GB1585079A

GB1585079A - Microwave circuits incorporating transistors

Info

Publication number: GB1585079A
Application number: GB4615776A
Authority: GB
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1977-10-25
Filing date: 1977-10-25
Publication date: 1981-02-25

Description

(54) IMPROVEMENTS IN OR RELATING TO MICROWAVE CIRCUITS INCORPORATING TRANSISTORS (71) We, THE GENERAL ELECTRIC COMPANY LIMITED, of 1 Stanhope Gate, London W1A 1EH, a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and b the following state ment:- This invention relates to microwave circuits incorporating transistors.

An important feature of such a circuit, is the impedance of the lead to the common electrode of the transistor, i.e. that electrode which is common to input and output circuits of the transistor. This lead impedance gives rise to unwanted feedback which at microwave frequencies can be large enough to give rise to gain and stability limitations. Furthermore, since a significant part of this lead impedance is often attributable to a conductor within the transistor package, it is often not possible to eliminate its effect by capacitive decoupling.

It is an object of the present invention to overcome this difficulty.

According to the invention there is provided a microwave transistor circuit arrangement comprising a pair of matched microwave transistors which are connected in identical circuit arrangements so as to operate as a push-pull pair in response to anti-phase, but otherwise identical, signals applied to their respective inputs, and whose common electrodes share a common lead whose impedance constitutes a major part of the total impedance of the lead to each other said common electrode.

In a circuit arrangement in accordance with the invention said common lead will clearly carry zero current of input signal frequency, so making the operation of the circuit arrangement totally independent of the impedence of said common lead at input signal frequencies.

It will be appreciated that in a circuit arrangement in accordance with the invention the impedence of the common lead will preferably constitute substantially the whole of the total impedance of the lead to each said common electrode, i.e. the impedances contributed to total impedance by conductors at either end of said common lead are made as small as possible.

This can easily be achieved in respect of any impedance contributed contributed to total impedance by conductors at the end of said common lead further from the transistors, but at the other end of the common lead, i.e. in respect of any impedance contributed to total impedance by an conductor individual to a respective transistor, can only be achieved by using transistors of suitable construction.

Hence, in a preferred microwave transistor arrangement in accordance with the invention, said common electrodes of said transistors are constituted by an electrically conductive member of substantially zero impedance which serves as an electrode for one region of one transistor and for the corresponding region of the other transistor, said common lead being directly connected to said electrically conductive member.

In one such arrangement said transistors are of the kind whose different regions comprise regions of material extending into a generally planar substrate, said one region of one transistor and the corresponding region of the other transistor are constituted by the same single region in the substrate and said electrically conductive member comprises a unitary area of electrically conductive material in ohmic contact with said single region. Typically, the transistors are field-effect transistors.

Several circuit arrangements in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a circuit arrangement intended for use in an amplifying circuit; Figure 2 is a perspective view of a fieldeffect transistor arrangement suitable for use in the circuit arrangement of Figure 1; Figure 3 is a schematic diagram of a circuit arrangement intended for use in an oscillating circuit; Figure 4 is a plan view of a structural embodiment of the circuit of Figure 3; Figure 5 is a plan view of an oscillator circuit incorporating the structure of Figure 4; and Figure 6 is a schematic diagram of the circuit of Figure 5.

Referring to Figure 1 the first circuit to be described comprises a pair of matched microwave field-effect transistors 1 and 2 having their sources S connected via a common land lead 3 to a line 4. The drains D of the transistors are connected via respective inductances 5 and 6 to a line 7, an energising voltage V being applied between the lines 4 and 7 in operation. The drains D of the transistors are futher respectively connected to terminals 8 and 9 and the gates G of the transistors are- connected to respective terminals 10 and 11.

In operation, anti-phase, but otherwise identical, microwave input signals are applied respectively between the terminal 10 and the line 4 and between the terminal 11 and the line 4.

The transistors operate as a push-pull pair providing balanced outputs between the terminals 8 and 9 and the line 4.

Since the input signal frequency source currents of the transistors are at all times of equal magnitude and opposite direction, the input signal frequency current in the common lead 3 is zero. Hence any impedance which the lead 3 may have at input signal frequencies will not affect the operation of the circuit. Thus the designer is freed from the layout restrictions which exist in known comparable transistor microwave amplifiers because of the need to provide the best possible, i.e. lowest possible impedance, input signal frequency grounding for the transistor common electrodes.

