Tunnel diodes provide a means of low-noise microwave amplification, with the amplifiers using the negative resistance of the tunnel diode to a.chieve amplification by reflection. Th e tunnel diode and its assumed equivalent circuit are discussed. The concept of negative-resistance reflection amplifiers is discussed from the standpoints of stability, gain, and noise performance. Two amplifi,er configurations are shown. of which the circulator-coupled type 1'S carried further into a design fo/' a C-band amplifier. The result 1'S an amph'fier at 6000 mc/s with a 5.S-db noise figure over 380 mc/s. An X-band amplifier is also reported. C. T. Munsterman Tunnel-Diode Microwave Am.plifiers ecent advances in tunnel-diode fabrication where the gain of the ith stage is denoted by G i techniques have made the tunnel diode a and its noise figure by F i. This equation shows that Rpractical, low-noise, microwave amplifier. Small stages without gain (G < 1) contribute greatly to size, low power requirements, and reliability make the overall system noise figure, especially if they these devices attractive for missile application, es­ are not preceded by some source of gain. If a pecially since receiver sensitivity is significantly low-noise-amplification device can be located near improved, with resulting increased homing time. the source of the signal, the contribution from the Work undertaken at APL over the past year has successive stages can be minimized by making G] resulted in the unique design techniques and hard­ sufficiently large, and the overall noise figure is ware discussed in this paper.-Y.· then that of the amplifier Fl. If low-noise ampli­ fication is not available until the second or third stages, then a high-noise figure results. This latter Low-Noise Microwave Amplification case pertains to many microwave systems in which The need for low-noise microwave amplification down conversion is necessary before amplification can be seen when the system noise figure is con­ can take place. The inclusion of tunnel-diode sidered. The overall noise figure, F, of a system amplifiers in systems formerly using down con­ consisting of n stages is given by the following version and then amplification results in noise expression: figure improvements generally greater than 6 db. When tunnel-diode amplifiers are used instead of F = FI + F2 - 1 + F3 - 1 + ... traveling wave tubes, they simplify greatly the G] GIG'!, required power supplies, reduce power dissipation, and make warm-up time negligible. It should be pointed out, however, that traveling wave tubes are superior in available gain and maximum out­ put power. Thus, tunnel-diode amplifiers are usu­ * The author wishes to acknowledge the assistance of E. E. Skelton ally considered only for receiver applications where and S. W. May of APL in the fabrication and testing of the tunnel diode amplifiers. signal levels are well below tunnel-diode saturation. 2 APL Technical Digest The Tunnel Diode ratio must be constant over the band; for stability it cannot be unity since unity match represents an The discovery of the tunnel diode was made in unstable condition. Tunnel-diode amplifiers are the 1950's during research on back diodes, that is, usually designed for less than 20-db gain to assure on diodes whose reverse conduction is greater than stable operation with expected diode resistance forward conduction. 1 High reverse conduction can variation. best be pictured as the limiting case of the zener A low-impedance DC source is required for stable breakdown. As theory had predicted, this reverse biasing; this is illustrated in Fig. 1A. The two load breakdown voltage could be decreased by higher lines shown represent the circuit resistance shunt­ doping concentrations of the semiconductor mate­ ing the tunnel diode. The high-impedance load rial. The result of these high impurity levels was line intersects the voltage-versus-current curve of not only an immediate reverse conduction, but un­ the tunnel diode in three places. The two positive expected negative resistance. 