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Report No " 272 yymsct JUL 24 1968 DESIGN AND APPLICATION OF A DIFFERENTIAL PHOTO AMPLIFIER STEPHEN EARL WHITESIDE June, 1966 REPORT NO. 272 DESIGN AND APPLICATION OF A DIFFERENTIAL PHOTO AMPLIFIER by STEPHEN EARL WHITESIDE B.S., University of Illinois, 1964 THESIS Submitted in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering in the Graduate College University of Illinois, 1966 Urbana, Illinois 618OI ACKNOWLEDGEMENT The author extends sincere thanks to his advisor, Professor W. J. Poppelbaum, for his encouragement and support. Thanks also are extended to Mrs. Barbara Holmer for typing the thesis . TABLE OF CONTENTS Page I. INTRODUCTION ........... 1 II. DIFFERENTIAL PHOTO AMPLIFIER .............. 2 11. 1 Usefulness and Circuit Requirements ....... 2 11. 2 Differential Amplifier Figures of Merit ..... 2 11. 3 Design of the Circuit ..... 4 11. 4 Tests and Adjustments 9 III. PULSED RUBY LASER DIFFRACTION PATTERN ......... 11 111.1 The Pulsed Ruby Laser .............. II 111. 2 Integrator Circuit .......... 13 111. 3 Diffraction Pattern Measurement ......... 14 IV. PROBLEMS IN ELECTRO-OPTICAL RESEARCH .......... 19 V. SUMMARY AND CONCLUSIONS ................. 21 BIBLIOGRAPHY ....................... 22 APPENDIX I . 23 APPENDIX II ...................... 25 -1- I . INTRODUCTION The combination of light and electronic circuitry has an extreme fascination for many of today's engineer-scientists: The many efforts directed toward visual display of information is proof of this. Research has also been stimulated by the introduction of much smaller solid state electro-optic circuit elements } I.e., light- emitting diodes, and lasers as veil as fiber light guides. The coupling of a diode through a fiber bundle to a phototransistor allows circuit interactions with nearly perfect electrical isolation and D-C coupling is therefore no problem. Furthermore, there are no low frequency limits (as imposed by transformers) and the upper frequency limits are sufficient to handle television frequencies. Some of the more promising optical storage devices lie in the photochemical area. Photochromic materials change color due to flashes of strong light. Their high resolution suggests use in information storage and retrieval systems. This resolution exceeds that of microfilm. Still such systems will need read-in circuitry if it is to be used with present day computers and it is here that we return to the electro-optic circuit elements. The purpose of this work was to design and build a differ- ential photo amplifier and demonstrate its application in electro- optic research by using it to measure a simple diffraction pattern generated by a pulsed ruby laser. _2- II . DIFFERENTIAL PHOTO AMPLIFIER II. 1 Usefulness and Circuit Requirements In much of electro-optical 'work one needs a means of making relative measurements of light intensities. For example , an experimenter may need to measure the light output from a ■weak source which is near a much stronger source. This is where a differential photo amplifier is needed. By observing both sources -with one input and the strong source alone with the other input the experimenter may determine the output intensity of the weak source provided linear response is built into the amplifier. In other cases the experimenter may need an accurate comparison of two fairly intense source s } or perhaps the spatial output of a source. The differential photo amplifier will give a rapid measurement in both cases. Also undesirable effects due to source variations may be cancelled out by using such a design. Requirements for the design stem from these possible applications: The circuit should be linear over a large range of input light intensities. The inputs must be small glass fibers in order to obtain sufficient discrimination in exploring light patterns and low-noise and low-drift design is mandatory in order to make low-level operation possible. -3- II .2 Differential Amplifier Figures of Merit for Differentia l Amplifiers The differential amplifier has two inputs. One input. called the reference input, usually directly views some standardized source. The other input, called the modulated input, usually views the source through some modulating device, from another angle, etc. It is a characteristic of the differential amplifier that only the difference between its two inputs is amplified and appears at the output: The amount of amplification