Contains reports on three research projects

Friedman, M. F.

Hurtig, C. R.

Wing, H. E., Jr.

Zimmermann, Henry J.

DSpace@MIT

XI. TRANSISTOR CIRCUITSProf. H. J. ZimmermannProf. R. B. AdlerProf. S. J. MasonM. F. FriedmanJ. GrossC. R. HurtigH. E. Wing, Jr.A. INTRODUCTIONThe transistor circuit research is aimed at problems of general interest in com-munication engineering. Fundamental principles of transistor circuit design and practi-cal limitations imposed by available devices are being investigated for circuits usefulin a wide variety of applications.Present studies include work on sweep generators, temperature effects in dc ampli-fiers, and methods of stabilization. A miniature refrigeration unit is under developmentfor stabilizing temperatures of critical circuits.The study of noise in semiconductors is a closely related program.H. J. ZimmermannB. BIAS STABILIZATION OF JUNCTION TRANSISTORSSince one of the most serious problems encountered in the application of junctiontransistors to ac amplifiers is the drift of the operating point with temperature, a studyhas been made of various methods of reducing this variation of the operating point. Themajor portion of the variation is caused by the temperature dependence of the saturationcurrent (Ico). The simplest means of achieving bias stabilization employs reactiveelements. These reactive elements cause the transistor to be in a grounded-base con-nection for bias currents; in grounded-emitter or grounded-collector connection forac signals. Several methods of achieving bias stabilization have been studied and willbe presented in a future technical report.C. R. HurtigC. TEMPERATURE CONTROLIn many circuits where transistors and germanium diodes are employed, a satis-factory dynamic range cannot be achieved if the ambient temperature range is large.Since, in a number of applications,the tolerances allowed, for obtainingHARGING REFRIGERANT to CONTROL a satisfactory dynamic range, areHAR CONTAINER VALVE -EXHAUSTVALVEt0 ti greater at lower mean values of theNSULATION controlled temperature, a study ofmethods of achieving temperatureFig. XI-1 control at low temperatures has beenAn open-loop system. started. Initial work has been aimed at-42-C(XI. TRANSISTOR CIRCUITS)oabarz50 60 70 80 90 100 110 120 130 140 150AMBIENT TEMPERATURE, ,-0 FFig. XI-2Plot of t. vs t for values of K .1 o oExpressing the variables in terms of tobtained.a temperature control of +3°F at a mean valueof 30'F. The units to be controlled are assumedto dissipate power of the order of 50-100 mw.The cooling system must satisfy three require-ments: small power input, small physical size,and the temperature control mentioned above.A simple method of obtaining a coolingdevice is to expand a refrigerant through acontrol valve. An open-loop system, employingthis method, was designed. It is shown inFig. XI-1. This system has a limited oper-ational time, but requires zero work inputduring expansion of the refrigerant.With component power levels of the orderof 100 mw, the greatest heat source is con-duction through the insulation surrounding thecomponents. For a given ambient temperature(ta), the internal temperature (ti) will be afunction of the cross-sectional area of theexpansion valve. An analysis for finding thesteady-state relation between ta and t. wasa1made by means of a heat balance for the system.and t., the following approximate relation wasa 1(ta i o /380 1 1(t - t ) 5 / 4 = K [1.2 t 3 .7  tl'9 + 1800 tl.8 + 560, 0001  (70.9 - 0.088 ta)a o a a a a(1)This relation agreed with the data in the lower range of ambient temperature, butnot at the higher values. It was believed that the fluid flow was reaching a "choked" ormaximum rate. Equation 1 was modified to account for a constant or "choked" con-dition, with the following result applicable above 1140 F.t. = ta - [K(70.9 - 0.088 t a ) 4/5 (z)where K and K' are constants that express system dimensions, area of the valveo oopening, and conversion factors. A family of curves for different valve openings,or K° values is shown in Fig. XI-2. Experimental values of K agree with calcu-lated values; values of K' are empirical. Test runs, covering a large part of theo-43-(XI. TRANSISTOR CIRCUITS)operating region, were made. In general, the results confirmed the theoretical values.H. E. Wing, Jr.D. TRANSISTOR SAWTOOTH GENERATORA study of methods of generating sawtooth waveforms has been started. One method,employing junction transistors, appears capable of providing high-current waveforms.This method employs a junction transistor operated in the grounded-emitter connection.The collector load consists of an inductance and resistance. Upon application of asquare waveform of current to the input, the collector diode may be switched to providean exponential variation of current in the inductance.Particular attention has been focused on the factors that govern the rise-and-fallcharacteristics of the current waveform. The factors that control these characteristicsare: the size of the inductance, the collector capacitance, the current gain, and thecutoff frequency. To generate high-frequency sawtooth waveforms, the size of theinductance is usually small, with the result that the resonant frequency of the LC com-bination is much larger than the frequency cutoff of the grounded-emitter connection.Thus, the latter determines the decay time of the waveform. To obtain long-sweepdurations, large inductances are employed. The maximum length of sweep obtainableis limited by the inductors. The decay time of such sweeps is limited by the resonanceof the inductance and collector capacitance.The use of a grounded-emitter connection yields high current gain. With com-mercially available transistors current gains from 10 to 100 may be readily obtained.Furthermore, many transistor types nominally rated at 5 ma are capable of conductingat least 17.5 ma if the collector dissipation is kept below maximum ratings. Fourtransistors have been paralleled to yield a maximum sweep current of 70 ma with acurrent gain of approximately 12.M. F. Friedman-44-

Transistor Circuits

http://dspace.mit.edu/bitstream/1721.1/51154/1/RLE_QPR_032_XI.pdf

Transistor Circuits

Abstract

Similar works

Full text

Available Versions

DSpace@MIT