The line 4 will normally be constructed so as not to have any appreciable impedance at input signal frequencies. However, if this is not the case, then the source of energising voltage may be connected to the line 4 so that the source currents of both transistors flow between the lead 3 and the voltage source via the same part of the line 4 which part of the line then constitutes an extension of the lead 3.

It will be understood, however, that any impedance at input signal frequencies which may be present in the connections between the actual source regions of the transistors and the adjacent end of the lead 3 will give rise to feedback and affect the operation of the circuit. It is therefore desirable to minimise the impedance of such connections.

Figure 2 shows a transistor arrangement for use in the circuit of Figure 1 wherein such mini misation of impedance is achieved.

Referring to Figure 2, the arrangement comprises a semi-insulating substrate 12 having a rectangular mesa 13 of n-type material on one of its main faces. On the exposed main face of the mesa 13 there are formed three rectangular metallised areas 14, 15 and 16 in ohmic contact with the underlying material. Between the central area 15 and the outer area 14 there is formed a relatively narrow rectangular metallised area 17 in Schottky contact with the underlying material, the area 17 extending beyond the mesa 13 at one end to make contact with a further rectangular metallised are 18 formed on the substrate 12 to one side of the mesa 13. A similar metallised area 19 making Schottky contact with the underlying material and connected to a metallised area 20 on the substrate is provided on the mesa 13 between the metallised areas 15 and 16.

The substrate 12 typically consists of gallium arsenide, the mesa 13 being formed by growing a suitably doped epitaxial layer on one main face of the substrate and etching away appropriate peripheral regions of the epitaxial layer.

The structure described with reference to Figure 2 constitutes a pair of field-effect transistors whose source regions are constituted by the material underlying the metallised area 15, the area 15 thus serving as the source electrode for both transistors. The metallised areas 14 and 16 respectively constitute the drain electrodes of the two transistors and the underlying material the drain regions of the transistors.

The metallised areas 17 and 19 respectively constitute the gate electrodes of the transistors, and the areas 18 and 20 constitute connection pads for the gate electrodes.

To facilitate external connections to the electrodes, the metallised areas 14, 15, 16, 18 and 20 are provided with bond wires 21.

In use in the circuit arrangement of Figure 1, the bond wire 21 between the source electrode 15 and the associated terminal effectively forms part of the lead wire 3. Thus any impedance at signal frequencies which this bond wire 21 may have will not affect the operation of the circuit. The source electrode 15 itself has substantially zero impedance so that it makes a negligible contribution to the total common electrode lead of each transistor in the circuit arrangement.

The input signals required for the circuit arrangement of Figure 1 are suitably obtained by means of a 180 hybrid or by means of a field-effect transistor connected so as to provide anti-phase outputs at its source and drain.

The balanced outputs at terminals 8 and 9 may suitably be combined in a 1800 hybrid. If desired, a load may be simply connected between one of the terminals 8 and 9 and the line 4, but this would, of course, involve a 3 db loss of gain. Alternatively the balanced outputs may be used directly to drive a load which is capable of accepting a push-pull drive, e.g. another push-pull amplifying stage.

Referring now to Figure 3, the second circuit to be described comprises a pair of matched microwave field effect transistors 22 and 23 having their sources S connected via a common lead 24 to a line 25 which is maintained at ground potential in operation. The drains D of the transistors are connected via respective inductances 26 and 27 to a line 28. The gate G of the transistor 22 is connected to the junction between two resistors 29 and 30 connected in series between the drain of transistor 23 and a line 31, and the gate G of the transistor 23 is connected to the junction between two resistors 32 and 33 connected in series between the drain of transistor 22 and the line 31. The drains of the transistors are further connected to respective terminals 34 and 35.

In operation the lines 28 and 31 are maintained at opposite potentials V1 and V2 with respect to ground and a resonator (not shown) is connected between the terminals 34 and 35.

The circuit then osicillates in known manner at a frequency determined by the resonator.

The effect of impedance at the oscillation frequency in the common lead 24 is eliminated as described above in relation to lead 3 of Figure 1. This has the advantage that the gain of the transistors 22 and 23 is not appreciably reduced by feedback due to impedance in their common electrode leads so that a minimum of the output is used in providing the feedback required to maintain oscillations.