here is called the differe ntial mode gain . Practically, of course , an output will be observed even if one applies equal signals at the inputs. Such equal signals are referred to as common mode signals . The ratio of the output produced by differential signals to that produced by common mode signals is called the common mode rejection fac tor. This rejection factor is one possible figure of merit for a differential amplifier, i.e., it is a measure of how closely an amplifier approaches the ideal. It is incidentally usual to measure the outputs as a diffe rence between the collectors of the differential stage in the above discussion . Another figure of merit is obtained by considering the potential of one of the collectors with respect to ground as an output. Here both types of inputs would produce a signal even in a perfectly balanced stage , and visibly the gain in the two types of use will be different: Here the ratio of the differential mode -It- gain to the common mode gain is called the discrimination factor . This discrimination factor is quite often used as a figure of merit since it is relatively easy to measure -with single-ended equipment . II. 3 Design of the Circuit Let us design the amplifier itself by incorporation of all conditions mentioned above. A first question is: shall we immediately put the phototransistors into the standard differential circuit form? We must first decide how flexible the system is to be: With the phototransistors placed in the standard differential circuit form one has no way to compensate electronically for different light input levels. Now,, in our suggested system, the reference input comes directly from the source while the modulated input may travel quite a different optical path. Possibly the reference input will be much more intense than the modulated input. The most convenient way to adjust for this is through the electronic circuit itself. This leads us to disregard the differential photo- transistor circuit and replace it with a photO' input differential circuit. Here then we shall use the phototransistors as adjustable detectors which control a. differential circuit. The detector stage that we built and used is shown below in Figure 1. This stage allows adjustment of operating level and of gain. The 2N1309 and associated circuitry serve mainly as an adjustable voltage source. The minimum beta of the 2N1309 is listed •5- as 80. If we assume that the voltage at point A is adjusted to -lOv and take f3 = 80, we may write a few equations and quickly see that the output impedance of our voltage source in this condition is approximately 30P» . The worst case is when the voltage at A is adjusted to -7-5 volts . Then the impedance is approximately 60ft . Compared with the 20k load resistor for the phototransistor this is quite negligible. If*, for a particular application., one should find the 60 ohms output impedance annoying, merely replacing the 10k potentiometer by a 5k one will reduce the 60 ohms to 30 ohms. The 20k potentiometer allows us to adjust the gain of our detector. The adjustable voltage source, of course, also allows adjustment of the output voltage level. The phototransistor is operated with an open base. This is its most sensitive configuration. Naturally it is also quite sensitive to temperature variations when in this configuration and therefore should be mounted with some care in the physical environment A differential stage was designed and added to the two photo detector stages to give us the complete differential photo amplifier. The circuit is given in Figure 2. If one considers the differential stage only, it becomes apparent that there is no provision for balancing the two 2N1309's. The Ik potentiometer adjusts the total bias current, but not the -6- LIGHT INPUT 10 K^ -15v VOLTAGE LEVEL ADJUST 0CP71 o OUTPUT X 25K (R L ) GAIN ADJUST Figure 1. Adjustable Gain Detector Stage -7- MODULATED LIGHT IN OV OUTPUT TO NTEGRATOR o V -15v REFERENCE LIGHT IN Figure 2. Differential Photo Amplifier. ■8- individual collector currents. In this configuration, then, the only provision for "balancing is the adjustable voltage sources in the detector stages. Should this he inconvenient, a balance potentiometer could be added at point E. Each end of the poten- tiometer -would be connected to one emitter and the wiper would be connected to the present bias potentiometer . See Figure 3° The detector voltage sources would then be used for coarse balancing