the paper describes low noise first stage optical receivers.Analysis of operating conditions affecting signal-to-noise ratio has been carried out.Each preamplifier was carefully optimised to work with particular type of the detector

Bielecki, Z.

Borejko, M.

Dufrene, R.

Kolosowski, W.

AMS Acta

Low noise optical receiversZ. Bielecki 1, W. Kolosowski 1, R. Dufrene 2, M. Borejko 31 Military University of Technology, 2 Kaliskiego Str. 00-908 Warszaw, Poland, phone (+48 22) 683-97-89 2 R. Dufrene. Telecommunications Research Institute. Warsaw, Poland3 M. Borejko. Scientific Industrial Centre of Professional Electronics RADWAR, Warsaw, PolandAbstract – the paper describes low noise first stageoptical receivers. Analysis of operating conditions affectingsignal-to-noise ratio has been carried out. Each preamplifier was carefully optimised to work with particular type of thedetector.Keywords – optical detectors, low noise preamplifier.I. INTRODUCTIONOptical radiation receivers are used in many up-to-date fields of science and technology determining current level of technological progress. The most important fields of their applications are industrial automation, robotics,space technology, medicine, and military technology.Photodetectors and low-noise signal processing arethe main components of a receiver of optical radiation.Signal amplitude at the detector output is of very lowvalue and in many cases is similar to noise level value.Thus, for signal amplification the low-noise systemsshould be used. Preamplifiers should ensure adequateamplification, their own noise should be low and theirbandwidth should be broad enough to obtain non-distoredreproduction of a shape of input signals.The noise produced in detectors and systems of signal processing circuits  constrained detection of low intensity signals (Fig. 1).Fig. 1. Main noise sources in photoreceiver.Optical radiation from an object is detected by aphotoreceiver. Total radiation received from any object is the sum of the emitted, reflected and transmitted radiation [1,2]. Objects that are not blackbodies emit only fractionof blackbody radiation, and the remaining fraction iseither transmitted or, for opaque objects, reflected. When the scene is composed of the objects and backgrounds ofsimilar temperatures, the reflected radiation tends toreduce the available contrast. However, reflections ofhotter or colder objects significantly effect theappearance of a thermal scene. The reflected sunlight isnegligible for 8 – 14 µm imaging, but it is important inUV, VIS and 3-5 µm band [3].An optical block in IR receiver produces an image ofthe observed objects in plane of the detector. The opticalelements like windows, domes and filters can be used toprotect the receiver from environment or to modifydetector spectral response.The most popular materials used for manufacturing of refractive optics of IR systems are: germanium (Ge),silicon (Si), fused silica (SiO2), zinc selenide (ZnSe),glass BK-7, and zinc sulphide (ZnS).Photons may originate from a variety of sources:scene background, path radiance, optics, warm filters,cold filter, and housing. The background noise can bereduced by using cold shied housing (Fig. 2). Photonnoise is unique to thermal imaging systems. It isnonexistent for visible sensors since the background inthe visible does not provide any photons. If the coldshield is isolated from the cooler, it may be at an elevated temperature. A warm filter (not shown) can alsocontribute to photon noise.Fig. 2. Cold shield housing decreases photon noise.There is no essential difference in design rules ofoptical objectives for visible and IR ranges. Designer ofIR optics has significantly fewer materials suitable for IR optical elements, in comparison with those for visiblerange, particularly for wavelengths over 2.5 µm.II. MAXIMISATION SIGNAL-TO-NOISE RATIOFigure 1 shown a simple block diagram of the firststage of a photoreceiver (photodetector plus low noisepreamplifier and biasing circuit of the detector). Theoptical signal and background radiation impinge on thephotodetector inducing a current in the external loadresistor.   