U.S. patent application number 10/093670 was filed with the patent office on 2002-08-08 for preamplifier circuit for a photodetector.
Invention is credited to Arachtingi, John William, Buescher, Kevin Scott, Lauffenburger, James Harold.
Application Number | 20020105385 10/093670 |
Document ID | / |
Family ID | 25109669 |
Filed Date | 2002-08-08 |
United States Patent
Application |
20020105385 |
Kind Code |
A1 |
Buescher, Kevin Scott ; et
al. |
August 8, 2002 |
Preamplifier circuit for a photodetector
Abstract
An amplifier circuit (20) for a photodetector includes a
transconductance variable gain stage (32). The transconductance
variable gain stage (32) has an input (34) capable of connecting to
the photodetector and an output (40). A transconductance gain stage
(44) has an input (42) connected to the output (40) of the
transconductance variable gain stage (32). A feedback resistor (46)
is connected between an output (48) of the transconductance gain
stage (44) and the input (42) of the transconductance gain stage
(44).
Inventors: |
Buescher, Kevin Scott;
(Colorado Springs, CO) ; Lauffenburger, James Harold;
(Colorado Springs, CO) ; Arachtingi, John William;
(Colorado Springs, CO) |
Correspondence
Address: |
Law Office of Dale B. Halling, LLC
Suite 311
24 South Weber St.
Colorado Springs
CO
80906
US
|
Family ID: |
25109669 |
Appl. No.: |
10/093670 |
Filed: |
March 8, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10093670 |
Mar 8, 2002 |
|
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09777235 |
Feb 5, 2001 |
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6396351 |
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Current U.S.
Class: |
330/308 |
Current CPC
Class: |
H03F 2203/30156
20130101; H03F 2203/7236 20130101; H03G 1/0023 20130101; H03F
2203/30054 20130101; H03G 1/0088 20130101; H03F 2203/45651
20130101; H03F 3/45183 20130101; H03F 2203/30151 20130101; H03F
3/3023 20130101; H03F 2203/45658 20130101 |
Class at
Publication: |
330/308 |
International
Class: |
H03F 003/08 |
Claims
What is claimed is:
1. An amplifier circuit for a photodetector comprising: a
transconductance variable gain stage having an input capable of
connecting to the photodetector and an output; a transconductance
gain stage having an input connected to the output of the
transconductance variable gain stage; and a feedback resistor
connected between an output of the transconductance gain stage and
the input of the transconductance gain stage.
2. The amplifier circuit of claim 1, wherein a gain of the
transconductance variable gain stage is set to have a predetermined
zero gain frequency crossing.
3. The amplifier circuit of claim 1, wherein the transconductance
variable gain stage includes a cascoded differential amplifier.
4. The amplifier circuit of claim 3, wherein the transconductance
variable gain stage includes a current mirror connected to the
cascoded differential amplifier.
5. The amplifier circuit of claim 4, wherein the current mirror
includes a positive cascoded current mirror and a negative cascoded
current mirror.
6. The amplifier circuit of claim 4, wherein the current mirror has
a gain switch.
7. The amplifier circuit of claim 6, wherein the gain switch is
controlled by a recording signal.
8. The amplifier circuit of claim 1, wherein the transconductance
variable gain stage has a second input connected to a ground.
9. An amplifier circuit for a photodetector comprising: a
transconductance variable gain stage having an input capable of
connecting to the photodetector; a transconductance gain stage
having an input connected to an output of the transconductance
variable gain stage; a feedback resistor connected between an
output of the transconductance gain stage and the input of the
transconductance variable gain stage; and a variable feedback
resistor connected to the input of the transconductance variable
gain stage and to the output of the transconductance gain
stage.
10. The amplifier circuit of claim 9, wherein the transconductance
variable gain stage includes a differential amplifier.
11. The amplifier circuit of claim 9, wherein the differential
amplifier is a cascoded differential amplifier.
12. The amplifier circuit of claim 10, wherein the transconductance
variable gain stage includes a current mirror connected to the
differential amplifier.
13. The amplifier circuit of claim 12, wherein the current mirror
includes a gain switch.
14. The amplifier of claim 12, wherein the current mirror includes
a positive current mirror and a negative current mirror.
15. The amplifier of claim 12, wherein the current mirror is a
cascoded current mirror.
16. An amplifier circuit for a photodetector comprising: a
differential amplifier having an input capable of connecting to the
photodetector; a variable gain positive current mirror connected to
the differential amplifier; and a variable gain negative current
mirror connected to the differential amplifier.
17. The amplifier of claim 16, wherein the variable gain positive
current mirror includes a positive gain switch.
18. The amplifier of claim of 16, further including a
transconductance gain stage connected to an output of the variable
gain positive current mirror.
19. The amplifier of claim 18, further including a resistor
connected between the input of the transconductance gain stage and
the output of the transconductance gain stage.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to the field of
amplifier circuits and more particularly to an amplifier circuit
for a photodetector.