and the balance potentiometer would provide a fine adjustment. This added resistance in the emitter circuit would decrease the gain somewhat, but it would also make the circuit less sensitive to the particular transistor used. The added resistance would aiso allow the differential stage to accept larger inputs before saturating: This corresponds directly to the decrease in gain, since a lower gain amplifier should naturally accept a larger input before reaching saturation. The magnitude of this acceptable input would be approximately equal to the added resistance in each emitter circuit times the total bias current (the sum of the two collector currents) . As the circuit is given ; the magnitude of acceptable inputs depends upon the individual transistor emitter-base characteristics. This is not as bad as it may seem since we are expecting rather small voltage changes at the input anyway. -9- 20011 wv 2N1309 TO EMITTER OF T5 soon - 15 v 6 10K TO EMITTER OF T6 CURRENT SOURCE BALANCE POTENTIOMETER Figure 3- Possible Circuit Refinements. •10- A further refinement of the circuit -would be the replace- ment of the Ik bias potentiometer with a current source transistor. This would greatly improve the differential action of the circuit since the impedance from point E to ground would now be on the order of 10-100 kilohm rather than the present 1 kilohm. The necessary circuitry is shown in Figure 3° In view of the low cost objective neither the current source nor the balance pot was used in the actual circuity i.e., the actual circuit used was the one given in Figure 2. The experimental tests and results are discussed in the following section. II. k Tests and Adjustments To increase the versatility of the amplifier a number of potentiometers were used. These all affect the output so that one needs to define a procedure for adjusting the amplifier. It is less confusing to set one half of the amplifier, say the unprimed half, to some set of values and then always adjust the other half, the primed side. No attempt was made during our applications to balance the gain of the two halves of the amplifier since we desired only relative readings. Several phototransistors were tested using a Philco i+0U infrared emitting diode as a standard source and the results were averaged to give a typical _i o electro- optical differential mode gain of (l.O + .l) X 10 -11- coulomb/photon Q,Q .9 micron. (See Appendix i) . This corresponds to a one percent change in intensity producing a 1.2 volt output. This may be extended to other wavelengths by using the published special sensitivity characteristics for the 0CP71 phototransistor . (See Appendix II ) . These values are merely typical and -would be expected to vary -with the particular transistors used. As -we stated before, most of our application required only relative measurements so the circuit was merely adjusted for balance and to give a readable output -without saturation. To. adjust the circuit we set R = R^ Z 15k, V = V = 4.5 volts, R^ = 60&Q . With these settings and virtually no light input V„ = 8.00 volts and R^ was adjusted so that V = V » For stronger light inputs V, and V. was increased as necessary to maintain V„ = V^ = 8 volts. A A J The frequency response of this amplifier was not tested: The published characteristics of the 0CP71 phototransistor indicate an upper frequency limit of three kilocycles and this undoubtedly establishes the frequency limit for the entire amplifier. Improved frequency response could be obtained by using some of the newer phototransistors of photodiodes. However, the present circuit was considered adequate for the diffraction pattern measurement. -12- III. PULSED RUBY LASER DIFFRACTION PATTERN III.l The Pulsed Ruby Laser The source for the diffraction pattern measurement •was a pulsed ruby laser. The use of the laser gave several advantages over use of a conventional source. First the high intensity of the laser beam produces an easily observed diffraction pattern even in the presence of side-light. The intense pattern allows lowering of the differential photo amplifier gain and a subsequent decrease in noise pick-up and thermal drift. Secondly, with the laser there is no problem of masking the source so as to obtain coherence. Thirdly, the laser beam is virtually monochromatic and hence no filters are necessary. Use of the pulse laser source created some problems . The laser's output consists of a series of very narrow pulses which