performanceanalysisofbootstraptransimpedanceamplifierforla...

2006 INTERNATIONAL RF AND MICROWAVE CONFERENCE PROCEEDINGS, SEPTEMBER 12 - 14, 2006, PUTRAJAYA, MALAYSIA

Performance Analysis of Bootstrap Transimpedance Amplifier For LargeWindows Optical Wireless Receiver

S. M. Idrus, N. Ngajikin, N. N. N. A. Malik and S. I. A. Aziz

Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Darul Takzim,MALAYSIA

sevia ftke.utm.my, nhafizah ftke.utm.my

Abstract- Due to optical wireless link power budgetconsiderations, the receiver is required to have alarge collection area. One of the main noisemechanisms in wideband preamplifiers employinglarge area detectors is the noise due to the low passfilter formed by the detector capacitance and theinput impedance to the preamplifier. Typical largephotodetection area commercial detectors hascapacitance are around 100-300 pF compared to50pF in fiber link. Hence, techniques to reduce theeffective detector capacitance are required in order toachieve a low noise and wide bandwidth design. Inthis paper analysis on the bootstrap transimpedanceamplifier (BTA) for input capacitance reduction willbe reported. This technique offers the usualadvantages of the transimpedance amplifier togetherwith an effective capacitance reduction technique foroptical wireless detector.

Keyword: Wireless optical communication,Transimpedance amplifier, photodetector, Bootstrap

1. Introduction

Since, the optical wireless link operates withlimited transmitter power, due to safetyconsiderations, in relatively high noise environmentsas a result of ambient light levels. Thus, theperformance of the optical receiver has a significantimpact on the overall system performance. In orderto reduce shot noise in the detector due to ambientlight an optical filter is required, whilst thepreamplifier should allow shot-noise limitedoperation.

Due to link budget considerations, the receiver isrequired to have a large collection area, which maybe achieved through the use of an opticalconcentrator (effectively noiseless gain) [1], a largearea photodetector or a combination of the two. Sinceindoor optical transceivers are intended for masscomputer and peripheral markets, the receiver designis extremely cost sensitive, which can makesophisticated optical systems unattractive.

The design of an optical receiver depends on themodulation format used by the transmitter. Theoptical wireless receiver system are, essentially

consists of the photo detector plus a pre-amplifierwith possibly additional signal processing circuit.Therefore, it is necessary to consider the propertiesof this device in the context of the associatedcircuitry combined in the receiver. It is essential thatthe detector perform efficiently with the followingamplifying and signal processing.

However for all optical receivers, fiber andwireless alike, their sensitivity is a trade off betweenphotodiode parameters and circuit noise.Applications that require a good sensitivity and abroad bandwidth will invariably use a small areaphotodiode, which means that the aperture is small.Receivers for long distance point-to-point fibersystems generally fall into this category. Conversely,for wireless optic applications require a largeaperture and so must use a large area photodiode,where upon sensitivity and speeds are reduced [2].As expected the sensitivity improves (i.e., reduces innumerical value) as the photodiode area reducesbecause of the correspondingly lower capacitance.However, small area photodiodes incur a greatercoupling loss due to the small aperture they presentto the incoming beam, so a careful trade off betweenthese factors is necessary to optimize the overallperformance.

Figure 1 shows that a receiver with an APDgives an 10dB sensitivity advantage over acorresponding PIN receiver, which is consistent with

Figure 1: Receiver sensitivity (at 155Mbps) in relationto photodiode type and detection area [2].

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observations on optical fiber receivers. APDreceivers however are more costly and require highoperating voltages, hence are predominantly used inspecialist systems where performance is key.Oppositely, for indoor systems which economy is apriority, favor PIN receivers.

2. Preamplification Technique

The current from the detector is usuallyconverted to a voltage before the signal is amplified.The current to voltage converter is perhaps the mostimportant section of any optical receiver circuit. Animproperly designed circuit will often suffer fromexcessive noise associated with ambient lightfocused onto the detector. To get the most from theoptical signal through the air system, the right front-end circuitry design must be considered. An opticalreceiver's front-end design can be usually groupedinto these pre-amplification techniques: low-impedance voltage amplifier; a high impedanceamplifier; and a trans-impedance amplifier. Any ofthe configurations can be built using contemporaryelectronics devices i.e. bipolar junction transistors(BJT), field effect transistors (FET), or high electronmobility transistors (CMOS). The receiverperformance that is achieved will depend on thedevices and design techniques used.

An equivalent circuit of a PN junctionphotodetector with and input the preamplifier stage isshown in Figure 2. The diode shunt resistance, Rj, ina reverse biased junction is usually very large(>106Q), compared to the load impedance RI, and canbe neglected. The resistance R, represents ohmiclosses in the bulk p and n regions adjacent to thejunction, and Cd represent the dynamic photodiodecapacitance. ~~~~~~~~~~~~A11

Figure 2: Simple equivalent circuit for PN or PINphotodetector.

The design of the front-end requires a trade-offbetween speed and sensitivity. Since using a largeload resistor RL can increase the input voltage to thepreamplifier, high impedance front-end is often used.