If Ph is average optical power, then Iph, theaverage photocurrent, is given byDetector- Dark - Background- SignalCurrent:Thermal noisePreamp. noisePhotoreceiverPreamplifierLoadResistorOpticalsignalBack-groundOutput signaland noiseDetectorCold aperture0Field of view  [degree]Detectivity*D1020 40 60 80 100 120 140 160180101001211th GAAS Symposium - Munich 2003 137id hhqPI ph =. (1)where q is the electron charge, d is the quantumefficiency, h is the Planck constant  and i is the opticalfrequency of light. To convert this to volts, simply multiply by the loadresistor. For 100 % sinusoidal modulated signal, the rmsoptical power is given by 2/hP , and the rmsphotocurrent isid hhqPI ph2= . (2)Generation and recombination noise results fromstatistical fluctuation in the rate of generation and raterecombination of charged particles into an upper statewithin the detector material. The statistical fluctuation inthe concentration of carriers produces the white noise,which has electrical frequencies less than the inverse ofcarrier lifetime. The current noise generated andrecombined in the bulk before reaching electrodes. Thecurrent noise expression for generation-recombinationnoise is( ) fgGqfAgqI thbsRG 6+6\+\=< 2222 44 d ,    (3)where \s is the radiant incident power from the signal,\b is the radiant incident power from the background,A is the detector area, g is the photoconductive gain, 6f is the electronic frequency bandwidth, Gth is the thermalconductance.Johnson noise of the detector (Rd) and load resistance (RL) is the fluctuation caused by the thermal motion ofthe charge carriers in resistive element. The Johnsonnoise current of the detector and load resistance RL isgiven respectively,44LLLJdddJ RfkTIandRfkTI6=6= <<      (4)where Td is the detector temperature, Rd is the detectorresistance, TL is the load resistance temperature.Shot noise is associated with the dc current flowingacross a potential barrier and is a series of independentevents. Because the dc current is proportional to thecross-selectional area of the detector, this noise is alsoproportional to the square root of the detector area, if the potential barrier cross-selectional area is the opticalactive area, as is typical for a photodiode. The current-noise expression for shot noise is( ) 2/12 fqII sh 6= .    (5)If I is the result of photons, that is, a photocurrent,then the noise of the detector is photon-noise limited. 1/f noise is a strong function of frequency, and mostimportant at low audio frequencies. The noise power isapproximately inversely proportional to the frequency`_fkII bf =/1 , (6)where k is the Boltzmann constant, Ib is the bias of thedetector, f is the electrical frequency.The one-over-f noise magnitude is affected by nonohmiccontacts at electrodes, and surface-state traps that causecurrent flow to be interrupted by variations in thetrapping time constant. One-over-f noise is alwayspresent in both photoconductors and bolometers, because there is always a dc-bias current present.The main purpose was to analyse the input stages ofphotoreceivers to optimise them providing maximal value of signal-to-noise ratio.A general noise model of the detector and load circuit has been accepted (Fig. 3).Fig. 3. Equivalent noise model of a detector and load of thephotoreceiver.(where Iph is the photocurrent, Ind is the detector noise, Inb is the background noise, Cd, Rd are the capacity and resistance of adetector respectively).Preamplifiers used for photoreceivers can beclassified as three categories: low impedancepreamplifiers, high impedance preamplifiers andtransimpedance preamplifiers. The active device of apreamplifier can be bipolar, JFET, or it can be an IC with bipolar, FET or MOSFET input stageSelection of active device primarily depends ondetector resistance and frequency range. It is difficult tosay exactly where device of each type should be used.The input active device of the preamplifier, is usually the dominant noise contributor of the readout. The noisecontribution of the input transistor is a function of thesource impedance exhibited by the detector. Thus, it isimportant to match the detector with an appropriatereadout input transistor [4]. Preamplifier noise is presented as two sources,voltage and current ones. Basing on the analysis of noisemodels of the receivers with photoconductive detectors,the following expression for S/N ratio can be written [5]22222/12aLJdJRGfphIIIIIINS++++=<<<   (7)where Ia is the preamplifier noise.The counter exhibits the squared average value of aphotocurrent and the dominator shows the totalequivalent input noise. The first term of the denominatordetermines the noise of 1/f type, the second one -generation-recombination noise