BACKGROUND OF THE INVENTION
[0002] There are a number optical storage standards such as CD
(compact disks), DVD (digital video disks), CD-RW (Write/Read CDs),
etc. All of these products require photodetector preamplifiers to
sense and amplify the reflection from the disks.
[0003] There is a trend to build a single machine that can read all
of the different standards. Each of these different standards have
different amounts of reflectance of the interrogating laser. Thus,
a preamplifier circuit designed for one standard is not optimum for
a second standard. This can result in misread bits and degrade the
performance of the optical storage system. Generally, a
preamplifier with adjustable gain is desirable for best
performance.
[0004] However, the photodetector/preamplifier circuit has a number
of other important constraints that make adjustable gain more
difficult. The main other constraints include: low-noise
amplification, wide signal bandwidth, DC accuracy, relatively large
photodetector size, high responsivity, and low cost. These other
constraints increase the challenge of providing adjustable
gain.
[0005] This problem can be demonstrated by an example. A typical
photodetector preamplifier circuit is shown in FIG. 1. The
photodetector is represented by the current source 14 and the
capacitor Cpd, 16. The preamplifier, 10, is represented in its
simplest form by the amplifier symbol and the feedback resistor Rf.
The gain of the preamplifier is approximately set by the resistor
Rf; which converts the input photocurrent to an output voltage,
where this conversion factor is called the transimpedance gain. The
bandwidth is set by the amplifier characteristics and by the
capacitance of the photodiode (plus other parasitic capacitances).
The amplifier, plus the resistor RF and the photodetector
capacitance Cpd form a feedback loop. This loop is potentially
unstable, and the standard stability criteria and calculations
(Nyquist, Bode, etc) must be used to guarantee stability over all
process, temperature and other production variations. The dominant
pole in the loops is usually created by the time constant of the
feedback resistor and the photodetector capacitance. In order to
achieve the low noise objective, the feedback resistor must be
large. In order to achieve wide bandwidth, the amplifier gain must
be large enough to move the dominant pole to a high frequency in
the closed loop. And, to achieve DC accuracy, the amplifier gain
must be quite high at low frequency.
[0006] Normally, to achieve DC accuracy and high gain, an amplifier
will be constructed in the standard industry practice of an input
differential stage, followed by a high gain 2.sup.nd stage and then
frequency-compensated by "Miller" a feedback capacitor across the
2.sup.nd stage. This produces a standard operational amplifier with
DC accuracy, high gain, and possibly wide closed-loop bandwidth.
However, using this in a photodetector preamplifier will lead to
either reduced bandwidth or to instability. The dominant pole of
the operational amplifier, combined with the dominant pole of the
Rf, Cpd will result in a 2.sup.nd order loop. If additional poles
and time delays are introduced (as a result of parasitics or other
portions of the amplifier), then the result is an oscillator.
[0007] The normal methods to reduce this oscillation are: 1) reduce
the bandwidth of the amplifier so that it is the dominant pole of
the system; 2) reducing the size of Rf to increase the frequency of
the Rf Cpd pole; 3) compensate Rf by placing a capacitor in
parallel with it; and/or 4) compensate Cpd by placing a resistor in
series with it. These solutions all result in sub-optimal
preamplifiers; 1 and 2 result in much lower bandwidth, while 3 and
4 result in much higher noise levels.
[0008] These problems are made much more difficult when the
feedback resistor, Rf needs to be variable. Then, any of these
compensation methods is more difficult due to the multiple criteria
that must be simultaneously optimized.
[0009] It is the object of this present invention to provide: a) an
amplifier topology that allows for optimum dynamic range,
bandwidth, transimpedance gain, signal to noise ratio, and DC
accuracy; b) a means of allowing the transimpedance gain to be
varied while the other criteria are optimized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a schematic diagram of a transimpedance amplifier
circuit connected to an optical detector in accordance with one
embodiment of the invention;
[0011] FIG. 2 is a schematic diagram of an amplifier circuit in
accordance with one embodiment of the invention;
[0012] FIG. 3 is a block diagram of a transconductance variable
gain stage in accordance with one embodiment of the invention;
and
[0013] FIG. 4 is a circuit diagram of a transconductance variable
gain stage in accordance with one embodiment of the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0014] An amplifier circuit for a photodetector includes a
transconductance variable gain stage. The transconductance variable
gain stage has an input capable of connecting to the photodetector
and an output. A transconductance gain stage has an input connected
to an output of the transconductance variable gain stage. A
feedback resistor is connected between an output of the
transconductance gain stage and the input of the transconductance
gain stage. The transconductance variable gain varies the overall
gain of the amplifier circuit. The loop bandwidth is determined by
the loop gain from the frequency rolloff of the Rf Cpd
timeconstant. As a result the loop bandwidth can be adjusted to
compensate for changes of Rf. The net result is that when Rf is
changed (in order to vary the transimpedance gain), the
transconductance variable gain can also be adjusted in order to
maintain the same optimum loop bandwidth. This makes the amplifier
circuit ideal for machines designed to handle different optical
data formats.
