GB2147472A - RF-solid-state arrangement - Google Patents

Abstract

An RF solid-state amplifier arrangement comprises four balanced transmission lines (1,2,3,4) each having a balanced pair of substantially coplanar conductors, suitably finlines, extending towards a common region at which a solid-state two- port amplifier device (5), such as an FET, is disposed. The finlines (1,2,3,4) and the amplifier device (5) are coupled together, for example by a strip (11) on the reverse side of the finline substrate (6), in such a manner that the two ports are mutually coupled to substantially only via the device (5) and that one port is connected across one pair of lines (1,2) in parallel and the other port across the other pair of lines (3,4) in parallel. One line from each pair (1,3) respectively form input and output lines, and the other line of each pair (2,4) is terminated to provide a substantially pure reactance across each port to assist in matching the input and output impedances of the device (5) to the desired characteristic impedance. <IMAGE>

Description

SPECIFICATION RF solid-state arrangement The invention relates to an RF solid-state amplifier arrangement comprising a plurality of balanced transmission lines each of the kind having a balanced pair of substantially coplanar conductors separated by a gap across which the electric field of microwave energy propagating along the transmission line in operation predominantly extends, and further comprising a solid-state amplifier device such as a field-effect transistor (generally referred to as an FET), the arrangement being particularly suitable for operation at millimetre wavelengths. Such transmission lines, which will be referred to herein as transmission lines of the kind set forth, suitably are finlines or slot lines.

An FET amplifier arrangement using waveguide and microstrip is known from the paper "A 60 GHz GaAs FET Amplifier" by E.T.

Watkins et al., 1 983 IEEE MTT-S Digest, pages 145-147. It has been found that such an arrangement, requiring waveguide/microstrip mode transducers, tends to have rather high losses and to have a rather narrow bandwidth.

According to the invention, an RF solidstate amplifier arrangement comprises four transmission lines of the kind set forth extending towards a common region and further comprises a solid-state amplifier device which is disposed at said common region and which has two ports, wherein the transmission lines and the solid-state amplifier device are coupled together in such a manner that in the operating frequency range of the amplifier arrangement, said two ports are mutually coupled substantially only via the solid-state amplifier device, a first of said ports is connected across a first and across a second of said transmission lines, and the second of said ports is connected across a third and across the fourth of said transmission lines, and wherein each of the second and fourth transmission lines is terminated so as to present at said region a substantially pure reactance, whereby said first and third transmission lines constitute respectively one an input for supplying RF energy to the solid-state amplifier device and the other an output for deriving RF energy from the solid-state amplifier device, and whereby the input and output impedances of the amplifier arrangement are dependent on the reactances presented at said common region by respectively one and the other of the second and fourth transmission lines.

Suitabiy, the solid-state amplifier device has three terminals of which a first and a second are respective terminals of the first and second ports and the third is common to both ports, the first terminal being connected to one of said pair of conductors of each of the first and second transmission lines, the second terminal being connected to one of said pair of conductors of each of the third and fourth transmission lines, and the third terminal being connected to one of said pair of conductors of each of the four transmission lines, and wherein said pair of conductors of the first transmission line are respectively connected to said pair of conductors of the second transmission line and said pair of conductors of the third transmission line are respectively connected to said pair of conductors of the fourth transmission line.

Finline is proving to be a particularly suitable transmission line for use at millimetre wavelengths, and a wide range of active and passive arrangements have been constructed in finline. However, a difficulty in realising a finline solid-state amplifier arrangement is that the input and output impedances of a solidstate amplifier device such as an FET are generally much lower than a typical standard characteristic impedance of a finline (e.g. 1 50 ohms) and that finline can only be conveniently realised with a somewhat restricted range of characteristic impedances, for example (depending on the operating frequency range) 120-300 ohms; lower impedances may require the gap between the fins (unilateral and bilateral finline) to be so small as to be difficult to realise accurately and reproducibly, and higher impedances may require the gap to be so wide as to approach the height of the waveguide in which the fins are disposed. Consequently, it is generally difficult to match the widely different typical impedances of the FET and finline. In amplifier arrangements embodying the invention and using finline for the transmission lines, this difficulty is mitigated by the presence of the second and fourth finlines respectively in parallel one with the input and the other with the output.The second and fourth finlines each provide an extra degree of freedom in matching: the reactances repectively presented by these finlines may be selected to respectively transform the impedances respectively presented by the FET at its two ports to values which can more readily be matched along the input and output lines to the desired value or respective desired values.

Similar considerations apply to slot lines.

