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A Fiber-Optically Triggered Avalanche Transistor Matic Test Equipment Involving Ac Bridges [8[

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A Fiber-Optically Triggered Avalanche Transistor Matic Test Equipment Involving Ac Bridges [8[ IEEE TRA~"SACTlO0:S ON INSTRU:vIEl\TATlON AND MEASUREMENT. VOL. 40. NO.3. JUl\E 1'191 25 Input signal ; 500 jJV 20 Amplifier gain ~ A Simulation result continuous curves Measured error points x x x A ; 200 o ::J 10 '"0 A 400 (f) ClJ 750 D:::: o 20 40 60 80 100 120 140 160 180 Phase difference <r (deg) Fig. 6. Extent of pha.'c compensation as a function of amplifier gain A. effcct [7 J. The schemc can also be successfully employed in auto­ A Fiber-Optically Triggered Avalanche Transistor matic test equipment involving ac bridges [8[. R. Jacob Baker, Gregory T. Perryman. and Phillip W. Watts Y. CONCLUSION It has been demonstrated that using feedback technique for phase Abstract-A silicon bipolar transistor operating in the avalanche re­ compensation in a PSD, the amplitudc of thc measurand can bc got gion is optically triggered into secondary breakdown, This transistor has been given the name fiber-optically triggered avalanche transistor directly without the need for a post-processing of the in-pha,e and (FOTAT), The FOTAT acts as an optical power discriminator. That quadrature components. Experimental results presented serve to is, secondar) breakdown occurs when the triggering optical power ex­ establish the easy applicability of the method to obtain a high de­ ceeds tbe triggering tbreshold of tbe FOT AT, This secondary break­ gree of phase compensation coupled with a fine resolution. Quan­ down is seen as a negative resistance between the collector and emitter. High voltage (> 100 V) nanosecond transition duration pulses arc gen­ titative comparison of the extent of compensation obtained exper­ erated using this negative resistance, imentally with that estimated by PC-simulation indicates good agreement. I. iNTRODlICTIOi': ACKl\;OWLEDG:vtEl\;'I Avalanche transistors have found many uses in the design of in­ The second author is obliged to Prof. A. ]. Rogers of King's strumentation [1]-[4], but some problems exist when trying to use College. London, U.K .. for kindling his interest in this area. The these transistors in electro-optic instrumentation. Time jitter re­ authors are grateful to Prof. Y. G. K. Murti of lIT Madras for quircments for this instrumentation can be much more stringent encouragement. valuable suggestions. and criticism. than in electrical instruments. The jitter developed when triggering these transistors electrically is mainly due to the stray inductance REFERENCES and capacitance. Also. when trying to generate a trigger frulll an optical signal a photodetector of some sort is usually required. A II J K. L. Smith. "Thc ubiquitous phase sensitive detector.·· Wireless fiber-optically triggered avalanche transistor (FOT A T) can help World. voL 78. pp. 367-.. 50. Aug. 1972. solve both of these problems. By injecting the light directly into 12J 1. G. Lac). "A current ,witching phase sensitive detector and it, ap­ the transistor, not only can the secondary breakdown mechanism plication to a resolved components voltmeter." EleClwlL EI1~ .. voL 39. pp. 1.. 8-151. Mar. 1967. be triggered. but the jitter is theoretically nonexistent. 13J D. P. Blair and P. H. Syndenham. "Phase ,en,itive detection a, a The optical triggering of an avalanche transistor was first shown me am to recoYer signals buried in noise." I PIII'sics E: Scientific In- by Thomas and Coleman [5]. A lens was used to focus part of a struments. YoL 8, pp. 621-627, 1975. laser signal onto a transistor die to start the second brcakdow n pro­ I"J v. Brunstein. "Frequency doubler and flip-flop make adju'table phase cess. This method of triggering is diffieult to incorporate into a shifter." Declrol1ics. \01. .. 9. p. 100. 1976. 15J 1. Austin and 1. R. Forre,\. "A preci,c digital phase shifter using portable piece of instrumentation due to the alignment necessary in PLL." Eiecrwl/. Lell .. vol. 14. pp. 254-255. Apr. 1978. 16J E. W. Tay and V. G. K. Murti, "Unity gain frequcncy independent Manu,cript received June 16. 1990: revised November 17, 1990. Thi, phase shifter.' Decrroll. Lett .. vol. 20. no. 10, pp. 431-432. May work was performed under the au'pice, of the U.S. Department of Energy 19~". by E.G.&G. Energy Measurements Inc. under Contract DE­ 17J A. J. Rogers. "Optical methods for mea,urement of voltage and cur­ AC08-X8NV 10617. rent on power systems." 01'[. Laser. Tech .. pp. 273-283, Dec. 1977. The authors are with E.G.&.G. Energy Mca,uremcnts. Las Vegas. NV 18J H. K. P. Neubert. 1l1Slnllllellf Transducers. London: Oxford Univer­ 89125. sity Press. 