[0015] FIG. 1 is a schematic diagram of a transimpedance amplifier
circuit 10 connected to an optical detector 12 in accordance with
one embodiment of the invention. The optical detector 12 is
represented as a current source 14 with a parallel capacitance 16.
The current source 14 has an output current that is proportional to
an input light. The output of the current source 14 is connected to
an inverting input 18 of the transimpedance amplifier (operational
amplifier) 20. A non-inverting input 22 of the operational
amplifier 20 is connected to a ground 24. A feedback resistor 26 is
connected between an output 28 of the operational amplifier 20 and
the inverting input 18 of the operational amplifier 20. In one
embodiment the feedback resistor is a variable resistor.
[0016] FIG. 2 is a schematic diagram of an amplifier circuit 20 in
accordance with one embodiment of the invention. The overall
transimpedance amplifier 20 has a transconductance variable gain
stage 32. An input 34 of the transconductance variable gain stage
32 is capable of connecting to a photodetector. A second input 36
of the transconductance variable gain stage 32 is connected to
ground 38. An output 40 the transconductance variable gain stage 32
is connected to an input 42 of a transconductance gain stage 44. A
feedback resistor 46 is connected between an output 48 and an input
42 of the transconductance gain stage 44. The feedback resistor 46
is not the same as resistor 26 of FIG. 1.
[0017] In one embodiment, the total gain of the amplifier is set to
have a predetermined zero gain crossing. This is accomplished by
changing the feedback resistance 26 when the gain of the
transconductance variable gain stage 32 is changed.
[0018] FIG. 3 is a block diagram of a transconductance variable
gain stage 32 in accordance with one embodiment of the invention.
The transconductance variable gain stage 32 has a cascoded
differential amplifier 60. The cascoded differential amplifier
(differential amplifier) 60 has an inverting input 62 and a
non-inverting input 64. A first output 66 of the cascoded
differential amplifier 60 is connected to a positive current mirror
(positive cascoded current mirror, cascoded current mirror) 68. A
second output 70 of the cascoded differential amplifier 60 is
connected to the negative current mirror (negative cascoded current
mirror) 72. The positive and negative current mirror together may
be referred to as the current mirror. A current sink 74 is
connected to the cascoded differential amplifier 60. A current
generator/cascoded voltages circuit 76 is connected to the positive
and negative current mirrors 68, 72. The output 78 of the positive
current mirror 68 and the output 80 of the negative current mirror
72 are connected together to form an output 82. The gain of the
current mirrors 68, 72 can be changed by a gain signal 84. This is
how the gain of the transconductance gain stage 32 is varied. In
one embodiment the gain signal 84 is a recording signal. Once the
format of the optical storage device is known the recording signal
reflects this format.
[0019] FIG. 4 is a circuit diagram of a transconductance variable
gain stage 32 in accordance with one embodiment of the invention.
The inverting input 62 of the transconductance variable gain stage
32 is connected to a first transistor 100. The non-inverting input
64 is connected to a second transistor 102. The first transistor
100 and the second transistor 102 form a differential amplifier.
The transistors 104, 106, 108, 110 in combination with the
differential amplifier form the cascoded differential amplifier 60.
The current sink 74 is implemented as the transistor 112. The
positive current mirror 68 is formed by transistors 114, 116, 118,
120. The gain signal 84 controls transistors (gain switch, positive
gain switch) 122, 124. An inverter 126 causes one of the
transistors 122, 124 to be open while the other transistor is
closed. When transistor 124 is closed, transistor 120 does not draw
current as a result the positive current mirror only draws current
from transistors 114, 116. When transistor 124 is open and
transistor 122 is closed transistor 120 draws current. As a result,
the positive current mirror draws current from transistors 114,
116, 118, 120. In one embodiment, the current capacity of
transistors 118, 120 is twice the current of transistors 114, 116.
As a result the high gain mode draws three times the current of the
low gain mode.
[0020] The transistors 128, 130, 132 transfer the current from
transistors 104, 106 to the negative current mirror 72. The
negative current mirror 72 is formed by transistors 134, 136, 138,
140. The gain switch (negative gain switch) is formed by
transistors 142, 144. The negative current mirror works essentially
the same as the positive current mirror. The current
generator/cascoded voltages circuit 76 powers the current mirrors
and various transistors. Power is provided by the input voltage
146.
[0021] Thus there has been described an amplifier circuit for a
photodetector that has a variable gain necessary to compensate for
varying input light levels. When this circuit is used with optical
storage devices, it allows a single device to accurately play a
variety of different optical storage devices (formats).
[0022] While the invention has been described in conjunction with
specific embodiments thereof, it is evident that many alterations,
modifications, and variations will be apparent to those skilled in
the art in light of the foregoing description. Accordingly, it is
intended to embrace all such alterations, modifications, and
variations in the appended claims.
* * * * *