2 Figure 1A shows a typical tunnel-diode voltage-current characteristic. The high reverse conduction and negative resist­ -- TUNNEL DIO DE ance region can be compared to the conventional --- CONVEN TIONAL diode curve shown by the broken line. DI O DE Design Considerations The concept of a negative resistance producing gain can best be shown by using simple transmis­ sion line theory. A transmission line of character­ istic impedance Zo is terminated in an impedance of value Zd. The loss of power because of reflec­ tion is given by the square of the voltage reflection coefficient, f. This coefficient is defined in terms of the impedances by the relationship f = Zd - Zo . Zd + Zo For posItlve impedance this is less than unity, as TYP ICAL VALU ES FOR A expected. But if Zd is a negative impedance, as a GE RMAN IUM TUNN EL DIO DE I­ tunnel diode could be, a voltage reflection coeffi­ II' = I 7 ma WZ« cient greater than unity can result. The power Vl l- Ron = - 78 !l (5 Vl gain, or voltage reflection coefficient squared, is Z6 ~o :~~ ~ 8~ then greater than one (Zd is now assumed nega­ u ~a=l.4 tive) : - DE SI RED OPE RA~~: I ~~IN T OL-________V~I _______ O~ F _M_I _N_IM_U_M__ N_ O_I_SE_______ C~ Zd 2 . I-Zd - Zol2 11 + Z O/ I VOLTAGE Gam = 1[/2 = -Zd + Zo = 1 - Z O/ Zd Fig. I--Tunnel-diode bias voltage plotted against The ratio of Z O/Zd for high gam is close to one. (A) current, (8) resistance, and (C) noise constant. For a perfect match (Zd = Z o), oscillation re­ resistance intercepts are stable and are utilized in sults. The necessity of impedance control is then computer applications of the tunnel diode. For the main problem of designing tunnel-diode am­ amplification a stable negative resistance is desired. plifiers. For a certain desired gain a certain imped­ A load line that results in a single valued bias point ance ratio must exist. For a given bandwidth this within the negative resistance region is also shown in Fig. lA. The requirement for this load line is 1 L. Esaki, "New Phenomenon in Narrow Germanium pen Junc­ that the external circuit resistance (load line) be tions," Phys. Rev., 109, 1958, 603-604. less than the magnitude of the minimum negative 2 L Esaki "Fundamentals of Esaki Tunnel Diode in Circuit Appli­ resistance, R m , available from the tunnel diode. cati~ns," Monograph on Radio Waves and Circuits (ed. S. Silver) Elsevier Publishing Co., Amsterdam, 1963, 359-373. This minimum resistance is indicated in Fig. 1B. May - June 1965 3 The low-resistance shunt on the tunnel diode re­ C j is the inherent capacitance of the quired for biasing introduces two problems. The junction. The shunting effect of this capaci­ first is that noise currents from this resistor must tance on the negative resistance limits the be isolated from the tunnel diode in order to pre­ frequency range. This is easily seen if the serve low-noise performance. Secondly, this resist­ parallel combination of R j and C j is put in a ance must be isolated from the tunnel diode at series-equivalent circuit. This is the first step desired frequencies of amplification in such a way in reducing the equivalent circuit to a two­ that a negative resistance is available for amplifi­ clement shunt network that is used in tunnel­ cation. diode amplifier design. Further reductions can This second problem can be illustrated by con­ be shown with an example. A typical tunnel sidering the equivalent resistance, R eq, of two diode for amplification at C-band, 5000 to parallel resistors, Rl and R '.!.. 6000 mc/ s, would have the following param­ eters: R,R .. C j = 0.44 pf Cp = 0.4 pf t Reo = R, + -R e Ls = 0.3 X lO-fI h - R j = 99.30 R.~ = 4.50 - Rm = 78.30 If R2 is a negative resistance, R eq will be negative The equivalent circuit reduction at 5800 mc/ s, IS provided R] is greater than IR 21: If these resistors shown below: are isolated by an inductance, AC stability becomes a problem. 0.3 X 10-9 h Tunnel-Diode Equivalent Circuit 0.3 x 10-' h D 0.82 The accepted equivalent circuit of a tunnel p 4.5 n-- o; diode is shown below : : :B~ : tJ-;6 ~I----'-_...J - 99.3 nD 0.614 pf 0.545 ~ -~.I 0.4 =~ pf .945 -70.7 pf pf - 7g7 _ n R ,q is the resistance of the bulk material =~ 1 1.1 - f)A OR Do_B_----'T pf and contacting plates. It is minimized by using thin semiconductor pellets, high-conductivity packaging materials, and high semiconductor Circuit B is appropriate for use when series tuning mobility. It is frequency-dependent because is employed. The design of a tunnel-diode ampli­ of the "skin effect." fier using shunt tuning, for which circuit A is best, R j is the negative resistance of the tunnel will be discussed in a later section. diode. It is bias-dependent, as shown in Fig. Two frequencies, resistive and reactive cutoff, 1B, and frequency-dependent only when car­ can be used in defining the practical frequency rier lifetime is approached; this frequency is range of a tunnel diode and, because they are about lOG mc/ s. R is the predominant tem­ j figures of merit, in tunnel-diode selection. pera ture-sensi ti ve parameter. L s is the parasitic series inductance. This RESISTIVE CUTOFF FREQuENcy- The resistive cut­ is caused by the ribbon material used to con­ off frequency, j,ro , is that frequency at which the nect the contact plate with the junction mate­ real part of the input impedance, neglecting Cp , is rial.