By using transistors having significant gain at the odd harmonics of the fundamental frequency of the resonator, the arrangement can be operated as a class D oscillator at a higher efficiency than that of conventional sinusoidal microwave oscillator arrangements.

Referring also to Figure 4, the part of the circuit of Figure 3 witlun the rectangle 56 is conveniently fabricated as a monolithic structure on a substrate 36. The transistors 22 and 23 are constituted by a similar structure to that shown in Figure 2 comprising a mesa of n-type material 37, metallised areas 38, 39 and 40 which are in ohmic contact with the mesa and serve as the source and drain electrodes of the transistors, and metallised areas 41 and 42 which are in Schott ky contact with the mesa and serve as the gate electrodes of the transistors.

The registers 29 and 30 are constituted by a second mesa 43 having a central metallised area 44 which is electrically connected to the gate metallised area 41, a metallised area 45 at one end which is electrically connected to the drain metallised area 40 and provided with a bond wire 46, and a metallised area 47 at its other end provided with a bond wire 48. Thus the part of the mesa 43 between metallised areas 44 and 45 constitutes the resistor 29 and the part of the mesa 43 between the metallised areas 44 and 47 constitutes the resistor 30.

The resistors 32 and 33 are similarly constituted by a third mesa 49 having a central metallised area 50 electrically connected to the gate metallised area 42, a metallised area 51 at one end electrically connected to the drain metallised area 38 and provided with a bond wire 52, and a metallised area 53 at its other end provided with a bond wire 54.

The source metallised area 39 is provided with a bond wire 55 which, together with the other bond wires 46, 48, 52 and 54 facilitates external connection to the structure.

Figure 5 shows a microstrip oscillator circuit arrangement incorporating the structure 56 of Figure 4.

In this arrangement, the bond wires 48 and 54 are connected to microstrip conductors 57 and 58 which are connected to a source of negative potential -V2 in operation, and the bond wire 55 is connected to a microstrip conductor 59 connected to ground in operation.

The bond wires 46 and 52 are connected to opposite ends of a microstrip resonator 60 of known form, the central point on the resonator microstrip being connected to a source of positive +V1 in operation via a microstrip conductor 61.

The output of the arrangement is derived by means of a further microstrip conductor 62 incorporating a capacitor 63, the conductor 62 making connection with the resonator 60 at a point chosen in dependence on the required output impedance of the arrangement.

An equivalent circuit diagram of the arrangement of Figure 4 is shown in Figure 6, the elements of Figure 6 being given the same reference numerals as the corresponding elements in Figure 5.

It is pointed out that, whilst in the arrangement described above, by way of example, the common electrodes of the transistors are their source electrodes, in other arrangements in accordance with the invention the common electrodes may be other electrodes of the transistors. Similarly, whilst field-effect transistors are employed in the arrangements described by way of example, other kinds of transistors may be employed in other arrangements in accordance with the invention.

WHAT WE CLAIM IS: 1. A microwave transistor circuit arrangement comprising a pair of matched microwave transistors which are connected in identical circuit arrangements so as to operate as a pushpull pair in response to anti-phase, but otherwise identical, signals applied to their respective inputs, and whose common electrodes share a common lead whose impedance constitutes a major part of the total impedance of the lead to each said common electrode.

2. A circuit arrangement according to Claim 1 wherein the impedance of the common lead constitutes substantially the whole of the total impedance of the lead to each said common electrode.