last for approximately one-half mi Hi sec. This output may be explained if we consider briefly laser operation. The laser is pumped by discharging a large capacitor through a xenon flash tube. This flash of light is concentrated on a specially- cut pink ruby crystal (which when inverted gives off a coherent pulse) . The pumping flash has a duration of approximately one millisecond and supplies energy to excite the electrons associated with the chromium atoms in the crystal. The excited electrons jump -13- to broad upper levels, and then make rapid transitions to one sharp lover level. When the number of electrons sitting on the sharp level exceeds the number of electrons at the ground level, the crystal will lase . In doing so the electrons -will make transitions from the sharp level to the ground state level "with each electron emitting one photon. This depopulation of the sharp level occurs much faster than the level can be filled by transitions from the broad upper levels. Hence, after a very short period of laser light, laser action stops until the population of the sharp level is built back up to the threshold value again. The pulses of laser light start approximately one -ha If millisecond after the xenon flash starts and repeat at approximately microsecond intervals until the end of the xenon flash. In order to take readings on an ordinary oscilloscope it was necessary to integrate these palses. Since the xenon flash tubes are operated very near their maximum output, they must cool a specified length of time between firings: This time length used was approximately 2-l/2 minutes. This makes data taking slow since one may take a reading once every 2-1/2 minutes. A time delay relay was used to fire the laser at that interval. If the laser is not fired at an approximately constant interval, tne ruby crystal temperature will be different and the laser output will vary. Another problem involved the charging circuit for the firing capacitors. The circuit of the particular laser -lo- used allowed about a five percent variation in charge, and this definitely produced variations in the laser output. A continuous laser would probably have eliminated part of these problems. III. 2 Integrator Circuit In order to measure the 0.5 ms pulse output from the differential amplifier caused by the laser beam an integrator circuit was designed. Readings were to be made on an oscilloscope and a very long time constant integrator was necessary to allow accurate readings. The circuit is given in Figure h. The time constant of this circuit is: t = RC , f or h = ~- + -> 1 ' r, + — h R '" 2R d (p + l)\ r c (P + ^c where C = capacitor r = collector resistance of transistor c R, = reverse resistance of diodes d R^ = load impedance = oscilloscope impedance. 2 Now (p + l) R_ » r and R, » r so that practically T = r C. L c d c c Experimentally this measured about 15 seconds indicating that r c was about k-5 megohm. A larger time constant up to ((3 + l) r C could be obtained by boot -strapping R to the input transistor Li -15- -3v V O SG5428 SG5428 W W — l €) 2N2904 2N2904 o — q= C=0.33/if v£o R L = 10M (OSCILLOSCOPE) Figure h. Integrator Circuit. -16- collector with a zener diode and adding a large collector resistor here, but this would require a higher source voltage. This circuit is analyzed by Joyce and Clark. The smaller time constant was adequate for our measurements . III. 3 Diffraction Pattern Measurement An 80-line film diffraction grating was used to produce a diffraction pattern. This pattern was observed by fiber optic light guides connected to the differential photo amplifier. One light guide was held stationary near the center of the pattern and the second light guide was moved across the pattern to take relative readings. The stationary and moveable light guides were mounted on a stage which was installed on an enclosed optical bench. The differential photo amplifier was enclosed in a brass box to reduce electrical noise pick-up. The pulsed ruby laser with its firing circuit, an oscilloscope, and power supplier complete the experi- mental equipment. (See Figure 5)- For a simple diffraction pattern: sin G = — where 6 = angle between the central maxima and additional maxima (the vertex is at the diffraction grating) D = wavelength d = spacing of lines in diffraction grating. •17- For the 80-line grating d = .003 nun., \ = 69^+3 angstroms. The theoretical and experimental curves are graphed in Figure 6. As shown, the