Distributed amplifier

A distributed amplifier with a simple structure that permits miniaturization of circuit in which the input-side transmission line of the distributed amplifier is constituted by a coplanar line having a signal line and a ground face formed on the upper face of a dielectric substrate and the output-side transmission line of the distributed amplifier is constituted by a microstrip line having a signal line formed on the upper face of the dielectric substrate and a ground face formed on the lower face of the dielectric substrate. In addition, a plurality of amplification transistors is formed on the upper face of the dielectric substrate and each of the electrodes is connected to the signal line or the ground face that is formed on the upper face of the dielectric substrate. Because a large transistor-drive current does not flow to the input-side transmission line, the signal line can be made narrow and there is no increase in the required surface area even in the case of a coplanar line. Further, because the ground face of the coplanar line can be formed on the upper face of the dielectric substrate, the source electrodes or similar of the transistors can be grounded easily with a simple structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a distributed amplifier having a broadband characteristic that is employed in an optical communication system, for example.

2. Description of the Related Art

The prior art includes Japanese Patent Kokai No. H7-183428 (Patent Document 1) and Japanese Patent Kokai No. 2003-152476 (Patent Document 2).

FIG. 1is a constitutional view of a conventional distributed amplifier.

As per the circuit constitution shown inFIG. 1, this distributed amplifier comprises an input-side transmission line2connected to an input terminal1that is supplied with input signals over a wide range. The input-side transmission line2serially connects seven line elements2a,2b, . . . ,2g, for example. The leading terminal of line element2ais connected to input terminal1and the end terminal of line element2gis connected to the ground potential GND via a terminator3.

This distributed amplifier also comprises an output-side transmission line4that corresponds with the input-side transmission line2. The output-side transmission line4serially connects seven line elements4a,4b, . . . ,4g. The leading terminal of the line element4ais connected to the ground potential GND via a terminator5and the end terminal of the line element4gis connected to an output terminal6.

Six transistors7a,7b, . . . ,7f, which are amplification units, are also connected in the form of a distribution circuit via six transistors7a,7b, . . . ,7fconstituting amplification units and a transmission line8between the input-side transmission line2and the output-side transmission line4. That is, the gate electrode of the transistor7ais connected to the connection point of the line elements2aand2band the drain electrode of the transistor7ais connected to the connection point of the line elements4aand4bvia the line element8aof the transmission line8. Likewise, hereinafter, the gate electrode of the transistor7fis connected to the connection point of the final line elements2fand2gand the drain electrode of the transistor7fis connected to the connection point of the line elements4fand4gvia the line element8f. The source electrodes of the transistors7ato7fare connected to the ground potential GND. Further, a supply voltage VDD is supplied to the output terminal6via an inductor9.

Representative forms that constitute the input-side transmission line2, output-side transmission line4and amplification-unit transmission line8include the microstrip line and coplanar line.

As shown inFIG. 2, the microstrip line is constituted such that the whole of the rear face of the dielectric substrate SUB1is the ground face G1and the signal line S1is disposed on the upper surface of the dielectric substrate SUB1. Supposing that the width of the signal line S1is w1and the thickness of the dielectric substrate SUB1is t, the characteristic impedance Z0of the microstrip line can be expressed as Z0=A×t/w1(where A is the proportionality constant).

Meanwhile, as shown inFIG. 3, the coplanar line has a signal line S2disposed on the upper surface of a dielectric substrate SUB2and there is a ground face G2on both sides of the signal line S2. Supposing that the width of the signal line S2is w2and the interval between the signal line S2and ground faces G2is w3, the characteristic impedance Z0of the coplanar line can be expressed as Z0=B×w3/w2(where B is the proportionality constant).

The distributed amplifier is normally designed with an I/O impedance of 50Ω. For this reason, the characteristic impedance Z0of the input-side transmission line2or the like must be set as 70 to 90Ω in consideration of the static capacitance of the transistors7ato7fof the amplification unit.

However, the distributed amplifier is subject to the following problems.

(1) When the input-side transmission line2and output-side transmission line4are both constituted by a microstrip line, the signal line and ground face are on different faces with a dielectric substrate interposed therebetween. Therefore, viaholes must be provided in the dielectric substrate in order to reach ground via the shortest route as per the source electrodes of the transistors7ato9finFIG. 1, which makes the wiring process complicated.