resulted fromfluctuations of radiation from signal and background, thethird term - generation-recombination noise produced bythermally excited carriers in semiconductors, next termsrepresent thermal noise of detector resistance, loading,and preamplifier.The noise of 1/f type depends on technology ofdetector manufacturing and the value of a bias current.RI I I I Cph nph d nd nb d RLViInRLoadDetector11th GAAS Symposium - Munich 2003138The second term can be reduced due to narrowing thedetector’s field of view, application of cooled diaphragms and optical filters. Lowering the detector temperaturecauses reduction of the third and fourth terms. High loadresistance gives thermal noise minimisation. Theoptimised preamplifiers of low noise should be used.To analysis of S/N ratio of the receivers withphotodiode, the following expression for S/N ratio wasobtained [5]( ) nLTdbsbphphFRfkTFMIIFMIIfqIMNS6+µµ³³+++6=< 42)(2 103002222(8)where M is the multiplication factor, F is the photodiodenoise factor, V is the bias voltage of the photodiode, andFn is the noise factor of the preamplifier.The counter includes squared average value ofphotocurrent multiplied by squared multiplication factorof an avalanche photodiode. Denominator represents thetotal substitute input noise of a receiver. The first term of Eq. (8) describes shot noise, the second one - thermalnoise of load resistance and preamplifier noise. Shotnoise depends on photocurrent, background current, aswell as on superficial and volumetric components of dark current.The results of the above mentioned activities wereused for experimental investigations, i.e., several IRdetection devices were designed, performed, and tested[6-8].III. EXAMPLES OF PREAMPLIFIER AND EXPERIMENTAL RESULTSMany problems have to be overcome duringconstruction of low-noise preamplifier for low-resistancedetectors (e.g., HgCdTe of resistance below 100 1). It isdue to the fact that the detectors of resistance of the order of 20 1 produce noise voltage lower than 0.6 nV/Hz1/2,i.e., below the noise voltage generated in the best(available) amplifying elements [9]. So, the questionarises; it is possible to build a preamplifier of noisevoltage below a detector noise? It appears that it can beachieved when several identical preamplifiers areconnected in parallel and next the output signals areadded.Such a system of signal processing ensures reductionof final input signal according to the relationship( ) 2/1<u= nVV ntotaln ,                         (9)where Vn is the noise voltage of a single preamplifier and n isthe number of amplifying stages.Figure 4 presents a diagram of transimpedancepreamplifier that is recommended for amplification of the signals received from low-resistance IR detectors. Lownoise transistors MAT 03 type were used at the inputstage of an preamplifier. This preamplifier can be applied both for photodiodes and photoresistors. When aphotoresistor is connected, the polarising resistor RL isrequired.For this system the noise voltage was 0.35 nV/Hz1/2 atf = 10 kHz.For amplification of the signals from detectors ofresistances of above 10 M1, the most frequently used are voltage and transimpedance amplifiers. The basic idea ofincrease in input impedance of preamplifier is reductionof thermal  noises. However, the high resistances RLcause narrowing of a band of the input stage of aphotoreceiver.     A preamplifier of high input impedance is significantload resistance for a detector so, it does not ensure widerange of signal changes. The problem of serious changesof a signal has been solved in transimpedancepreamplifiers.R7R6C2Output-VZC6C5+VZC4C3T1InputT2 T3 T4 T5 T6C1OP27R4R5R3R2C7MAT03R1T72SD737D1D2Fig. 4. Preamplifier for low-resistance infrared detectors.For amplification of the signals from high-resistancedetectors, the system was used the simplified scheme ofwhich is shown in Fig. 5. In the preamplifier, the integrated circuit of OPA 129 type was used.The output voltage to the op-amp is then,dphin ZIV =  (10)where Zd is the detector impedance.The detector impedance is given bydddd CRjRZt+=1.     (11)Since the feedback impedance and detectorimpedance act as a voltage divider between Vout andground, and since we can assume that Vin<<Vout, theoutput voltage is,dfinout ZZVV <=  (12)whereffff CRjRZt+=1.   (13)From equation 10 and 13 we obtainfffphout CRjRIVt+<=1.    (14)The amplitude of Vout is its absolute value,11th GAAS Symposium - Munich 2003 139( ) ( ) 2/1222/1*1 ffphoutoutoutRIVVVot+== ,   (15)where of = RfCf.The signal and cutoff frequency ff is the frequency atwhich the signal has decreased by a factor of 2ffff CRf/o/ 2121== . (16)Notice that the output voltage is independent of Rdand Cd. As long as Rf is linear and Iph is a linear functionof the photon flux, the detector-amplifier system is linear. Moreover, even if Rd Cd >>of, the frequency response ofthe system is determined by of, the limit imposed by thefeedback resistor. As the detector impedance decreaseswith increasing frequency, the transimpedance gainincreases to compensate (equation 12). The highfrequency performance is improved substantially forphotodiodes because the capacitance of the feedbackresistor can be made much smaller than that of thedetector.Fig. 5. Preamplifier for high-resistance infrared detectors.The noise analysis of the preamplifier was alsoperformed by mean of the computer programme CLIENT 99 of the Protel International PLD Ltd. Firm. Theprogramme ensures determination of approximate noiseparameters already at the stage of a system design. It isespecially important for design of low-noise amplifiersused in measuring or detecting systems of very lowsignals. Thus, due to computer simulation we can avoiddesigning the input stages of measuring or detectingsystems, that do not fulfil the assumed requirements.Next, we have measured noise level of a preamplifierusing the method presented in Fig. 6.Fig. 6. Noise measurement system.This is a measurement system recommended by MIL-STD-202 standard. Noise was measured using homodyne nanovoltmeter SR 830 DSP type. The received inputnoise current was 0.25 fA/Hz1/2 for f = 10 kHz.Measurement noise is about 1.4 times higher than thecalculated one.Further works on improvement in preamplifiers for low-resistance HgCdTe and GaN detectors are being made.IV CONCLUSIONResearch and design works were limited to the signalprocessing systems emploing photovoltaic infrareddetectors. An analysis of noise models of photodetectorpreamplifier systems was carried out. The preamplifiersare designed for detectors. Each preamplifier is carefullyoptimised to work with particular type of the detector.Current mode configuration results in excellent stabilityof the first stage in all working conditions. Specialattention has been paid to ensure maximum signal tonoise ratio in wide range of detectors resistance. All input stages of the AC preamplifiers employ ultra low noisetransistors so the bias point can be precisely trimmed toobtain the optimal matching with the detector. Themodels of preamplifiers are placed in integrated housingwhich provides for the thermoelectric cooler of thedetector, effective EMI shielding, and is more convenient for customer. Our preamplifiers are indyviduallyoptimised to work with optical detectors. However, theycan be used with many types of detectors commerciallyavailable.The eleborated preamplifiers can be used in IRpyrometers, IR sensors, thermal scanners, IR thermalcameras, smart munitions, and in many otherapplications.REFERENCES[1]. A. Rogalski and J. Piotrowski, “Intrinsic infraredphotodetectors,” Prog. in Quant. Electr. 12, 1988.[2]. G.H. Rieke, Detection of Light: from the Ultraviolet to the Submillimeter, Cambridge, Cambridge University Press,1994.[3]. C.T. Elliot, D. Day, and D.J. Wilson, “An integratingdetector for serial scan thermal imaging,” Infrared Phys.22, 31–42 (1982). [4]. C.D. Mothenbacher, J.A. Connelly. Low-noise electronic system design. New York, Willey 1995.[5]. Z. Bielecki, “Maximisation of signal to noise ratio ininfrared radiation receivers,” Opto-electron. Rev. 10, 209–216 (2002). [6]. Z. Bielecki. “Photoreceiver with avalanche C-30645photodiode” IEE Proceedings Optoelectronics, 147, 234-236 (2000).[7]. Z. Bielecki, “Photoreceiver with popcorn-noise reductionsystems,” 26th International Conference on Infrared andMillimeter Waves, 10–14 September, Toulouse, France, 72 - 73 (2001).[8]. Z. Bielecki, “Elimination of popcorn noise in receiverswith pyroelectric detectors”. International ConferenceInfrared Sensors & Systems, Erfurt, Proc. IRS2, 173–177(2002).[9]. Catalog Rohm Co. Ltd. Epitaxial planar npn silicontransistors 2SD 786 type (1990).C ~1pFf+OPA129+15VR 1000M1f~0,1 Fµ23 4870,1 Fµ60,1 Fµ1000M1-15VPhotodiodePreamp. Amp.OscyloscopeGeneratorDetectorAttenuatormABiassupplyNano--voltmeter11th GAAS Symposium - Munich 2003140