It may be noted that a finline RF solid-state oscillator arrangement comprising four finlines extending towards a common region and further comprising an FET disposed at said common region, wherein the two fins of a first finline are respectively connected to the two fins of a second finline and wherein the two fins of a third finline are respectively connected to the two fins of the fourth finline, is known from the paper "Stabilised Fin-Line FET-Oscillators" by A. Jacob and C. Ansorge, Proceedings of the 1 3th European Microwave Conference, pages 303-307.In that arrangement, however, a metal strip provides feedback between the drain and the gate of the FET so that there is coupling between all four finlines; only one finline is accessible for coupling to other equipment, the other three finlines being terminated by respective shortcircuits, the positions of which can be adjusted so as (in combination with the slot width and the feedback strip) to set the oscillating frequency and maximise the output power. Another finline RF solid-state oscillator arrangement is known from the paper "A 30 GHz FET-Oscillator using Fin-Line Circuitry" by H. Meinel, Proceedings of the 11th European Microwave Conference, pages 297-300. That arrangement comprises three finlines extending towards a common region and further comprises a three-terminal FET disposed at the common region.The fins of adjacent lines are connected together pairwise to form three sets of fins to which the three terminals of the FET are respectively connected; two of the finlines are terminated by adjustable short-circuits, and the third (across which the gate and source are connected) constitutes the output. The frequency of operation mainly depends on the length of the finline across which the gate and source are connected; the finline across which the gate and drain are connected provides feedback, and its length can be adjusted for maximum power output.By contrast, in embodiments of the present invention using an FET as the solid-state amplifier device, the drain and gate are mutually coupled substantially only via the FET (and analogously for other solid-state amplifying devices): two of the transmission lines constitute respectively input and output lines of the amplifier arrangement, and the other two transmission lines are respectively in parallel one with the input line and the other with the output line to provide reactances which may be used in matching the input and output impedances of the FET to desired values.

Suitably, in an embodiment of the invention, the first and/or third transmission line comprises means for matching to a desired value the impedance presented at the first and/or second port respectively by the combination of the solid-state amplifier device and the second and/or fourth transmission line respectively. When the real part of the impedance presented by the solid-state amplifier device at a said port connected across a transmission line comprising said matching means is less than the real part of said desired value, said matching means may comprise a transformer which is substantially a quarter wavelength long at a frequency in the operating frequency range of the amplifier and which is spaced from said common region by substantially a quarter wavelength at said frequency.

As an alternative, the matching means may comprise a series stub.

At least one of the second and fourth transmission lines may be a finline comprising two conductive fins on only one major surface of a dielectric substrate, said two conductive fins being mutually isolated at DC, and to provide said substantially pure reactance the finline may be terminated by a conductive layer which is disposed on the other major surface of the substrate and which bridges the gap between the two fins of the finline on said one major surface.

An amplifier arrangement embodying the invention may comprise means for inhibiting the production of energy in the operating frequency range resulting from self-oscillation.

Embodiments of the invention will now be described, by way of example, with reference to the diagrammatic drawings, in which: Figure 1 is a plan view of a dielectric substrate bearing conductive fins and an FET, for use in an embodiment of the invention: Figure 2 is a plan view on a larger scale of a central region of Fig. 1, and Figure 3 is a partial plan view of a modification to the substrate of Figs. 1 and 2.

Referring to Figs. 1 and 2. a finline RF solid-state amplifier arrangement embodying the invention comprises four finlines, 1. 2, 3 and 4 respectively, arranged symmetrically so as to extend towards a common central region at which an FET 5 is disposed. The finlines are formed by conductive coatings on one major surface (the upper surface in the drawings) of a thin dielectric substrate 6 of fairiy low dielectric constant (for example 2.2), the substrate being disposed in a metal housing (not shown) having cavities such that the four finlines extend along central longitudinal Eplanes of respective rectangular waveguides meeting at the common region, the waveguides of finlines 1 and 3 being collinear and being orthongonal to the collinear waveguides of finlines 2 and 4. The disposition of the broad walls of the waveguides relative to the conductive coatings are indicated by the pair of dashed lines WG adjacent each edge of the substrate.

Each finline comprises two fins separated by a narrow gap (the width of which has been exaggerated in Fig. 1 for the sake of clarity), adjacent fins of adjacent finlines being connected together pairwise; the fins are formed by four discrete conductive coatings 7, 8, 9 and 10 respectively. A conductive strip 11 (shown only in Fig. 2) extending across the common central region on the lower major surface of the substrate is conductively connected through the substrate to the coatings 8 and 10 (by means not shown) so as to interconnect them.As a result, in the operating frequency range of the amplifier (and in this case generally at Ref), finlines 1 and 2 are connected in parallel, as are finlines 3 and 4, but the two pairs of parallel-connected finlines are substantially mutually isolated, there being only a very small degree of coupling due to fringing fields and to the inductance of the strip 11 which causes it to be a slightly imperfect short-circuit.