(2) When the input-side transmission line2and output-side transmission line4are both constituted by a coplanar line, the width w2and internal w3must both be wide in order to provide characteristic impedance Z0of 70 to 90Ω and to secure the current capacitance flowing to the transistors7ato9f.

For example, when a microstrip line with a signal line width of 50 μm is created by using a GaAs substrate that is 220 μm thick, the characteristic impedance is then 77Ω. In contrast, in order to create a coplanar line with the same signal line width and characteristic impedance by means of the same GaAs substrate, the interval between the signal line and ground faces must be 140 μm. Therefore, miniaturization of the circuit is problematic.

(3) As disclosed in Patent Document 1, when a microstrip line and a coplanar line are mixed and the signal lines of the microstrip line and coplanar line respectively are arranged on different faces, the structure is a complex one.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a distributed amplifier with a simple structure that permits miniaturization.

The distributed amplifier of the present invention uses a coplanar line and a microstrip line as transmission lines. The signal line and ground faces of the coplanar line, the signal line of the microstrip line, and a plurality of amplification transistors are formed on the upper face of the dielectric substrate and the ground faces of the microstrip line are formed on the lower face of the dielectric substrate.

According to the present invention, the signal line and ground faces of the coplanar line, the signal line of the microstrip line, and the amplification transistors are formed on the upper face of the dielectric substrate. Therefore, all of the respective electrodes of the transistors can be connected to the signal lines and ground faces, and so forth, on the upper face of the dielectric substrate. Accordingly, viaholes need not be provided in the dielectric substrate in order to connect the transistors. As a result, a distributed amplifier with a simple structure that permits circuit miniaturization is obtained, and so forth.

The input-side transmission line of the distributed amplifier is constituted by a coplanar line having a signal line and ground faces formed on the upper face of a dielectric substrate and the output-side transmission line is constituted by a microstrip line having a signal line formed on the upper face of the dielectric substrate and ground faces formed on the lower face of the dielectric substrate. In addition, a plurality of amplification transistors is formed on the upper face of the dielectric substrate and the respective electrodes of the transistors are connected to the signal line or ground faces thus formed on the upper face of the dielectric substrate.

Further objects and new characteristics of the invention will surely become more completely clear upon reading the following description of the preferred embodiments in conjunction with the attached drawings. However, the drawings merely permit an understanding of the invention and do not limit the scope thereof.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4is a constitutional view of the distributed amplifier illustrating the embodiment of the present invention.FIGS. 5 and 6show the structure of the distributed amplifier inFIG. 4, whereFIG. 5is a planar view thereof andFIG. 6is a partial cross-sectional view thereof along the line VI-VI inFIG. 5.

As the circuit constitution inFIG. 4shows, the distributed amplifier comprises an input-side transmission line10that is connected to an input terminal1to which broad-range input signals are supplied and that constitutes an input-side filter. As shown inFIG. 6, the input-side transmission line10is a coplanar line that is constituted by the signal line S2and ground faces G2that are formed on the surface of the dielectric substrate SUB. In addition, the ground face G1is provided over the whole of the rear face of the dielectric substrate SUB.

The input-side transmission line10serially connects seven line elements11,12, . . . ,17such that the leading terminal of the line element11is connected to the input terminal1and the end terminal of the line element17is connected to the ground potential GND via the terminator3.

Further, the distributed amplifier comprises an output-side transmission line20constituting an output-side filter that corresponds with an input-side transmission line10. As shown inFIG. 6, the output-side transmission line20is a microstrip line that is constituted by a signal line S1, which is formed on the upper surface of the dielectric substrate SUB, and the ground face G1that is provided over the whole of the rear face of the dielectric substrate SUB.

The output-side transmission line20serially connects seven line elements21,22, . . . ,27such that the leading terminal of the line element21is connected to the ground potential GND via the terminator5and the end terminal of the line element27is connected to the output terminal6.