English

Low noise optical receivers

AMS Acta - Alm@DL - Università di Bologna

Low noise optical receivers
Z. Bielecki 1, W. Kolosowski 1, R. Dufrene 2, M. Borejko 3
1 Military University of Technology, 2 Kaliskiego Str. 00-908 Warszaw, Poland, phone (+48 22) 683-97-89
 2 R. Dufrene. Telecommunications Research Institute. Warsaw, Poland
3 M. Borejko. Scientific Industrial Centre of Professional Electronics RADWAR, Warsaw, Poland
Abstract – the paper describes low noise first stage
optical receivers. Analysis of operating conditions affecting
signal-to-noise ratio has been carried out. Each preamplifier 
was carefully optimised to work with particular type of the
detector.
Keywords – optical detectors, low noise preamplifier.
I. INTRODUCTION
Optical radiation receivers are used in many up-to-
date fields of science and technology determining current 
level of technological progress. The most important fields 
of their applications are industrial automation, robotics,
space technology, medicine, and military technology.
Photodetectors and low-noise signal processing are
the main components of a receiver of optical radiation.
Signal amplitude at the detector output is of very low
value and in many cases is similar to noise level value.
Thus, for signal amplification the low-noise systems
should be used. Preamplifiers should ensure adequate
amplification, their own noise should be low and their
bandwidth should be broad enough to obtain non-distored
reproduction of a shape of input signals.
The noise produced in detectors and systems of signal 
processing circuits  constrained detection of low intensity 
signals (Fig. 1).
Fig. 1. Main noise sources in photoreceiver.
Optical radiation from an object is detected by a
photoreceiver. Total radiation received from any object is 
the sum of the emitted, reflected and transmitted radiation 
[1,2]. Objects that are not blackbodies emit only fraction
of blackbody radiation, and the remaining fraction is
either transmitted or, for opaque objects, reflected. When 
the scene is composed of the objects and backgrounds of
similar temperatures, the reflected radiation tends to
reduce the available contrast. However, reflections of
hotter or colder objects significantly effect the
appearance of a thermal scene. The reflected sunlight is
negligible for 8 – 14 µm imaging, but it is important in
UV, VIS and 3-5 µm band [3].
An optical block in IR receiver produces an image of
the observed objects in plane of the detector. The optical
elements like windows, domes and filters can be used to
protect the receiver from environment or to modify
detector spectral response.
The most popular materials used for manufacturing of 
refractive optics of IR systems are: germanium (Ge),
silicon (Si), fused silica (SiO2), zinc selenide (ZnSe),
glass BK-7, and zinc sulphide (ZnS).
Photons may originate from a variety of sources:
scene background, path radiance, optics, warm filters,
cold filter, and housing. The background noise can be
reduced by using cold shied housing (Fig. 2). Photon
noise is unique to thermal imaging systems. It is
nonexistent for visible sensors since the background in
the visible does not provide any photons. If the cold
shield is isolated from the cooler, it may be at an elevated 
temperature. A warm filter (not shown) can also
contribute to photon noise.
Fig. 2. Cold shield housing decreases photon noise.
There is no essential difference in design rules of
optical objectives for visible and IR ranges. Designer of
IR optics has significantly fewer materials suitable for IR 
optical elements, in comparison with those for visible
range, particularly for wavelengths over 2.5 µm.
II. MAXIMISATION SIGNAL-TO-NOISE RATIO
Figure 1 shown a simple block diagram of the first
stage of a photoreceiver (photodetector plus low noise
preamplifier and biasing circuit of the detector). The
optical signal and background radiation impinge on the
photodetector inducing a current in the external load
resistor.   If P? is average optical power, then Iph, the
average photocurrent, is given by
Detector
- Dark 
- Background
- SignalCurrent:
Thermal noise
Preamp. noise
Photoreceiver
PreamplifierLoadResistor
Optical
signal
Back-
ground
Output signal
and noise
Detector
Cold aperture
0
Field of view  [degree]
D
et
ec
tiv
ity
*
D
10
20 40 60 80 100 120 140 160180
10
10
0
1
2
11th GAAS Symposium - Munich 2003 137
?
? ?
h
qP
I ph =
. (1)
where q is the electron charge, ? is the quantum
efficiency, h is the Planck constant  and ? is the optical
frequency of light. 
To convert this to volts, simply multiply by the load