Furthermore, a large RL reduces the thermal noiseand improves the receiver sensitivity. The maindrawback of high impedance frond-end is its lowbandwidth given by Af= (27rRLCT )-1, where Rs << RLis assumed and CT includes the contributions fromthe photodiode (Cd ) and the transistor used foramplification (Ca). A high-impedance front-endcannot be used if Af is considerably less than the bitrate. An equalizer is sometime used to increase thebandwidth. The equalizer acts as a filter thatattenuates low-frequency components of the signalmore than the high-frequency components, therebyeffectively increase the front-end bandwidth. If thereceiver sensitivity is not of concern, one can simplydecrease RL to increase the bandwidth, resulting in alow impedance front-end. Transimpedance front endsprovide a configuration that has high sensitivitytogether with a large bandwidth. Its dynamic range isalso improved compared with high-impedance frontends.

Optical fiber receivers mostly employ atransimpedance design because this affords a goodcompromise between bandwidth and noise, both ofwhich are influenced by the capacitance of thephotodiode. However, the large area photodiodes thatare essential in optical wireless require designs thatare significantly more tolerant of high devicecapacitances. A design that is will use in opticalwireless receivers combines transimpedance withbootstrapping, the latter of which reduces theeffective photodiode capacitance as perceived bysignals. This allows a relatively high feedbackimpedance to be used, which reduces noise andincreases sensitivity.

3. Bootstraps TransimpedanceAmplifier

Due to optical wireless link power budgetconsiderations, the receiver is required to have alarge collection area. One of the main noisemechanisms in wideband preamplifiers employinglarge area detectors is the noise due to the low passfilter formed by the detector capacitance and theinput impedance to the preamplifier. Typical largephotodetection area commercial detectors hascapacitance are around 100-300 pF compared to50pF in fiber link. Hence, techniques to reduce theeffective detector capacitance are required in order toachieve a low noise and wide bandwidth design.

Significantly, in any photodetector application,capacitance is a major factor, which limits responsetime. Decreasing load resistance improves thisaspect, but at the expense of sensitivity. In thesubsequent amplifier, positive feedback may be usedwith caution. It is possible to combine the effectivestability of negative feedback with the desirablefeatures of the positive type. Beside that, the input

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capacitance in effect constitutes part of the feedbacknetwork of the op-amp and hence reduces theavailable loop gain at high frequencies. In somecases a high input capacitance can cause the circuit tohave a lightly damped or unstable dynamic response.Lag compensation by simply adding feedbackcapacitance is generally used to guarantee stability,however this approach does not permit the full gain-bandwidth characteristic of the op-amp to be fullyexploited. An alternative approach, the bootstraptransimpedance amplifier (BTA) for inputcapacitance reduction has been reported by [3, 4] waspreviously intended for receiver bandwidthenhancement. This technique offers the usualadvantages of the transimpedance amplifier togetherwith an effective capacitance reduction technique foroptical wireless detector mentioned above.

4. BTA Circuit Design and Simulation

The basic bootstrapping principle is to use anadditional buffer amplifier to actively charge anddischarge to input capacitance as required. By doingso the effective source capacitance is reduced,enabling the overall bandwidth of the circuit to beincreased. There are four possible bootstrapconfigurations (series or shunt bootstrapping modes,with either floating or grounded sources), both areshown in Figure 2 (a) and (b) respectively, which canbe applied to the basic circuit. The seriesconfiguration and shunt technique can be found in[5].

Cf

A RfCc

A much improved version of the circuit,incorporated within a transimpedance amplifierreported in [4] has been use to simulate the BTAbandwidth performance and the effect feedbackcapacitance to reduce effective photodiodecapacitance and. This bootsrap transimpedanceamplifier arrangement is consisted of four stages.There is unity gain, FET buffer, cascade amplifierand buffer output. The type of this circuit is FloatingSource and Series Bootstrap TransimpedanceAmplifier.

In this arrangement, the gain of transistor J1 wasfound not necessary, as Qi/Q2 provided the diodecurrent. Qi itself acted as an emitter follower fromthe source of J2 and Q2 was a current source, drivenfrom the source of J3. The photodiode capacitancewas bootstrapped by the J1, stage in conjunction withQ1. An FET buffer, J3, drove a dipolar cascadecircuit Q3/Q4, buffered by Q5. Overall feedback wasgiven from the emitter of Q5 to the gate of JI.Capacitance Cf was used to reduce the effects ofstray capacitance of the feedback resistor. R4 and C2were used to bootstrap the input to J2 to keep its inputimpedance high [4].

Figure 4: The schematic circuit of Floating Source andSeries Bootstrap Transimpedance Amplifier.

it = C.o Ci, V.

(a)

tI

Figure 3: Equivalent circuit for shunt BTA (a) groundedsource and (b) floating source.

The photodiode and detected optical signal wasmodel as a current source in the front-end opticalreceiver equivalent circuit as shown in Figure 4. Themodel was simulated using PSPICE, where thepositions for each node photodiode and feedbackcapacitance are highlighted. The photodiode andfeedback capacitance are varied to observe theperformance characteristics of the BTA. Thetransistors used in this circuit are BSX 20 - NPNbipolar transistor and BF256A - n-channel JFETdepletion.

