MODELING OF HYBRID RF AMPLIFIERS BASED ON TUNNEL DIODES AND MINIATURIZATION ISSUES

N. S. Butov, N. F. Trofimov

(Presented by the scientific and technical seminar of the Department of Radio Engineering)

Introduction

Printed RF amplifiers, owing to their small dimensions, high potential suitability for series production, and low cost, may be of considerable interest for onboard devices. As active elements in such RF amplifiers it is advisable to use tunnel diodes, since over most of the microwave range they have a lower noise figure than, for example, transistors and vacuum triodes, and they do not require a bulky power supply. In noncirculator circuits they also possess good temperature stability, high reliability, and very low weight. However, in implementing them one encounters practical difficulties associated with the complexity of providing tuning under the particular conditions of the printed structure.

It is possible that this is precisely what explains the absence of publications on the experimental characteristics of implemented printed RF amplifiers based on tunnel diodes. Meanwhile, many of the difficulties mentioned can be overcome by preliminary development on models. The proposed article sets forth certain principles and experience in modeling hybrid tunnel-diode RF amplifiers whose high-frequency elements and interconnections, with the exception of the diode, are made by the printed method. The structure and characteristics of these RF amplifiers and of their models are presented. Ways of miniaturizing tunnel amplifiers as a whole are considered.

Some Principles of Modeling

When developing hybrid devices with active elements, it is necessary to take into account that the equivalent circuits of microwave units manufactured by means of printed technology, as a rule, differ from the calculated ones. Usually this discrepancy is due to the neglect of certain specific reactive parameters of printed structures.

Such parameters arise, for example, when sections of the metallic surface of the structure having a potential different from zero are used for grounding circuit elements; when the symmetry of a strip line is disturbed by connecting tuning capacitors, diodes, and other devices to the central conductor; when the shape of high-frequency elements is changed, and in a number of other analogous cases. As a result, the expected effect may turn out to be greatly distorted or may even not be observed.

In this respect, stripline and coaxial structures of conventional manufacture have been better mastered and appear to be less critical and “capricious” than printed ones; they can easily be tuned even when the parameters deviate significantly from the calculated values and usually ensure that the calculated effect is obtained. This makes it possible to use them as models of printed circuits both for solving fundamental questions and for identifying the structural features of a given printed circuit. However, it is often known in advance that, for technological reasons and from considerations of electrical and mechanical strength, certain parts of stripline structures must be modified when fabricated as printed circuits. Unfortunately, the parasitic parameters mentioned above may also arise at the same time.

Naturally, such changes must first of all be reflected in the structure of the model, since otherwise, when switching to printed technology, difficulties may arise in implementation and, later, difficulties in operation. Thus, when modeling a printed unit it is necessary to have a sufficiently clear idea of its proposed structure, taking into account specific limitations.

Of no small importance is the method of approach to the tuning of models, since circuits with active two-terminal networks possess a certain criticality of their characteristics with respect to internal couplings and to the matching of elements. One may, for example, consider generally accepted such a process for tuning these circuits in which matching of the diode unit is carried out only “in the presence of negative resistance,” that is, under conditions of combating oscillation. But another process is also possible, in which, at the stage of impedance matching, the tunnel diode is replaced by an equivalent passive two-terminal network, and only the final tuning is performed “with negative resistance.”

Such an approach provides much more information about the parameters of the circuit and its stability than the generally accepted one; it facilitates the process of adjusting the circuit and corresponds precisely to the very idea of modeling.

Summing up all that has been said, one may write the following conditions (or principles) for modeling hybrid circuits:

1. Taking into account the limitations of the printed structure (i.e., maximum correspondence) in modeling.

2. Division of model tuning into “passive” and “active” stages.

3. Maximum approximation of the structure of the hybrid unit to the structure of the model.

4. Fulfillment of the general rules for designing microwave printed circuits (for example, direct and symmetrical connections of the circuit grounding points with the shielding plates, symmetrization of inhomogeneities, allowance for the dependence of the effective dielectric constant on the shape of the pattern, rounding of the corners of the pattern, etc.).

Experiment

The stated modeling principles were taken as the basis in creating hybrid microwave amplifiers using tunnel diodes of the transmission type (Fig. 1), with a stabilizing network.

The frequency characteristics of the model and of the samples of printed amplifiers manufactured from it are shown in Fig. 2.

The dashed line shows the characteristics of printed-amplifier mock-ups whose dimensions or shapes do not correspond to the models.

From examination of the indicated curves it is evident that the characteristics of the samples and of the corresponding models agree satisfactorily.

Thus, in modeling, the process of developing the hybrid amplifiers under consideration acquires a regular character; elements of empirical refinement and adjustment are largely eliminated from this process, and it is substantially accelerated.

As an example, one may mention the development process for a printed VHF through-type amplifier, which, although it bypassed the modeling stage,

Fig. 1. Structure of the printed elements

nevertheless required various refinements and adjustments that took twice as much time and effort, while the result obtained was much worse (see the dotted line, Fig. 2) than that of the modeled sample. The structure of the printed elements corresponding to Fig. 1 has dimensions not exceeding \(45 \times 10 \times 65\), and can readily

Fig. 2. Power-gain characteristics: 1 — sample without modeling, without stabilizing circuit; 2 — the same, but with a stabilizing circuit; 3 — sample according to the model; 4 — model

be reduced by half by increasing the dielectric constant of the filler, the capacitance of the tunnel diode, and through constructive refinement.

Apparently, further miniaturization of tunnel UHF amplifiers in the low-frequency and middle parts of the decimeter band is advisable in connection with the use of microminiature nonreciprocal devices, and also on the basis of modeling circulatorless circuits with decoupling by means of bridge devices.

REFERENCES

1. “Stripline Systems of Superhigh Frequencies.” Collection of articles edited by V. I. Sushkevich. Moscow, Foreign Literature Publishing House, 1959.