resistor. For 100 % sinusoidal modulated signal, the rms
optical power is given by 2/?P , and the rms
photocurrent is
?
? ?
h
qP
I ph
2
=
 . (2)
Generation and recombination noise results from
statistical fluctuation in the rate of generation and rate
recombination of charged particles into an upper state
within the detector material. The statistical fluctuation in
the concentration of carriers produces the white noise,
which has electrical frequencies less than the inverse of
carrier lifetime. The current noise generated and
recombined in the bulk before reaching electrodes. The
current noise expression for generation-recombination
noise is
( ) fgGqfAgqI thbsRG ?+??+?=? 2222 44 ? ,    (3)
where ?s is the radiant incident power from the signal,
?b is the radiant incident power from the background,
A is the detector area, g is the photoconductive gain, ?f is 
the electronic frequency bandwidth, Gth is the thermal
conductance.
Johnson noise of the detector (Rd) and load resistance 
(RL) is the fluctuation caused by the thermal motion of
the charge carriers in resistive element. The Johnson
noise current of the detector and load resistance RL is
given respectively
,
44
L
L
LJ
d
d
dJ R
fkT
Iand
R
fkT
I
?
=
?
= ??      (4)
where Td is the detector temperature, Rd is the detector
resistance, TL is the load resistance temperature.
Shot noise is associated with the dc current flowing
across a potential barrier and is a series of independent
events. Because the dc current is proportional to the
cross-selectional area of the detector, this noise is also
proportional to the square root of the detector area, if the 
potential barrier cross-selectional area is the optical
active area, as is typical for a photodiode. The current-
noise expression for shot noise is
( ) 2/12 fqII sh ?= .    (5)
If I is the result of photons, that is, a photocurrent,
then the noise of the detector is photon-noise limited. 
1/f noise is a strong function of frequency, and most
important at low audio frequencies. The noise power is
approximately inversely proportional to the frequency
?
?
f
kI
I bf =/1 , (6)
where k is the Boltzmann constant, Ib is the bias of the
detector, f is the electrical frequency.
The one-over-f noise magnitude is affected by nonohmic
contacts at electrodes, and surface-state traps that cause
current flow to be interrupted by variations in the
trapping time constant. One-over-f noise is always
present in both photoconductors and bolometers, because 
there is always a dc-bias current present.
The main purpose was to analyse the input stages of
photoreceivers to optimise them providing maximal value 
of signal-to-noise ratio.
A general noise model of the detector and load circuit 
has been accepted (Fig. 3).
Fig. 3. Equivalent noise model of a detector and load of the
photoreceiver.
(where Iph is the photocurrent, Ind is the detector noise, Inb is the 
background noise, Cd, Rd are the capacity and resistance of a
detector respectively).
Preamplifiers used for photoreceivers can be
classified as three categories: low impedance
preamplifiers, high impedance preamplifiers and
transimpedance preamplifiers. The active device of a
preamplifier can be bipolar, JFET, or it can be an IC with 
bipolar, FET or MOSFET input stage
Selection of active device primarily depends on
detector resistance and frequency range. It is difficult to
say exactly where device of each type should be used.
The input active device of the preamplifier, is usually the 
dominant noise contributor of the readout. The noise
contribution of the input transistor is a function of the
source impedance exhibited by the detector. Thus, it is
important to match the detector with an appropriate
readout input transistor [4]. 
Preamplifier noise is presented as two sources,
voltage and current ones. Basing on the analysis of noise
models of the receivers with photoconductive detectors,
the following expression for S/N ratio can be written [5]
22222
/1
2
aLJdJRGf
ph
IIIII
I
N
S
++++
=
???
   (7)
where Ia is the preamplifier noise.
The counter exhibits the squared average value of a
photocurrent and the dominator shows the total
equivalent input noise. The first term of the denominator
determines the noise of 1/f type, the second one -
generation-recombination noise resulted from
fluctuations of radiation from signal and background, the
third term - generation-recombination noise produced by
thermally excited carriers in semiconductors, next terms
represent thermal noise of detector resistance, loading,
and preamplifier.
The noise of 1/f type depends on technology of
detector manufacturing and the value of a bias current.
