
REC
for comparison with a pre-determined dc reference voltage V
REF
. The difference between the rectified/peak detected output voltage V
REC
and the reference voltage V
REF
is considered as an error signal which is amplified and integrated by a Miller Integrator
22
to provide a control signal V
CONTROL
. A Miller Integrator is a well known form of integrator incorporating an active device such as a transistor amplifier. The Miller Integrator
22
is required to have a high gain, in order to ensure that the error signal approaches zero (ie in order to ensure that the difference between the rectified/peak detected output voltage V
REC
and the reference voltage V
REF
becomes zero) by means of controlling the gain of the Tz Amp by varying the impedance of a feedback resistor
24
.
If the rectified/peak detected output voltage V
REC
is smaller than the dc reference voltage V
REF
, then the Tz Amp
18
must operate at high gain to provide high sensitivity of the optical receiver. When the rectified/peak detected output voltage V
REC
becomes just greater than the dc reference voltage V
REF
, then the on-set of AGC occurs and continues whilst the input channel I
IN
increases. When the feedback resistor
24
is at a minimum the Tz Amp is operating at very low gain and approaches an overload condition.
The most effective method of controlling the gain of the Tz Amp
18
is to vary the value Rf of the feedback resistor
24
, as shown by FIG.
1
. This can be achieved when using such technologies as CMOS, BiCMOS or GaAs MESFET were the availability of Field Effect Transistors (FETs) allows them to be configured as variable resistors connected in parallel with the feedback resistor
24
to vary the Tz Amp gain. To achieve the highest performance receiver GaAs MESFET technology is often used but at greatest expense. Alternatively, CMOS technology can offer the lowest cost but offers the poorest performance in terms of sensitivity. A good compromise is often provided by using BiCMOS technology, where a high performance is achieved through use of bipolar transistors for low noise while the gain control is achieved by the FET connected in parallel with the feedback resistor. The use of bipolar technology can achieve sensitivities comparable to GaAs MESFET technology, but poor gain control techniques may result in either lowering of the sensitivity of the receiver or providing an inadequate dynamic range.
OBJECTS OF THE INVENTION
The invention seeks to provide a bipolar IC transimpedance amplifier including gain control.
The invention also seeks to provide a bipolar IC transimpedance amplifier which obviates some of the problems encountered with other IC transimpedance amplifiers.
SUMMARY OF THE INVENTION
The present invention provides a transimpedance amplifier for an optical receiver comprising an input for receiving an input current signal from a photodetector and a gain stage for amplifying the input current signal. It also comprises a resistor network coupling the output and input of the gain stage, the resistor network comprising at least first and second resistive elements in a parallel arrangement and being arranged such that one of said resistive elements can be isolated from the output of the gain stage wherein the one of said resistive elements can be isolated from the output of the gain stage by means of a transistor switching network.
REFERENCES:
patent: 4540952 (1985-09-01), Williams
patent: 5012202 (1991-04-01), Taylor
patent: 5412346 (1995-05-01), Burger, Jr. et al.
patent: 5451904 (1995-09-01), Terada et al.
patent: 5786730 (1998-07-01), Hadley
patent: 6081603 (2000-06-01), Engh et al.
patent: 6114686 (2000-09-01), Funahashi
Flett Robin M
Visocchi Pasqualino Michelle
Lee Mann Smith McWilliams Sweeney & Ohlson
Nguyen Khanh Van
Nortel Networks Limited
Pascal Robert
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