A plurality of transistors31,32, . . . ,36constituting amplification units are connected in the form of a distribution circuit via a transmission line40that is constituted by a microstrip line between the input-side transmission line10and the output-side transmission line20. That is, the gate electrode of the transistor31is connected to the connection point between the line elements11and12and the drain electrode of the transistor31is connected to the connection point between the line elements21and22via a line element41of the transmission line40. Likewise thereafter, the gate electrode of the transistor36is connected to the connection point between the final line elements16and17and the drain electrode of the transistor36is connected to the connection point between the line elements26and27via the line element46. The source electrodes of the transistors31to36are each connected to the ground potential GND. Further, the output terminal6has the supply voltage VDD supplied thereto via the inductor9.

Here, a GaAs substrate for which the thickness t1is 220 μm, for example, is used as the dielectric substrate SUB and the signal lines S1and S2are formed from metal for which the thickness t2is 3 μm, for example. Further, the width w2of the signal line S2of the coplanar line, which is the input-side transmission line10, is 5 μm, and the interval w3between the signal line S2and ground faces G2is set at 27.5 μm. Further, the width w1of the signal line S1of the microstrip line, which is the output-side transmission line20and the amplification-unit transmission line40is then 40 μm.

As a result, the characteristic impedance Z0of the input-side transmission line10, output-side transmission line20, and transmission line40is then 82Ω and the permissible current of the output-side transmission line20and transmission line40is then 400 mA or more. Further, the transistors31to36and the terminators3and5are formed on the surface of the dielectric substrate SUB in the same manner as the signal lines S1and S2. Therefore, the respective source electrodes of the transistors31to36and the ground side of the terminators3and5are connected to the ground faces G2that are formed on the surface of the dielectric substrate SUB.

In the case of this distributed amplifier, the supply voltage VDD is supplied to the output terminal6via the inductor9and to the drain electrode of each of the transistors41to46respectively via the output-side transmission line20and transmission line40. An input signal on which a suitable bias voltage is superposed is also supplied from the input terminal1.

The input signal is amplified as a result of being supplied to the gate electrode of each of the transistors31to36respectively via the input-side transmission line10constituting the input-side filter. The signals outputted from each of the transistors31to36are transmitted to the output-side transmission line20constituting the output-side filter via the transmission line40. These signals are then synthesized by means of the output-side transmission line20and outputted from the output terminal6as an output signal.

Thus, with the distributed amplifier of this embodiment, a DC current barely flows and the input-side transmission line10, for which a broad signal line with a large current capacitance is not necessary, is constituted by a coplanar line. As a result, the pattern area of the input-side transmission line10barely increases even when a coplanar line is used. In addition, by employing a coplanar line, the ground faces G2can be provided on the surface of the dielectric substrate SUB. Therefore, the source electrodes of the transistors31to36can be connected directly to the ground faces G2without the interposition of viaholes. There is therefore the advantage that the structure is simple and the circuit miniaturization is possible.

Further, the output-side transmission line20, which requires a large DC current to flow, is constituted by a microstrip line. As a result, the long-term reliability of the metal forming the signal line can be assured and the output impedance can be set to a predetermined 50 ohms over a wide range without enlarging the circuit pattern.

In addition, circuit elements such as the signal lines S1and S2, the terminators3and5, and the transistors31to36are all formed on the surface of the dielectric substrate SUB and the rear face of the dielectric substrate SUB is the ground face G1, whereby the fabrication process can be simplified.

Further, the present invention is not limited to the embodiment above. Rather, a variety of modifications is possible. Such modified examples include the following, for example:

(a) The number of stages of the distributed amplifier is not limited to six: any number of stages is satisfactory;

(b) the widths w1and w2of the signal lines S1and S2respectively, the interval w3between the signal line S2and ground face G2, thicknesses t1and t2of the dielectric substrate SUB and signal line respectively and the value of the characteristic impedance Z0are not limited to those illustrated;

(c) in the case of the input-side transmission line10, which is a coplanar line, the ground face G1is also formed on the rear-face side of the dielectric substrate SUB but the ground face G1on the rear-face side of the input-side transmission line10may be dispensed with;

(d) as the transistors31to36, bipolar transistors can be used in addition to field effect transistors such as source/ground-type field effect transistors, cascode-connected field effect transistors, and dual-gate field effect transistors; and

(e) the dielectric substrate SUB is not limited to GaAs and may instead be silicon or the like.

This application is based on Japanese Patent Application No. 2004-188144 which is herein incorporated by reference.