RI I I I Cph nph d nd nb d RL
Vi
InR
LoadDetector
11th GAAS Symposium - Munich 2003138
The second term can be reduced due to narrowing the
detector’s field of view, application of cooled diaphragms 
and optical filters. Lowering the detector temperature
causes reduction of the third and fourth terms. High load
resistance gives thermal noise minimisation. The
optimised preamplifiers of low noise should be used.
To analysis of S/N ratio of the receivers with
photodiode, the following expression for S/N ratio was
obtained [5]
( ) n
L
T
dbsbph
ph
F
R
fkT
FMIIFMIIfq
IM
N
S
?
+
?
?
?
?
?
?
?
?
+++?
=
? 4
2)(2 10
300
22
22
(8)
where M is the multiplication factor, F is the photodiode
noise factor, V is the bias voltage of the photodiode, and
Fn is the noise factor of the preamplifier.
The counter includes squared average value of
photocurrent multiplied by squared multiplication factor
of an avalanche photodiode. Denominator represents the
total substitute input noise of a receiver. The first term of 
Eq. (8) describes shot noise, the second one - thermal
noise of load resistance and preamplifier noise. Shot
noise depends on photocurrent, background current, as
well as on superficial and volumetric components of dark 
current.
The results of the above mentioned activities were
used for experimental investigations, i.e., several IR
detection devices were designed, performed, and tested
[6-8].
III. EXAMPLES OF PREAMPLIFIER AND 
EXPERIMENTAL RESULTS
Many problems have to be overcome during
construction of low-noise preamplifier for low-resistance
detectors (e.g., HgCdTe of resistance below 100 ?). It is
due to the fact that the detectors of resistance of the order 
of 20 ? produce noise voltage lower than 0.6 nV/Hz1/2,
i.e., below the noise voltage generated in the best
(available) amplifying elements [9]. So, the question
arises; it is possible to build a preamplifier of noise
voltage below a detector noise? It appears that it can be
achieved when several identical preamplifiers are
connected in parallel and next the output signals are
added.
Such a system of signal processing ensures reduction
of final input signal according to the relationship
( ) 2/1??= nVV ntotaln ,                         (9)
where Vn is the noise voltage of a single preamplifier and n is
the number of amplifying stages.
Figure 4 presents a diagram of transimpedance
preamplifier that is recommended for amplification of the 
signals received from low-resistance IR detectors. Low
noise transistors MAT 03 type were used at the input
stage of an preamplifier. This preamplifier can be applied 
both for photodiodes and photoresistors. When a
photoresistor is connected, the polarising resistor RL is
required.
For this system the noise voltage was 0.35 nV/Hz1/2 at
f = 10 kHz.
For amplification of the signals from detectors of
resistances of above 10 M?, the most frequently used are 
voltage and transimpedance amplifiers. The basic idea of
increase in input impedance of preamplifier is reduction
of thermal  noises. However, the high resistances RL
cause narrowing of a band of the input stage of a
photoreceiver.
     A preamplifier of high input impedance is significant
load resistance for a detector so, it does not ensure wide
range of signal changes. The problem of serious changes
of a signal has been solved in transimpedance
preamplifiers.
R7
R6C2
Output
-VZ
C6C5
+VZ
C4C3
T1
Input
T2 T3 T4 T5 T6
C1
OP27
R4R5R3R2
C7
MAT03
R1
T7
2SD737
D1
D2
Fig. 4. Preamplifier for low-resistance infrared detectors.
For amplification of the signals from high-resistance
detectors, the system was used the simplified scheme of
which is shown in Fig. 5. 
In the preamplifier, the integrated circuit of OPA 129 
type was used.
The output voltage to the op-amp is then
,dphin ZIV =  (10)
where Zd is the detector impedance.
The detector impedance is given by
dd
d
d CRj
R
Z
?+
=
1
.     (11)
Since the feedback impedance and detector
impedance act as a voltage divider between Vout and
ground, and since we can assume that Vin<<Vout, the
output voltage is
,d
f
inout Z
Z
VV ?=  (12)
where
ff
f
f CRj
R
Z
?+
=
1
.   (13)
From equation 10 and 13 we obtain
ff
fph
out CRj
RI
V
?+
?=
1
.    (14)
The amplitude of Vout is its absolute value,
11th GAAS Symposium - Munich 2003 139
( ) ( ) 2/122
2/1*
1 f
fph
outoutout
RI
VVV
??+
== ,   (15)
where ?f = RfCf.
The signal and cutoff frequency ff is the frequency at
which the signal has decreased by a factor of 2
fff
f CR
f
??? 2
1
2
1
== . (16)
Notice that the output voltage is independent of Rd
and Cd. As long as Rf is linear and Iph is a linear function