3. A circuit arrangement according to Claim 1 or Claim 2 for use in an amplifying circuit.

4. A circuit arrangement according to Claim 1 or Claim 2 for use in an oscillating circuit.

5. A microwave transistor circuit arrangement according to any preceding claim wherein said common electrodes of said transistors are constituted by an electrically conductive member of substantially zero impedance which serves as an electrode for one region of one transistor and for the corresponding region of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. between two resistors 29 and 30 connected in series between the drain of transistor 23 and a line 31, and the gate G of the transistor 23 is connected to the junction between two resistors 32 and 33 connected in series between the drain of transistor 22 and the line 31. The drains of the transistors are further connected to respective terminals 34 and 35. In operation the lines 28 and 31 are maintained at opposite potentials V1 and V2 with respect to ground and a resonator (not shown) is connected between the terminals 34 and 35. The circuit then osicillates in known manner at a frequency determined by the resonator. The effect of impedance at the oscillation frequency in the common lead 24 is eliminated as described above in relation to lead 3 of Figure 1. This has the advantage that the gain of the transistors 22 and 23 is not appreciably reduced by feedback due to impedance in their common electrode leads so that a minimum of the output is used in providing the feedback required to maintain oscillations. By using transistors having significant gain at the odd harmonics of the fundamental frequency of the resonator, the arrangement can be operated as a class D oscillator at a higher efficiency than that of conventional sinusoidal microwave oscillator arrangements. Referring also to Figure 4, the part of the circuit of Figure 3 witlun the rectangle 56 is conveniently fabricated as a monolithic structure on a substrate 36. The transistors 22 and 23 are constituted by a similar structure to that shown in Figure 2 comprising a mesa of n-type material 37, metallised areas 38, 39 and 40 which are in ohmic contact with the mesa and serve as the source and drain electrodes of the transistors, and metallised areas 41 and 42 which are in Schott ky contact with the mesa and serve as the gate electrodes of the transistors. The registers 29 and 30 are constituted by a second mesa 43 having a central metallised area 44 which is electrically connected to the gate metallised area 41, a metallised area 45 at one end which is electrically connected to the drain metallised area 40 and provided with a bond wire 46, and a metallised area 47 at its other end provided with a bond wire 48. Thus the part of the mesa 43 between metallised areas 44 and 45 constitutes the resistor 29 and the part of the mesa 43 between the metallised areas 44 and 47 constitutes the resistor 30. The resistors 32 and 33 are similarly constituted by a third mesa 49 having a central metallised area 50 electrically connected to the gate metallised area 42, a metallised area 51 at one end electrically connected to the drain metallised area 38 and provided with a bond wire 52, and a metallised area 53 at its other end provided with a bond wire 54. The source metallised area 39 is provided with a bond wire 55 which, together with the other bond wires 46, 48, 52 and 54 facilitates external connection to the structure. Figure 5 shows a microstrip oscillator circuit arrangement incorporating the structure 56 of Figure 4. In this arrangement, the bond wires 48 and 54 are connected to microstrip conductors 57 and 58 which are connected to a source of negative potential -V2 in operation, and the bond wire 55 is connected to a microstrip conductor 59 connected to ground in operation. The bond wires 46 and 52 are connected to opposite ends of a microstrip resonator 60 of known form, the central point on the resonator microstrip being connected to a source of positive +V1 in operation via a microstrip conductor 61. The output of the arrangement is derived by means of a further microstrip conductor 62 incorporating a capacitor 63, the conductor 62 making connection with the resonator 60 at a point chosen in dependence on the required output impedance of the arrangement. An equivalent circuit diagram of the arrangement of Figure 4 is shown in Figure 6, the elements of Figure 6 being given the same reference numerals as the corresponding elements in Figure 5. It is pointed out that, whilst in the arrangement described above, by way of example, the common electrodes of the transistors are their source electrodes, in other arrangements in accordance with the invention the common electrodes may be other electrodes of the transistors. Similarly, whilst field-effect transistors are employed in the arrangements described by way of example, other kinds of transistors may be employed in other arrangements in accordance with the invention. WHAT WE CLAIM IS:

1. A microwave transistor circuit arrangement comprising a pair of matched microwave transistors which are connected in identical circuit arrangements so as to operate as a pushpull pair in response to anti-phase, but otherwise identical, signals applied to their respective inputs, and whose common electrodes share a common lead whose impedance constitutes a major part of the total impedance of the lead to each said common electrode.

the other transistor, said common lead being directly connected to said electrically conductive member.

6. A microwave transistor circuit arrangement according to Claim 5 wherein said transistors are of the kind whose different regions comprise regions of material extending into a generally planar substrate, said one region of one transistor and the corresponding region of the other transistor are constituted by the same single region in the substrate and said electrically conductive member comprises a unitary area of electrically conductive material in ohmic contact with said single region.

7. A microwave transistor circuit arrangement according to Claim 6 wherein said transistors are field-effect transistors.

8. A microwave transistor circuit arrangement according to Claim 7 wherein said transistors comprise a transistor arrangement substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

9. A microwave transistor circuit arrangement substantially as hereinbefore described with reference to Figure 1, Figure 3, Figure 4, Figure 5 or Figure 6 of the accompanying drawings.