Since wider photodetection area was needed foroptical wireless, that will incorporating largereffective photodiode capacitance as perceived bysignals. Therefore, photodiode capacitance Cd with0.1,IF was used in this simulation for variable valueof feedback capacitance, Cf. Figure 5 shows thefrequency response of the simulated BTA with Cd =

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0.1 ~IF and feedback capacitance, Cf= OF, and peaking

gain, MP is 9.0dB and 3dB bandwidth 1.90GHz were

obtained. By varying the Cf with fixed value Of Cd, it

was shown that the BW decreases and peaking gainwere reduce simultaneously. This is shown by Figure6 the effect the feedback capacitance, which the Cf

will improve system stability.

0

0

.........- (1.8631G,!-68.914)

..................

-80-

.................

..................

-104-- ----------------- ............... ..........1OH' 1 OOH, 10KH, 1.OMH, 10OMH, 10GH, 1.OTH,

DB(V(l t) V (i.))

F-q. ...y

Figure 5: BTA frequency response with feedbackcapacitance, Cf = OF and photodiode capacitance, Cd=

0. I .&.

Figure 6: BTA frequency responses with variablefeedback capacitance, Cf and photodiode capacitance, Cd=

0. 1IF~.

By measuring the bandwidth for each value offeedback capacitance, the relationship between thefeedback capacitance and the BTA frequencyresponses can be plotted as shown in Figure 7.Simultaneously, the peaking gain is also changing byvarying the feedback capacitance value as shown inFigure 8. Thus in general observation, it was foundthat the most effective value of feedback capacitance,let say for Cd= 0.1IgF will be 3.5pF because there isno peaking gain and the bandwidth considerably highat 33.4 MHz. However, it was shown that thebandwidth improved significantly for lower Cd asshown in Figure 9 for the BTA circuit with Cf 3.5pF. In order to obtain a critically damped responsethe shunt circuit required a smaller value of feedbackcapacitance, indicating that bootstrapping hadeffectively reduced the source capacitance. Theincrease in bandwidth can be attributed to thedecrease in feedback capacitor required to produce acritically damped response.

Figure 7: BTA bandwidth decreases with the feedback

capacitance.

Figure 8: Peaking gain decreased with the feedback

capacitance.

Figure 9: BTA frequency responses with 3.5pFfeedback capacitance.

5. Conclusion

In this work various optical front-end receiver

design were studied. Receivers for long distance

point-to-point fiber systems generally require a good

sensitivity and a broad bandwidth will invariably use

a small area photodiode. Oppositely, for wireless

optic applications require a large aperture and large

photodetection area, where upon sensitivity and

speeds are reduced. As expected the sensitivity

improves as the photodiode area reduces because of

the correspondingly lower capacitance. However,

419

2000

_ 1500

600

0 2 3 i 5 6Ct (p

10

. 6

-2 1 2 3 4 6

Cd 01

Cd=lp


small area photodiodes incur a greater coupling lossdue to the small aperture they present to theincoming beam. Optical fiber receivers mostlyemploy a transimpedance design because a goodcompromise between bandwidth and noise, both ofwhich are influenced by the capacitance of thephotodiode. However, the large area photodiodes thatare essential in optical wireless require designs thatare significantly more tolerant of high devicecapacitances. Hence, in optical wireless receiverscombines transimpedance with bootstrapping,whereby the bootstrapping reduces the effectivephotodiode capacitance as perceived by signals.

This paper has presented an overview of basicbootstrap configurations for the standardtransimpedance amplifier. The circuit was simulatedand frequency responses of the floating source andseries bootstrap transimpedance amplifier werepresented. The design has presented a simpleexample of a shunt bootstrap amplifier based on twooperational amplifiers of the same type and showsthat the techniques can be used to realized a fasterresponse than is possible with a single amplifieralone. The bootstrap method may provide a viabledesign option for applications with high gain andrequiring a wide bandwidth.

References

[1] R. Ramirez-Iniguez and R. J. Green, "Totallyinternally reflecting optical antennas for wirelessIR communication," IEEE Wireless DesignConference, London, UK, pp. 129-132, May2002, 2002.M.J.

[2] McCullagh and D.R. Wisely 155Mbit/s opticalwireless link using a bootstrapped silicon APDreceiver', Electronics Letter, 3rd March 1994Vol. 30 No. 5, pg 430-432.

[3] R. J. Green, "Experimental performance of abandwidth enhancement technique forphotodetectors". Electronics Letters, Vol. 22 (3),pp. 153-55, Jan. 1986.

[4] R. J. Green and M. G. McNeill, "Bootstraptransimpedance amplifier: a new configuration".IEE Proc. Pt G, Vol. 136 (2), pp. 57-61, April1989.

[5] C. Hoyle A. Peyton 'Bootstrapping TechniquesTo Improve The Bandwidth Of TransimpedanceAmplifiers', IEEE Proc. pg 711-716.

[6] D.J.T. Heatley And Ian Neild, "OpticalWireless: The Promise And The Reality" IEEEProc. pg 1/2 -1/6.

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