1975. pp. 196-197, 2nd ed. IEEE Log Number 9143082. 0018-9456/91 /0600-0649S01 .00 C 1991 IEEE 650 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT. VOL. 40. NO.3. JUNE 1991 ·V cc (, ~ R C (2 ~ (a) Fig. 2. FOTAT test circuit. junction is given by M= m I - (~)" where n is in the range of 2-6 for silicon, V is the potential across (b) the junction, and Vb is the breakdown voltage of the junction. If RB Fig. 1. (a) Epitaxial transistor used as a FOTAT. (b) Packaged FOT AT. is zero, some simplifying assumptions may be made. The break­ down voltage Vb is approximately BVCBO , the breakdown voltage of the collector-base junction with the emitter open. The emitter directing the laser signal and the requirement that the optical source current is negligible compared to MlcBo and can be assumed to be be close to the transistor. For these reasons, the optical triggering zero. of an avalanche transistor has been used very little. With the ma­ With the aid of Fig. 3 the dynamic operation of the transistor turing of fiber optics within the last decade these problems can be circuit shown in Fig. 2, with RB equal to zero, may be described. eliminated. Fig. I(a) shows the epitaxial transistor used as a FO­ A value of Rc is selected such that the static operating point of the TAT. The top of a transistor unit, packaged in a metal TO-39 type transistor is at point A, a voltage just slightly below BVCBO and at package, is removed. This exposes the transistor die. One end of a currenth. The value of Rc is typically> 100 kO. When the tran­ an optical fiber is cleaved, positioned over the die, and epoxied in sistor is triggered the operating point moves to point B. Point B is place, while the other end is terminated with a fiber optic connec­ determined by BVCBO , RL , and RAo where RA is the "on" resistance tor. The result, shown in Fig. I(b), is a versatile device that can of the transistor. The negative resistance of the transistor while be used in electro-optic equipment, performing functions from that switching is given by the slope of the load line between points A of a nonlinear detector to that of an optically triggered electrical andB, pulse generator. (3) II. DEVICE OPERATION The static characteristics of a bipolar transistor, as shown in Fig. The transition duration from A to B is essentially independent of 2, while operating in the avalanche mode, are given by (la)-(ld), the load resistance. The upper limit of the load resistance is deter­ mined such that the transistor will not enter the active region when (Ia) switching. The lower limit is set by the amount of power the tran­ sistor can dissipate without being destroyed. From point B the col­ (I b) lector current decays exponentially with a time constant of (RA + (Ie) Rd C until reaching point C. This is the point where the transistor is no longer in second breakdown. The collector to emitter voltage (I d) at this point is not great enough to sustain the avalanche condition and is denoted by VA' The collector current drops off quickly from The base current, I B , is assumed to be flowing out of the base. The point C to point D. From point D the collector capacitor voltage collector current, while operating in the avalanche region, is called charges back to BVCBO (point A). the leakage current, I L • The only parameter of operation not yet described analytically is As the collector-base junction becomes reverse biased a point is the optical trigger pulse. Some problems exist in fuJly character­ reached where avalanche multiplication, M, becomes greater than izing the effects of this pulse. The emitter and base metalizations one. The empirical expression for avalanche multiplication of a p-n shown in Fig. I present the first problem. The etched area between IEEE TRAl"SACTlOl"S ON I~STRUMENTATlON AND MEASURB1EKT. VOL. ~(). MJ. 1. JUNE IY~I 651 f­ Z W 0: 0: ::::> o 0: o Fig. 4. Output pubc of FOT AT -100 V / div and 20 ns / div. of­ W --' --' 8 ! value of voltage should be set very close to the breakdown voltage of the transistor so as to keep leakage current to a minimum. I The output of this circuit into a 50-n load is shown in Fig. 4. The output amplitude is approximately 500 V with a first transition duration of about 10 ns. The optical trigger pulse is a 1.0-ns FDHM. Gaussian. at 806 nm. Fig. 5. This optical pulse is coupled BV CBO to an attenuator. through a 50/125 graded index fiber. for adjust­ ment of the optical power. The output of the attenuator is con­ V CE (COLLECTOR-EMITTER VOLTAGE) nected through a 50/125 graded index fiber. to a beamsplitter. The Fig. 3. 1- V Characteristics of FOTA T. purpose of the beam splitter is to split the light pulse into two parts; one for monitoring the optical amplitude and the other for trigger­ ing the FOTAT.
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