The FET 5 in this case has four symmetrically-disposed beam-lead terminals: a gate G, two sources S1 and S2, and a drain D (Fig.

2), which are respectively conductively connected to the coatings 7, 8, 9 and 1 0. Owing to the interconnection of coatings 8 and 10 by strip 11, the two source terminals S1 and S2 are connected together (they are in this case not interconnected within the FET). The FET is operated in the common-source mode: the gate and drain are respective terminals of input and output ports of the FET, and either or each of the sources may be considered as a terminal common to these two ports of the FET.

The strip 11 may be connected to the conductive coatings 8 and 10 by respective metal foil tapes each of which extends through a respective slot cut in the substrate by means of a laser and which is connected to the respective conductive coating on each major surface by conductive epoxy resin.

Such adhesive may similarly be used to connect the terminals of the FET 5 to the coatings 7-10.

Each of the four fin lines comprises, at its end remote from the common central region, a finline/waveguide mode transducer. Each of the finlines 2 and 4 is terminated by a respective short-circuit 12, 1 3 which in this case is adjustable and is disposed in the waveguide to which the respective finline is coupled by its mode transducer (the short-circuits being indicated schematically in Fig. 1); the finlines 2 and 4 thus present substantially pure variable reactances across the input and output ports respectively of the FET.

Conductive coatings 8 and 10 are in conductive contact with the waveguide housing in which the substrate 6 is mounted, while coatings 7 and 9 are isolated from the housing at DC but coupled to it in the operating frequency range of the amplifier by means of thin dielectric sheets (not shown) interposed between the coatings and the housing. Respective low-pass filters 14, 1 5 are connected to coatings 7 and 9 respectively so that respective direct bias voltages may be applied to the gate and drain of the FET.One terminal of a respective capacitor 16, 1 7 is also connected to each of the coatings 7 and 9 close to the common central region, the other termi nal of each capacitor being connected through the substrate to a conductive coating (not shown) on the lower surface that in use is in contact with the housing: the fins formed by the coatings 7 and 9 are thus short-circuited to the housing at radio frequencies well below the operating frequency range of the amplifier and below the cut-off frequency of the finlines.

The input finline 1 and the output finline 3 comprise respective transformers 18, 1 9 which are each a quarter wavelength long at the centre of the operating frequency range of the amplifier arrangement and which in this case are spaced a quarter wavelength from the input and output ports of the FET at the common central region.

As explained above, the finlines 2 and 4 are respectively in parallel with the input and output finlines 1 and 3, and the substantially pure reactances presented across the input and output ports of the FET by finlines 2 and 4 may be used for matching. While the reactance across the input port is dependent only on finline 2 and the reactance across the output port only on finline 4, the input and output impedances of the FET itself are interdependent, the impedance presented by the device at one port being dependent on the loading of the other port, and therefore in the first instance, matching suitably is performed empirically by an iterative process. As an example, a result of the matching process may be as follows.Assuming that the input impedance of the FET has a resistance component (i.e. real part) less than the characteristic impedance Z0 (purely real in this case) of the input finline 1 and has an inductive reactance component (i.e. imaginary part), the shortcircuited stub comprising finline 2 is adjusted to provide across the input port a capacitive reactance such that the impedance loading the finline 1 is purely real (the value of said resistance component). Moving a quarter wavelength along the finline 1 away from the input port, this is transformed to a purely real impedance greater than ZO: this is matched by the quarter-wave transformer 1 8 to the characteristic impedance Z0 of the finline immediately beyond the transformer (i.e. to the left in Fig. 1).Similar considerations apply to the output port and finlines 3 and 4.

If the real part of the impedance presented by the solid-state amplifier device at a port thereof is greater than the characteristic impedance of the finline which is connected to that port and which comprises a quarter-wave transformer, the transformer may be disposed adjacent that port.

The capacitors 16, 17 inhibit the production of energy in the operating frequency range of the amplifier resulting from selfoscillation, for example oscillation with a fundamental at a relatively low microwave frequency and with significant energy at one or more higher harmonics in the operating frequency range of the amplifier. As an alternative to the capacitors, open-circuited stubs a quarter wavelength long at such a fundamental frequency may be connected to the con ductive coatings 7 and 9. It has been found that such oscillation is apparently due to the conductive coatings which form the fins acting, below the cut-off frequency of the finlines, so as to provide at the ports of the FET reactive loads (effectively open-circuited stubs) suitable for oscillation to occur.The capacitors 1 6 and 1 7 or the alternative quarter-wave open-circuited stubs shouid preferably be mounted as close as possible to the FET so as substantially to short-circuit these reactive loads at the fundamental oscillating frequency.