pattern measured by the differential photo amplifier indicates primarily only five bright lines: To the naked eye approximately fifteen lines -were visible when the pattern was projected on a white card. Yet a Polaroid black and white photograph of the card synchronized with the laser flash also showed only five bright lines. So the amplifier is of approximately the same sensitivity as the film in this case. -18- MECHANICAL STAGE PULSED RUBY LASER -•» I DIFFRACTION GRATING DIFFERENTIAL PHOTO AMPLIFIER TIMER OSCILLOSCOPE Figure 5« Block Diagram of Experimental Set-up. in in z cr < LU _i o ■19- .. c\j ■- 00 X o II 00 -z. cr ro in -z. cr w G ^H CD -P -P 03 CD Ph C\J C w o o •H o •p U 1! CO fc CM tH ^ — ch •H 13 p a -p rH •H cO rH o Ph •H -P !j (D v • m o II CD £1 >> EH -P •H Td W a fl cO cu -p H ci cd H •P a rd CD CD § ^H •H d ^H co CD cO Ph jy X s W . CD VD -P O CD & 3 hO •H h -20- IV. PROBLEMS IN ELECTRO-OPTICAL RESEARCH There are numerous problems to be solved in almost any application of electro^optics . Many of these problems are of a geometrical nature. An optical bench is practically indispensible . Included in the geometrical area "would be the problem of signal power. Signal power received or transmitted depends upon the areas of the devices. Light from sources other than lasers radiates in all available directions. Much of it is "wasted even "when focused and reflected quite carefully. Optimizing the geometrical set-up is necessary in order to optimize the external quantum efficiency : For each quantum of light generated the external quantum efficiency indicates the probability of a photon being sensed at the receiver. The internal quantum efficiency of most electro-optical circuit elements is quite good but a large percentage of the photons generated do not escape the crystal due to internal reflection and absorption. Typically, even the laser transmits only 10 to 25 percent of its generated photons. The pulsed ruby laser has other problems too: It may be fired only once about every 2-3 minutes. When it is fired, its temperature rises rapidly, then it cools toward the ambient temperature. Since the laser output depends on the ruby temperature, the firing circuit must be accurately timed for the outputs to be constant. The voltage of the capacitors charged up to -21- fire the laser must also be maintained at a precise level in order to obtain constancy. Another problem is the nonlinearity of many of the electro- optical circuit elements, their gain being a function of the light intensity received. Also gain-band -width products of electro-optical circuit elements are still quite low. -22- V. SUMMARY AND CONCLUSIONS This research project led to the design of a differential photoamplifier for pulsed light input via two glass fibers. Adequate performance was obtained in exploring the relative intensities in a diffraction pattern produced by a ruby laser. The problems of temperature drift, sensitivity, bias adjustment, and less noise were resolved in satisfactory fashion. It is suggested that a higher frequency version of this amplifier be designed for future experiments -23- Bibliography 1. Joyce, M. V., and Clark, K. K„ Transistor Circuit Analysis , Addison-Wesley Publishing Company, Inc., 1961, Massachusetts 2. Keyes , R. J., et al. "Modulated Intrared Diode Spans 30 Miles". Electronics, Vol. 36, No. 14, April 5, 1963- 3- Lamort, M. F. , and Liebert, R. B. "P-N Junctions as Radiation Sources". Electronics , Vol. 37, No. 20, July 13, 1961+. k. Lengyel, Bela A. lasers , John Wiley and Sons, Inc., 1962, New York. 5- Middlebrook, R. D. Differential Amplifiers , John Wiley and Sons, Inc., 1963, New York. 6. Wunderman, I. "Optoelectronics at Work", Electronics Vol. 37, No. 21, July 27, 1964. -2k- APPENDIX I The A-C electro-optical differential mode gain of the amplifier is defined as the change in one collector current due to a change in the number of photons per second incident on one phototransistor . This gain was determined by using a Philco GAE kOk photodiode as a standard source for which the number of photons per second output as a function of the input current is known. Illuminating several phototransistors with this standard source allowed an average value of photon- induced current to be measured. (See Tables I-III.) Using this average value of the number of amperes per photon/second and an experimental measure of the change in one collector current due to a change in the incident intensity on one phototransistor the A-C electro-optical differential mode gain may be calculated. Considering Figure 7? a one percent change in incident intensity P produces a 1.2 volt change in collector voltage V . ~~ 1 O fl TV) T*) f-^ Y* f^ C2 This corresponds to a typical gain of 1.0 + . 