of the photon flux, the detector-amplifier system is linear. 
Moreover, even if Rd Cd >>?f, the frequency response of
the system is determined by ?f, the limit imposed by the
feedback resistor. As the detector impedance decreases
with increasing frequency, the transimpedance gain
increases to compensate (equation 12). The high
frequency performance is improved substantially for
photodiodes because the capacitance of the feedback
resistor can be made much smaller than that of the
detector.
Fig. 5. Preamplifier for high-resistance infrared detectors.
The noise analysis of the preamplifier was also
performed by mean of the computer programme CLIENT 
99 of the Protel International PLD Ltd. Firm. The
programme ensures determination of approximate noise
parameters already at the stage of a system design. It is
especially important for design of low-noise amplifiers
used in measuring or detecting systems of very low
signals. Thus, due to computer simulation we can avoid
designing the input stages of measuring or detecting
systems, that do not fulfil the assumed requirements.
Next, we have measured noise level of a preamplifier
using the method presented in Fig. 6.
Fig. 6. Noise measurement system.
This is a measurement system recommended by MIL-
STD-202 standard. Noise was measured using homodyne 
nanovoltmeter SR 830 DSP type. The received input
noise current was 0.25 fA/Hz1/2 for f = 10 kHz.
Measurement noise is about 1.4 times higher than the
calculated one.
Further works on improvement in preamplifiers for low-
resistance HgCdTe and GaN detectors are being made.
IV CONCLUSION
Research and design works were limited to the signal
processing systems emploing photovoltaic infrared
detectors. An analysis of noise models of photodetector
preamplifier systems was carried out. The preamplifiers
are designed for detectors. Each preamplifier is carefully
optimised to work with particular type of the detector.
Current mode configuration results in excellent stability
of the first stage in all working conditions. Special
attention has been paid to ensure maximum signal to
noise ratio in wide range of detectors resistance. All input 
stages of the AC preamplifiers employ ultra low noise
transistors so the bias point can be precisely trimmed to
obtain the optimal matching with the detector. The
models of preamplifiers are placed in integrated housing
which provides for the thermoelectric cooler of the
detector, effective EMI shielding, and is more convenient 
for customer. Our preamplifiers are indyvidually
optimised to work with optical detectors. However, they
can be used with many types of detectors commercially
available.
The eleborated preamplifiers can be used in IR
pyrometers, IR sensors, thermal scanners, IR thermal
cameras, smart munitions, and in many other
applications.
REFERENCES
[1]. A. Rogalski and J. Piotrowski, “Intrinsic infrared
photodetectors,” Prog. in Quant. Electr. 12, 1988.
[2]. G.H. Rieke, Detection of Light: from the Ultraviolet to the 
Submillimeter, Cambridge, Cambridge University Press,
1994.
[3]. C.T. Elliot, D. Day, and D.J. Wilson, “An integrating
detector for serial scan thermal imaging,” Infrared Phys.
22, 31–42 (1982). 
[4]. C.D. Mothenbacher, J.A. Connelly. Low-noise electronic 
system design. New York, Willey 1995.
[5]. Z. Bielecki, “Maximisation of signal to noise ratio in
infrared radiation receivers,” Opto-electron. Rev. 10, 209–
216 (2002). 
[6]. Z. Bielecki. “Photoreceiver with avalanche C-30645
photodiode” IEE Proceedings Optoelectronics, 147, 234-
236 (2000).
[7]. Z. Bielecki, “Photoreceiver with popcorn-noise reduction
systems,” 26th International Conference on Infrared and
Millimeter Waves, 10–14 September, Toulouse, France, 72 
- 73 (2001).
[8]. Z. Bielecki, “Elimination of popcorn noise in receivers
with pyroelectric detectors”. International Conference
Infrared Sensors & Systems, Erfurt, Proc. IRS2, 173–177
(2002).
[9]. Catalog Rohm Co. Ltd. Epitaxial planar npn silicon
transistors 2SD 786 type (1990).
C ~1pFf
+
OPA129
+15V
R 1000M?f~
0,1 Fµ
2
3 4
8
7
0,1 Fµ
6
0,1 Fµ
1000M?
-15V
Photodiode
Preamp. Amp.
Oscyloscope
Generator
Detector
Attenuator
mA
Bias
supply
Nano-
-voltmeter
11th GAAS Symposium - Munich 2003140


Abstract – the paper describes low noise first stage optical receivers. Analysis of operating conditions affecting signal-to-noise ratio has been carried out. Each preamplifier was carefully optimised to work with particular type of the detector

Low noise optical receivers

Abstract

Similar works

Full text

Available Versions

AMS Acta

AMS Acta - Alm@DL - Università di Bologna

CiteSeerX