Fig. 3 shows a modification of the substrate of Fig. 1 and 2 in which the adjustable waveguide short-circuits of the stubs comprising finlines 2 and 4 have been replaced by fixed terminations of those finlines themselves (the mode transducers on those finlines being of course no longer required). In order not to connect the conductive coatings 7 and 9 (to which the bias voltages for the FET are applied) to the coatings 8 and 10 (which are in contact with the housing), each of the terminations comprises a respective coating 20, 21 which is disposed on the lower surface of the substrate and which bridges the gap between the fins on the upper surface, thus acting substantially as a short-circuit.Such an arrangement may for example be used when suitable matching conditions for a particular type of FET have been established using the arrangement of Figs. 1 and 2, and provides the advantages of being more compact and of providing approximately the same reactance for matching purposes over a broader bandwidth, since the positions of the terminations may be the closest to the common central region that are suitable for providing the necessary reactances.

The solid-state amplifier device may be an FET with only three terminals, i.e. with only a single source. Using the finline arrangement described above, the single source may be connected to either of the conductive coatings 8 and 10. As an alternative, the amplifier device may be an FET device having a plurality of drain and/or gate terminals which may be connected together in parallel, suitably within the device; with a piurality of source terminals, at least two source terminals may similarly be connected together. As a further alternative, the solid-state amplifier device may comprise another kind of transistor such as a bipolar transistor.

An amplifier arrangement embodying the invention may be suitable for integration with another arrangement using a transmission line of the kind set forth, such as a mixer. By reducing transmission losses, the invention enables the overall noise figure of such a combination to be improved.

Claims (8)

1. An RF solid-state amplifier arrangement comprising four balanced transmission lines each of the kind having a balanced pair of substantially coplanar conductors separated by a gap across which the electric field of microwave energy propagating along the transmission line in operation predominantly extends, said transmission lines extending towards a common region, and further comprising a solid-state amplifier device which is disposed at said common region and which has two ports, wherein the transmission lines and the solid-state amplifier device are coupled together in such a manner that in the operating frequency range of the amplifier arrangement.

said two ports are mutually coupled substantially only via the solid-state amplifier device, a first of said ports is connected across a first and across a second of said transmission lines, and the second of said ports is connected across a third and across the fourth of said transmission lines.

and wherein each of the second and fourth transmission lines is terminated so as to present at said region a substantially pure reactance, whereby said first and third transmission lines constitute respectively one an input for supplying RF energy to the solid-state amplifier device and the other an output for deriving RF energy from the solid-state amplifier device. and whereby the input and output impedances of the amplifier arrangement are dependent on the reactances presented at said common region by respectively one and the other of the second and fourth transmission lines.

2. An amplifier arrangement as claimed in claim 1 wherein the solid-state amplifier device has three terminals of which a first and second are respective terminals of the first and second ports and the third is common to both ports, the first terminal being connected to one of said pair of conductors of each of the first and second transmission lines, the second terminal being connected to one of said pair of conductors of each of the third and fourth transmission lines, and the third terminal being connected to one of said pair of conductors of each of the four transmission lines, and wherein said pair of conductors of the first transmission line are respectively connected to said pair of conductors of the second transmission line and said pair of conductors of the third transmission line are respectively connected to said pair of conductors of the fourth transmission line.

3. An amplifier arrangement as claimed in claim 1 or 2 wherein the first and/or third transmission line comprises means for matching to a desired value the impedance presented at the first and/or second port respectively by the combination of the solid-state amplifier device and the second and/or fourth transmission line respectively.

4. An amplifier arrangement as claimed in claim 3 wherein the real part of the impedance presented by the solid-state amplifier device at a said port connected across a transmission line comprising said matching means is less than the real part of said desired value, and wherein said matching means comprises a transformer which is substantially a quarter wavelength long at a frequency in the operating frequency range of the amplifier and which is spaced from said common region by substantially a quarter wavelength at said frequency.

5. An amplifier arrangement as claimed in claim 2, or as claimed in claim 3 or 4 when appendant to claim 2, wherein at least one of the second and fourth transmission-lines is a finline comprising two conductive fins on only one major surface of a dielectric substrate, said two conductive fins being mutually isolated at DC, and wherein the fin line is terminated by a conductive layer which is disposed on the other major surface of the substrate and which bridges the gap between the two fins of the finline on said one major surface.

6. An amplifier arrangement as claimed in any preceding claim comprising means for inhibiting the production of energy in the operating frequency range resulting from selfoscillation.

7. An amplifier arrangement as claimed in any preceding claims wherein the solid-state amplifier device comprises a field-effect transistor, said first and second terminals being respectively one a gate and the other a drain, and the third terminal being a source.

8. A RF solid-state amplifier arrangement substantially as herein described with reference to the drawings.