1 X 10 photon/sec ■25- Table I. Data for Philco GAE kOk Photodiode (manufacturer's data) Bias Current 100 ma 200 ma No. of Photons /sec Into 30 Cone at 0.9 micron 5.2 X 10 Ik 1.2 X 10 15 Table II. Photo-Current Data - 5 . 2 X 10 Incident Photons /Sec Phototran- sistor Number 1 Dark Current (Average)* (micro-amps) Light Current (micro-amps) Light Induced Current (micro- amps ) Amperes/ Photon Second (X10 21 ) 1 9-5 16.0 6.5 12.5 2 9-5 13.5 4.0 7-7 ** 3 9-0 10.5 1.5 2.9 *-*l4. 6.3 8.0 1.7 3*3 5 9.0 12.0 3.0 5.8 6 AlfPT-f 8.0 13.0 rnrrpnt ft . Q Y 5 = -21 amperes 9-6 photons/sec * Average dark current ■ average of the dark current before illumination and the dark current immediately after illumination ** Samples 3 and h were not geometrically typical and were not averaged. -26- 15 Table III. Photo-Current Data - 1.2 X 10 Incident Photons /sec Light-Induced Current Amperes/ (micro-amps) photons /sec 13-5 11.2 7-0 5.8 5.5 h.G 8.2 6.8 12.0 10.0 10.0 8^3 Average light- induced current 8.8 X 10 . , 7 photons/sec Light Current (micro-amps) 1 22.0 2 16.5 **3 lU.5 -X-X-l! 1U.5 5 21.0 6 18.0 ** Samples 3 and k were not geometrically typical and were not averaged. -27- 7.5 v -15v 16K gP® ®ICB0 < _i UJ 100- 90- 80- 70 60 50 40 30 20 10 + 0, 1.0 1.5 2.0 WAVELENGTH (MICRONS) Figure 8. Spectral Sensitivity of Milliard 0CP71, (Manufacturer's Data) . UNCLASSIFIED Security Classification DOCUMENT CONTROL DATA • R&D (Security clmifiolior o/ title, body of mbmtrmcl and Indexing annotation null be entered whan the overall report ia claaailiad) 1 OPIGINATIN G ACTIVITY (Corporate author) Department of Computer Science University of Illinois Urbana, Illinois 61801 la REPORT JECJRITy CLASSIFICATION Unclassified Zb group 3 REPORT TITLE DESIGN AND APPLICATION OF A DIFFERENTIAL PHOTO AMPLIFIER 4 DESCRIPTIVE NOTES (Typa o/ report and tncluelva datee) Technical Report; M.S. Thesis 5 AUTHOR(S) (Laet name, tiretname. Initial) Whiteside, Stephen E, 6 REPO RT DATE June, I966 7a- TOTAL NO. OP RAOII 28 7b. no. of RIFI 6 8«. CONTRACT OR ORANT NO. 6. PROJECT NO. • a. ORIGINATOR'S REPORT NUMBeRfSj 9b. OTHER REPORT N o(S) (A ny other numbere that may be aealgnad thle report) 10 AVAILABILITY/LIMITATION NOTICES 11 SUPPLEMENTARY NOTES ., :ne 12. SPONSORING MILITARY ACTIVITY Department of the Navy Office of Naval Research Washington, D.C. 20360 13 ABSTRACT The combination of light and electronic circuitry has an extreme fascination for many of todays engineer-scientists: The many efforts directed toward visual display of information is proof of this. Research has alos been stimulated by the introduction of much smaller solid state electro-optical circuit elements, i.e. light-emitting diodes, and lasers as well as fiber light guides. The coupling of a diode through a fiber bundle to a phbtotransistor allows circuit interactions with nearly perfect electrical isolation and"D-C coupling is therefore no problem. Furthermore, there are no frequency limits (as imposed by transformers) and the upper frequency limits are sufficient to handle television frequencies. Some of the more promising optical storage devices lie in the photo- chemical area. Photochromic materials change color due to flashes of strong light. Their high resolution suggests use in information storage and retrieval systems. This resolution exceeds that of microfilm. Still such systems will need read- in and read-out circuitry if it is to be used with present day computers and it is here that we return to the electro-optic circuit elements. The purpose of this work was to design and build a differential photo amplifier and demonstrate its application in electro-optic research by using it to measure a simple diffraction pattern generated by a pulsed ruby laser. DD FORM 1 J AN 84 1473 UNCLASSIFIED Security Classification UNCLASSIFIED Security Classification KEY WORDS LINK A LINK 8 ROLE LINK C Electro-optics Differential amplifier Light amplifier Ruby laser INSTRUCTIONS 1. ORIGINATING ACTIVITY: Enter the name and address of the contractor, subcontractor, grantee, Department of De- fense activity or other organization (corporate author) issuing the report. 2a. 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